Simple Generic Data Types
=========================

This chapter describes those of Guile's simple data types which are primarily used for their role as items of generic data.  By simple we mean data types that are not primarily used as containers to hold other data --- i.e. pairs, lists, vectors and so on. For the documentation of such compound data types, see Compound Data Types.

Booleans
--------

The two boolean values are #t for true and #f for false. They can also be written as #true and #false, as per R7RS.

Boolean values are returned by predicate procedures, such as the general equality predicates eq?, eqv? and equal? (see Equality) and numerical and string comparison operators like string=? (see String Comparison) and <= (see Comparison).

     (<= 3 8)
     => #t

     (<= 3 -3)
     => #f

     (equal? "house" "houses")
     => #f

     (eq? #f #f)
     =>
     #t

In test condition contexts like if and cond (see Conditionals), where a group of subexpressions will be evaluated only if a CONDITION expression evaluates to ``true'', ``true'' means any value at all except #f.

     (if #t "yes" "no")
     => "yes"

     (if 0 "yes" "no")
     => "yes"

     (if #f "yes" "no")
     => "no"

A result of this asymmetry is that typical Scheme source code more often uses #f explicitly than #t: #f is necessary to represent an if or cond false value, whereas #t is not necessary to represent an if or cond true value.

It is important to note that #f is not equivalent to any other Scheme value.  In particular, #f is not the same as the number 0 (like in C and C++), and not the same as the ``empty list'' (like in some Lisp dialects).

In C, the two Scheme boolean values are available as the two constants SCM_BOOL_T for #t and SCM_BOOL_F for #f. Care must be taken with the false value SCM_BOOL_F: it is not false when used in C conditionals.  In order to test for it, use scm_is_false or scm_is_true.

 -- Scheme Procedure: not x
 -- C Function: scm_not (x)
     Return #t if X is #f, else return #f.

 -- Scheme Procedure: boolean? obj
 -- C Function: scm_boolean_p (obj)
     Return #t if OBJ is either #t or #f, else return #f.

 -- C Macro: SCM SCM_BOOL_T
     The SCM representation of the Scheme object #t.

 -- C Macro: SCM SCM_BOOL_F
     The SCM representation of the Scheme object #f.

 -- C Function: int scm_is_true (SCM obj)
     Return 0 if OBJ is #f, else return 1.

 -- C Function: int scm_is_false (SCM obj)
     Return 1 if OBJ is #f, else return 0.

 -- C Function: int scm_is_bool (SCM obj)
     Return 1 if OBJ is either #t or #f, else return 0.

 -- C Function: SCM scm_from_bool (int val)
     Return #f if VAL is 0, else return #t.

 -- C Function: int scm_to_bool (SCM val)
     Return 1 if VAL is SCM_BOOL_T, return 0 when VAL is SCM_BOOL_F, else signal a `wrong type' error.

     You should probably use scm_is_true instead of this function when you just want to test a SCM value for trueness.

Numerical data types
--------------------

Guile supports a rich ``tower'' of numerical types --- integer, rational, real and complex --- and provides an extensive set of mathematical and scientific functions for operating on numerical data.  This section of the manual documents those types and functions.

You may also find it illuminating to read R5RS's presentation of numbers in Scheme, which is particularly clear and accessible: see Numbers (R5RS).

Scheme's Numerical ``Tower''
............................

Scheme's numerical ``tower'' consists of the following categories of numbers:

integers
     Whole numbers, positive or negative; e.g. --5, 0, 18.

rationals
     The set of numbers that can be expressed as P/Q where P and Q are integers; e.g. 9/16 works, but pi (an irrational number) doesn't. These include integers (N/1).

real numbers
     The set of numbers that describes all possible positions along a one-dimensional line. This includes rationals as well as irrational numbers.

complex numbers
     The set of numbers that describes all possible positions in a two dimensional space. This includes real as well as imaginary numbers (A+Bi, where A is the real part, B is the imaginary part, and i is the square root of -1.)

It is called a tower because each category ``sits on'' the one that follows it, in the sense that every integer is also a rational, every rational is also real, and every real number is also a complex number (but with zero imaginary part).

In addition to the classification into integers, rationals, reals and complex numbers, Scheme also distinguishes between whether a number is represented exactly or not.  For example, the result of 2\sin(\pi/4),2*sin(pi/4) is exactly \sqrt2,2^(1/2), but Guile can represent neither \pi/4,pi/4 nor \sqrt2,2^(1/2) exactly. Instead, it stores an inexact approximation, using the C type double.

Guile can represent exact rationals of any magnitude, inexact rationals that fit into a C double, and inexact complex numbers with double real and imaginary parts.

The number? predicate may be applied to any Scheme value to discover whether the value is any of the supported numerical types.

 -- Scheme Procedure: number? obj
 -- C Function: scm_number_p (obj)
     Return #t if OBJ is any kind of number, else #f.

For example:

     (number? 3)
     => #t

     (number? "hello there!")
     => #f

     (define pi 3.141592654)
     (number? pi)
     => #t

 -- C Function: int scm_is_number (SCM obj)
     This is equivalent to scm_is_true (scm_number_p (obj)).

The next few subsections document each of Guile's numerical data types in detail.

Integers
........

Integers are whole numbers, that is numbers with no fractional part, such as 2, 83, and -3789.

Integers in Guile can be arbitrarily big, as shown by the following example.

     (define (factorial n)
       (let loop ((n n) (product 1))
         (if (= n 0)
             product
             (loop (- n 1) (* product n)))))

     (factorial 3)
     => 6

     (factorial 20)
     => 2432902008176640000

     (- (factorial 45))
     => -119622220865480194561963161495657715064383733760000000000

Readers whose background is in programming languages where integers are limited by the need to fit into just 4 or 8 bytes of memory may find this surprising, or suspect that Guile's representation of integers is inefficient.  In fact, Guile achieves a near optimal balance of convenience and efficiency by using the host computer's native representation of integers where possible, and a more general representation where the required number does not fit in the native form.  Conversion between these two representations is automatic and completely invisible to the Scheme level programmer.

C has a host of different integer types, and Guile offers a host of functions to convert between them and the SCM representation. For example, a C int can be handled with scm_to_int and scm_from_int.  Guile also defines a few C integer types of its own, to help with differences between systems.

C integer types that are not covered can be handled with the generic scm_to_signed_integer and scm_from_signed_integer for signed types, or with scm_to_unsigned_integer and scm_from_unsigned_integer for unsigned types.

Scheme integers can be exact and inexact.  For example, a number written as 3.0 with an explicit decimal-point is inexact, but it is also an integer.  The functions integer? and scm_is_integer report true for such a number, but the functions exact-integer?, scm_is_exact_integer, scm_is_signed_integer, and scm_is_unsigned_integer only allow exact integers and thus report false.  Likewise, the conversion functions like scm_to_signed_integer only accept exact integers.

The motivation for this behavior is that the inexactness of a number should not be lost silently.  If you want to allow inexact integers, you can explicitly insert a call to inexact->exact or to its C equivalent scm_inexact_to_exact.  (Only inexact integers will be converted by this call into exact integers; inexact non-integers will become exact fractions.)

 -- Scheme Procedure: integer? x
 -- C Function: scm_integer_p (x)
     Return #t if X is an exact or inexact integer number, else return #f.

          (integer? 487)
          => #t

          (integer? 3.0)
          => #t

          (integer? -3.4)
          => #f

          (integer? +inf.0)
          => #f

 -- C Function: int scm_is_integer (SCM x)
     This is equivalent to scm_is_true (scm_integer_p (x)).

 -- Scheme Procedure: exact-integer? x
 -- C Function: scm_exact_integer_p (x)
     Return #t if X is an exact integer number, else return #f.

          (exact-integer? 37)
          => #t

          (exact-integer? 3.0)
          => #f

 -- C Function: int scm_is_exact_integer (SCM x)
     This is equivalent to scm_is_true (scm_exact_integer_p (x)).

 -- C Type: scm_t_int8
 -- C Type: scm_t_uint8
 -- C Type: scm_t_int16
 -- C Type: scm_t_uint16
 -- C Type: scm_t_int32
 -- C Type: scm_t_uint32
 -- C Type: scm_t_int64
 -- C Type: scm_t_uint64
 -- C Type: scm_t_intmax
 -- C Type: scm_t_uintmax
     The C types are equivalent to the corresponding ISO C types but are defined on all platforms, with the exception of scm_t_int64 and scm_t_uint64, which are only defined when a 64-bit type is available.  For example, scm_t_int8 is equivalent to int8_t.

     You can regard these definitions as a stop-gap measure until all platforms provide these types.  If you know that all the platforms that you are interested in already provide these types, it is better to use them directly instead of the types provided by Guile.

 -- C Function: int scm_is_signed_integer (SCM x, scm_t_intmax min, scm_t_intmax max)
 -- C Function: int scm_is_unsigned_integer (SCM x, scm_t_uintmax min, scm_t_uintmax max)
     Return 1 when X represents an exact integer that is between MIN and MAX, inclusive.

     These functions can be used to check whether a SCM value will fit into a given range, such as the range of a given C integer type. If you just want to convert a SCM value to a given C integer type, use one of the conversion functions directly.

 -- C Function: scm_t_intmax scm_to_signed_integer (SCM x, scm_t_intmax min, scm_t_intmax max)
 -- C Function: scm_t_uintmax scm_to_unsigned_integer (SCM x, scm_t_uintmax min, scm_t_uintmax max)
     When X represents an exact integer that is between MIN and MAX inclusive, return that integer.  Else signal an error, either a `wrong-type' error when X is not an exact integer, or an `out-of-range' error when it doesn't fit the given range.

 -- C Function: SCM scm_from_signed_integer (scm_t_intmax x)
 -- C Function: SCM scm_from_unsigned_integer (scm_t_uintmax x)
     Return the SCM value that represents the integer X.  This function will always succeed and will always return an exact number.

 -- C Function: char scm_to_char (SCM x)
 -- C Function: signed char scm_to_schar (SCM x)
 -- C Function: unsigned char scm_to_uchar (SCM x)
 -- C Function: short scm_to_short (SCM x)
 -- C Function: unsigned short scm_to_ushort (SCM x)
 -- C Function: int scm_to_int (SCM x)
 -- C Function: unsigned int scm_to_uint (SCM x)
 -- C Function: long scm_to_long (SCM x)
 -- C Function: unsigned long scm_to_ulong (SCM x)
 -- C Function: long long scm_to_long_long (SCM x)
 -- C Function: unsigned long long scm_to_ulong_long (SCM x)
 -- C Function: size_t scm_to_size_t (SCM x)
 -- C Function: ssize_t scm_to_ssize_t (SCM x)
 -- C Function: scm_t_ptrdiff scm_to_ptrdiff_t (SCM x)
 -- C Function: scm_t_int8 scm_to_int8 (SCM x)
 -- C Function: scm_t_uint8 scm_to_uint8 (SCM x)
 -- C Function: scm_t_int16 scm_to_int16 (SCM x)
 -- C Function: scm_t_uint16 scm_to_uint16 (SCM x)
 -- C Function: scm_t_int32 scm_to_int32 (SCM x)
 -- C Function: scm_t_uint32 scm_to_uint32 (SCM x)
 -- C Function: scm_t_int64 scm_to_int64 (SCM x)
 -- C Function: scm_t_uint64 scm_to_uint64 (SCM x)
 -- C Function: scm_t_intmax scm_to_intmax (SCM x)
 -- C Function: scm_t_uintmax scm_to_uintmax (SCM x)
     When X represents an exact integer that fits into the indicated C type, return that integer.  Else signal an error, either a `wrong-type' error when X is not an exact integer, or an `out-of-range' error when it doesn't fit the given range.

     The functions scm_to_long_long, scm_to_ulong_long, scm_to_int64, and scm_to_uint64 are only available when the corresponding types are.

 -- C Function: SCM scm_from_char (char x)
 -- C Function: SCM scm_from_schar (signed char x)
 -- C Function: SCM scm_from_uchar (unsigned char x)
 -- C Function: SCM scm_from_short (short x)
 -- C Function: SCM scm_from_ushort (unsigned short x)
 -- C Function: SCM scm_from_int (int x)
 -- C Function: SCM scm_from_uint (unsigned int x)
 -- C Function: SCM scm_from_long (long x)
 -- C Function: SCM scm_from_ulong (unsigned long x)
 -- C Function: SCM scm_from_long_long (long long x)
 -- C Function: SCM scm_from_ulong_long (unsigned long long x)
 -- C Function: SCM scm_from_size_t (size_t x)
 -- C Function: SCM scm_from_ssize_t (ssize_t x)
 -- C Function: SCM scm_from_ptrdiff_t (scm_t_ptrdiff x)
 -- C Function: SCM scm_from_int8 (scm_t_int8 x)
 -- C Function: SCM scm_from_uint8 (scm_t_uint8 x)
 -- C Function: SCM scm_from_int16 (scm_t_int16 x)
 -- C Function: SCM scm_from_uint16 (scm_t_uint16 x)
 -- C Function: SCM scm_from_int32 (scm_t_int32 x)
 -- C Function: SCM scm_from_uint32 (scm_t_uint32 x)
 -- C Function: SCM scm_from_int64 (scm_t_int64 x)
 -- C Function: SCM scm_from_uint64 (scm_t_uint64 x)
 -- C Function: SCM scm_from_intmax (scm_t_intmax x)
 -- C Function: SCM scm_from_uintmax (scm_t_uintmax x)
     Return the SCM value that represents the integer X. These functions will always succeed and will always return an exact number.

 -- C Function: void scm_to_mpz (SCM val, mpz_t rop)
     Assign VAL to the multiple precision integer ROP. VAL must be an exact integer, otherwise an error will be signalled.  ROP must have been initialized with mpz_init before this function is called.  When ROP is no longer needed the occupied space must be freed with mpz_clear. See Initializing Integers (GNU MP Manual), for details.

 -- C Function: SCM scm_from_mpz (mpz_t val)
     Return the SCM value that represents VAL.

Real and Rational Numbers
.........................

Mathematically, the real numbers are the set of numbers that describe all possible points along a continuous, infinite, one-dimensional line. The rational numbers are the set of all numbers that can be written as fractions P/Q, where P and Q are integers. All rational numbers are also real, but there are real numbers that are not rational, for example \sqrt2, the square root of 2, and \pi,pi.

Guile can represent both exact and inexact rational numbers, but it cannot represent precise finite irrational numbers.  Exact rationals are represented by storing the numerator and denominator as two exact integers.  Inexact rationals are stored as floating point numbers using the C type double.

Exact rationals are written as a fraction of integers.  There must be no whitespace around the slash:

     1/2
     -22/7

Even though the actual encoding of inexact rationals is in binary, it may be helpful to think of it as a decimal number with a limited number of significant figures and a decimal point somewhere, since this corresponds to the standard notation for non-whole numbers.  For example:

     0.34
     -0.00000142857931198
     -5648394822220000000000.0
     4.0

The limited precision of Guile's encoding means that any finite ``real'' number in Guile can be written in a rational form, by multiplying and then dividing by sufficient powers of 10 (or in fact, 2).  For example, -0.00000142857931198 is the same as -142857931198 divided by 100000000000000000.  In Guile's current incarnation, therefore, the rational? and real? predicates are equivalent for finite numbers.

Dividing by an exact zero leads to a error message, as one might expect. However, dividing by an inexact zero does not produce an error. Instead, the result of the division is either plus or minus infinity, depending on the sign of the divided number and the sign of the zero divisor (some platforms support signed zeroes -0.0 and +0.0; 0.0 is the same as +0.0).

Dividing zero by an inexact zero yields a NaN (`not a number') value, although they are actually considered numbers by Scheme. Attempts to compare a NaN value with any number (including itself) using =, <, >, <= or >= always returns #f.  Although a NaN value is not = to itself, it is both eqv? and equal? to itself and other NaN values.  However, the preferred way to test for them is by using nan?.

The real NaN values and infinities are written +nan.0, +inf.0 and -inf.0.  This syntax is also recognized by read as an extension to the usual Scheme syntax.  These special values are considered by Scheme to be inexact real numbers but not rational.  Note that non-real complex numbers may also contain infinities or NaN values in their real or imaginary parts.  To test a real number to see if it is infinite, a NaN value, or neither, use inf?, nan?, or finite?, respectively. Every real number in Scheme belongs to precisely one of those three classes.

On platforms that follow IEEE 754 for their floating point arithmetic, the +inf.0, -inf.0, and +nan.0 values are implemented using the corresponding IEEE 754 values. They behave in arithmetic operations like IEEE 754 describes it, i.e., (= +nan.0 +nan.0) => #f.

 -- Scheme Procedure: real? obj
 -- C Function: scm_real_p (obj)
     Return #t if OBJ is a real number, else #f.  Note that the sets of integer and rational values form subsets of the set of real numbers, so the predicate will also be fulfilled if OBJ is an integer number or a rational number.

 -- Scheme Procedure: rational? x
 -- C Function: scm_rational_p (x)
     Return #t if X is a rational number, #f otherwise. Note that the set of integer values forms a subset of the set of rational numbers, i.e. the predicate will also be fulfilled if X is an integer number.

 -- Scheme Procedure: rationalize x eps
 -- C Function: scm_rationalize (x, eps)
     Returns the simplest rational number differing from X by no more than EPS.

     As required by R5RS, rationalize only returns an exact result when both its arguments are exact.  Thus, you might need to use inexact->exact on the arguments.

          (rationalize (inexact->exact 1.2) 1/100)
          => 6/5

 -- Scheme Procedure: inf? x
 -- C Function: scm_inf_p (x)
     Return #t if the real number X is +inf.0 or -inf.0.  Otherwise return #f.

 -- Scheme Procedure: nan? x
 -- C Function: scm_nan_p (x)
     Return #t if the real number X is +nan.0, or #f otherwise.

 -- Scheme Procedure: finite? x
 -- C Function: scm_finite_p (x)
     Return #t if the real number X is neither infinite nor a NaN, #f otherwise.

 -- Scheme Procedure: nan
 -- C Function: scm_nan ()
     Return +nan.0, a NaN value.

 -- Scheme Procedure: inf
 -- C Function: scm_inf ()
     Return +inf.0, positive infinity.

 -- Scheme Procedure: numerator x
 -- C Function: scm_numerator (x)
     Return the numerator of the rational number X.

 -- Scheme Procedure: denominator x
 -- C Function: scm_denominator (x)
     Return the denominator of the rational number X.

 -- C Function: int scm_is_real (SCM val)
 -- C Function: int scm_is_rational (SCM val)
     Equivalent to scm_is_true (scm_real_p (val)) and scm_is_true (scm_rational_p (val)), respectively.

 -- C Function: double scm_to_double (SCM val)
     Returns the number closest to VAL that is representable as a double.  Returns infinity for a VAL that is too large in magnitude.  The argument VAL must be a real number.

 -- C Function: SCM scm_from_double (double val)
     Return the SCM value that represents VAL.  The returned value is inexact according to the predicate inexact?, but it will be exactly equal to VAL.

Complex Numbers
...............

Complex numbers are the set of numbers that describe all possible points in a two-dimensional space.  The two coordinates of a particular point in this space are known as the real and imaginary parts of the complex number that describes that point.

In Guile, complex numbers are written in rectangular form as the sum of their real and imaginary parts, using the symbol i to indicate the imaginary part.

     3+4i
     =>
     3.0+4.0i

     (* 3-8i 2.3+0.3i)
     =>
     9.3-17.5i

Polar form can also be used, with an @ between magnitude and angle,

     1@3.141592 => -1.0      (approx)
     -1@1.57079 => 0.0-1.0i  (approx)

Guile represents a complex number as a pair of inexact reals, so the real and imaginary parts of a complex number have the same properties of inexactness and limited precision as single inexact real numbers.

Note that each part of a complex number may contain any inexact real value, including the special values +nan.0, +inf.0 and -inf.0, as well as either of the signed zeroes 0.0 or -0.0.

 -- Scheme Procedure: complex? z
 -- C Function: scm_complex_p (z)
     Return #t if Z is a complex number, #f otherwise.  Note that the sets of real, rational and integer values form subsets of the set of complex numbers, i.e. the predicate will also be fulfilled if Z is a real, rational or integer number.

 -- C Function: int scm_is_complex (SCM val)
     Equivalent to scm_is_true (scm_complex_p (val)).

Exact and Inexact Numbers
.........................

R5RS requires that, with few exceptions, a calculation involving inexact numbers always produces an inexact result.  To meet this requirement, Guile distinguishes between an exact integer value such as 5 and the corresponding inexact integer value which, to the limited precision available, has no fractional part, and is printed as 5.0.  Guile will only convert the latter value to the former when forced to do so by an invocation of the inexact->exact procedure.

The only exception to the above requirement is when the values of the inexact numbers do not affect the result.  For example (expt n 0) is 1 for any value of n, therefore (expt 5.0 0) is permitted to return an exact 1.

 -- Scheme Procedure: exact? z
 -- C Function: scm_exact_p (z)
     Return #t if the number Z is exact, #f otherwise.

          (exact? 2)
          => #t

          (exact? 0.5)
          => #f

          (exact? (/ 2))
          => #t

 -- C Function: int scm_is_exact (SCM z)
     Return a 1 if the number Z is exact, and 0 otherwise.  This is equivalent to scm_is_true (scm_exact_p (z)).

     An alternate approch to testing the exactness of a number is to use scm_is_signed_integer or scm_is_unsigned_integer.

 -- Scheme Procedure: inexact? z
 -- C Function: scm_inexact_p (z)
     Return #t if the number Z is inexact, #f else.

 -- C Function: int scm_is_inexact (SCM z)
     Return a 1 if the number Z is inexact, and 0 otherwise.  This is equivalent to scm_is_true (scm_inexact_p (z)).

 -- Scheme Procedure: inexact->exact z
 -- C Function: scm_inexact_to_exact (z)
     Return an exact number that is numerically closest to Z, when there is one.  For inexact rationals, Guile returns the exact rational that is numerically equal to the inexact rational.  Inexact complex numbers with a non-zero imaginary part can not be made exact.

          (inexact->exact 0.5)
          => 1/2

     The following happens because 12/10 is not exactly representable as a double (on most platforms).  However, when reading a decimal number that has been marked exact with the ``#e'' prefix, Guile is able to represent it correctly.

          (inexact->exact 1.2)
          => 5404319552844595/4503599627370496

          #e1.2
          => 6/5

 -- Scheme Procedure: exact->inexact z
 -- C Function: scm_exact_to_inexact (z)
     Convert the number Z to its inexact representation.

Read Syntax for Numerical Data
..............................

The read syntax for integers is a string of digits, optionally preceded by a minus or plus character, a code indicating the base in which the integer is encoded, and a code indicating whether the number is exact or inexact.  The supported base codes are:

#b
#B
     the integer is written in binary (base 2)

#o
#O
     the integer is written in octal (base 8)

#d
#D
     the integer is written in decimal (base 10)

#x
#X
     the integer is written in hexadecimal (base 16)

If the base code is omitted, the integer is assumed to be decimal.  The following examples show how these base codes are used.

     -13
     => -13

     #d-13
     => -13

     #x-13
     => -19

     #b+1101
     => 13

     #o377
     => 255

The codes for indicating exactness (which can, incidentally, be applied to all numerical values) are:

#e
#E
     the number is exact

#i
#I
     the number is inexact.

If the exactness indicator is omitted, the number is exact unless it contains a radix point.  Since Guile can not represent exact complex numbers, an error is signalled when asking for them.

     (exact? 1.2)
     => #f

     (exact? #e1.2)
     => #t

     (exact? #e+1i)
     ERROR: Wrong type argument

Guile also understands the syntax +inf.0 and -inf.0 for plus and minus infinity, respectively.  The value must be written exactly as shown, that is, they always must have a sign and exactly one zero digit after the decimal point.  It also understands +nan.0 and -nan.0 for the special `not-a-number' value. The sign is ignored for `not-a-number' and the value is always printed as +nan.0.

Operations on Integer Values
............................

 -- Scheme Procedure: odd? n
 -- C Function: scm_odd_p (n)
     Return #t if N is an odd number, #f otherwise.

 -- Scheme Procedure: even? n
 -- C Function: scm_even_p (n)
     Return #t if N is an even number, #f otherwise.

 -- Scheme Procedure: quotient n d
 -- Scheme Procedure: remainder n d
 -- C Function: scm_quotient (n, d)
 -- C Function: scm_remainder (n, d)
     Return the quotient or remainder from N divided by D.  The quotient is rounded towards zero, and the remainder will have the same sign as N.  In all cases quotient and remainder satisfy N = Q*D + R.

          (remainder 13 4) => 1
          (remainder -13 4) => -1

     See also truncate-quotient, truncate-remainder and related operations in Arithmetic.

 -- Scheme Procedure: modulo n d
 -- C Function: scm_modulo (n, d)
     Return the remainder from N divided by D, with the same sign as D.

          (modulo 13 4) => 1
          (modulo -13 4) => 3
          (modulo 13 -4) => -3
          (modulo -13 -4) => -1

     See also floor-quotient, floor-remainder and related operations in Arithmetic.

 -- Scheme Procedure: gcd x...
 -- C Function: scm_gcd (x, y)
     Return the greatest common divisor of all arguments. If called without arguments, 0 is returned.

     The C function scm_gcd always takes two arguments, while the Scheme function can take an arbitrary number.

 -- Scheme Procedure: lcm x...
 -- C Function: scm_lcm (x, y)
     Return the least common multiple of the arguments. If called without arguments, 1 is returned.

     The C function scm_lcm always takes two arguments, while the Scheme function can take an arbitrary number.

 -- Scheme Procedure: modulo-expt n k m
 -- C Function: scm_modulo_expt (n, k, m)
     Return N raised to the integer exponent K, modulo M.

          (modulo-expt 2 3 5)
             => 3

 -- Scheme Procedure: exact-integer-sqrt K
 -- C Function: void scm_exact_integer_sqrt (SCM K, SCM *S, SCM *R)
     Return two exact non-negative integers S and R such that K = S^2 + R and S^2 <= K < (S + 1)^2. An error is raised if K is not an exact non-negative integer.

          (exact-integer-sqrt 10) => 3 and 1

Comparison Predicates
.....................

The C comparison functions below always takes two arguments, while the Scheme functions can take an arbitrary number.  Also keep in mind that the C functions return one of the Scheme boolean values SCM_BOOL_T or SCM_BOOL_F which are both true as far as C is concerned.  Thus, always write scm_is_true (scm_num_eq_p (x, y)) when testing the two Scheme numbers x and y for equality, for example.

 -- Scheme Procedure: =
 -- C Function: scm_num_eq_p (x, y)
     Return #t if all parameters are numerically equal.

 -- Scheme Procedure: <
 -- C Function: scm_less_p (x, y)
     Return #t if the list of parameters is monotonically increasing.

 -- Scheme Procedure: >
 -- C Function: scm_gr_p (x, y)
     Return #t if the list of parameters is monotonically decreasing.

 -- Scheme Procedure: <=
 -- C Function: scm_leq_p (x, y)
     Return #t if the list of parameters is monotonically non-decreasing.

 -- Scheme Procedure: >=
 -- C Function: scm_geq_p (x, y)
     Return #t if the list of parameters is monotonically non-increasing.

 -- Scheme Procedure: zero? z
 -- C Function: scm_zero_p (z)
     Return #t if Z is an exact or inexact number equal to zero.

 -- Scheme Procedure: positive? x
 -- C Function: scm_positive_p (x)
     Return #t if X is an exact or inexact number greater than zero.

 -- Scheme Procedure: negative? x
 -- C Function: scm_negative_p (x)
     Return #t if X is an exact or inexact number less than zero.

Converting Numbers To and From Strings
......................................

The following procedures read and write numbers according to their external representation as defined by R5RS (see R5RS Lexical Structure (The Revised^5 Report on the Algorithmic Language Scheme)).  See the (ice-9 i18n) module, for locale-dependent number parsing.

 -- Scheme Procedure: number->string n [radix]
 -- C Function: scm_number_to_string (n, radix)
     Return a string holding the external representation of the number N in the given RADIX.  If N is inexact, a radix of 10 will be used.

 -- Scheme Procedure: string->number string [radix]
 -- C Function: scm_string_to_number (string, radix)
     Return a number of the maximally precise representation expressed by the given STRING. RADIX must be an exact integer, either 2, 8, 10, or 16. If supplied, RADIX is a default radix that may be overridden by an explicit radix prefix in STRING (e.g. "#o177"). If RADIX is not supplied, then the default radix is 10. If string is not a syntactically valid notation for a number, then string->number returns #f.

 -- C Function: SCM scm_c_locale_stringn_to_number (const char *string, size_t len, unsigned radix)
     As per string->number above, but taking a C string, as pointer and length.  The string characters should be in the current locale encoding (locale in the name refers only to that, there's no locale-dependent parsing).

Complex Number Operations
.........................

 -- Scheme Procedure: make-rectangular real_part imaginary_part
 -- C Function: scm_make_rectangular (real_part, imaginary_part)
     Return a complex number constructed of the given REAL-PART and IMAGINARY-PART parts.

 -- Scheme Procedure: make-polar mag ang
 -- C Function: scm_make_polar (mag, ang)
     Return the complex number MAG * e^(i * ANG).

 -- Scheme Procedure: real-part z
 -- C Function: scm_real_part (z)
     Return the real part of the number Z.

 -- Scheme Procedure: imag-part z
 -- C Function: scm_imag_part (z)
     Return the imaginary part of the number Z.

 -- Scheme Procedure: magnitude z
 -- C Function: scm_magnitude (z)
     Return the magnitude of the number Z. This is the same as abs for real arguments, but also allows complex numbers.

 -- Scheme Procedure: angle z
 -- C Function: scm_angle (z)
     Return the angle of the complex number Z.

 -- C Function: SCM scm_c_make_rectangular (double re, double im)
 -- C Function: SCM scm_c_make_polar (double x, double y)
     Like scm_make_rectangular or scm_make_polar, respectively, but these functions take doubles as their arguments.

 -- C Function: double scm_c_real_part (z)
 -- C Function: double scm_c_imag_part (z)
     Returns the real or imaginary part of Z as a double.

 -- C Function: double scm_c_magnitude (z)
 -- C Function: double scm_c_angle (z)
     Returns the magnitude or angle of Z as a double.

Arithmetic Functions
....................

The C arithmetic functions below always takes two arguments, while the Scheme functions can take an arbitrary number.  When you need to invoke them with just one argument, for example to compute the equivalent of (- x), pass SCM_UNDEFINED as the second one: scm_difference (x, SCM_UNDEFINED).

 -- Scheme Procedure: + z1 ...
 -- C Function: scm_sum (z1, z2)
     Return the sum of all parameter values.  Return 0 if called without any parameters.

 -- Scheme Procedure: - z1 z2 ...
 -- C Function: scm_difference (z1, z2)
     If called with one argument Z1, -Z1 is returned. Otherwise the sum of all but the first argument are subtracted from the first argument.

 -- Scheme Procedure: * z1 ...
 -- C Function: scm_product (z1, z2)
     Return the product of all arguments.  If called without arguments, 1 is returned.

 -- Scheme Procedure: / z1 z2 ...
 -- C Function: scm_divide (z1, z2)
     Divide the first argument by the product of the remaining arguments.  If called with one argument Z1, 1/Z1 is returned.

 -- Scheme Procedure: 1+ z
 -- C Function: scm_oneplus (z)
     Return Z + 1.

 -- Scheme Procedure: 1- z
 -- C function: scm_oneminus (z)
     Return Z - 1.

 -- Scheme Procedure: abs x
 -- C Function: scm_abs (x)
     Return the absolute value of X.

     X must be a number with zero imaginary part.  To calculate the magnitude of a complex number, use magnitude instead.

 -- Scheme Procedure: max x1 x2 ...
 -- C Function: scm_max (x1, x2)
     Return the maximum of all parameter values.

 -- Scheme Procedure: min x1 x2 ...
 -- C Function: scm_min (x1, x2)
     Return the minimum of all parameter values.

 -- Scheme Procedure: truncate x
 -- C Function: scm_truncate_number (x)
     Round the inexact number X towards zero.

 -- Scheme Procedure: round x
 -- C Function: scm_round_number (x)
     Round the inexact number X to the nearest integer.  When exactly halfway between two integers, round to the even one.

 -- Scheme Procedure: floor x
 -- C Function: scm_floor (x)
     Round the number X towards minus infinity.

 -- Scheme Procedure: ceiling x
 -- C Function: scm_ceiling (x)
     Round the number X towards infinity.

 -- C Function: double scm_c_truncate (double x)
 -- C Function: double scm_c_round (double x)
     Like scm_truncate_number or scm_round_number, respectively, but these functions take and return double values.

 -- Scheme Procedure: euclidean/ X Y
 -- Scheme Procedure: euclidean-quotient X Y
 -- Scheme Procedure: euclidean-remainder X Y
 -- C Function: void scm_euclidean_divide (SCM X, SCM Y, SCM *Q, SCM *R)
 -- C Function: SCM scm_euclidean_quotient (SCM X, SCM Y)
 -- C Function: SCM scm_euclidean_remainder (SCM X, SCM Y)
     These procedures accept two real numbers X and Y, where the divisor Y must be non-zero.  euclidean-quotient returns the integer Q and euclidean-remainder returns the real number R such that X = Q*Y + R and 0 <= R < |Y|.  euclidean/ returns both Q and R, and is more efficient than computing each separately.  Note that when Y > 0, euclidean-quotient returns floor(X/Y), otherwise it returns ceiling(X/Y).

     Note that these operators are equivalent to the R6RS operators div, mod, and div-and-mod.

          (euclidean-quotient 123 10) => 12
          (euclidean-remainder 123 10) => 3
          (euclidean/ 123 10) => 12 and 3
          (euclidean/ 123 -10) => -12 and 3
          (euclidean/ -123 10) => -13 and 7
          (euclidean/ -123 -10) => 13 and 7
          (euclidean/ -123.2 -63.5) => 2.0 and 3.8
          (euclidean/ 16/3 -10/7) => -3 and 22/21

 -- Scheme Procedure: floor/ X Y
 -- Scheme Procedure: floor-quotient X Y
 -- Scheme Procedure: floor-remainder X Y
 -- C Function: void scm_floor_divide (SCM X, SCM Y, SCM *Q, SCM *R)
 -- C Function: SCM scm_floor_quotient (X, Y)
 -- C Function: SCM scm_floor_remainder (X, Y)
     These procedures accept two real numbers X and Y, where the divisor Y must be non-zero.  floor-quotient returns the integer Q and floor-remainder returns the real number R such that Q = floor(X/Y) and X = Q*Y + R.  floor/ returns both Q and R, and is more efficient than computing each separately.  Note that R, if non-zero, will have the same sign as Y.

     When X and Y are integers, floor-remainder is equivalent to the R5RS integer-only operator modulo.

          (floor-quotient 123 10) => 12
          (floor-remainder 123 10) => 3
          (floor/ 123 10) => 12 and 3
          (floor/ 123 -10) => -13 and -7
          (floor/ -123 10) => -13 and 7
          (floor/ -123 -10) => 12 and -3
          (floor/ -123.2 -63.5) => 1.0 and -59.7
          (floor/ 16/3 -10/7) => -4 and -8/21

 -- Scheme Procedure: ceiling/ X Y
 -- Scheme Procedure: ceiling-quotient X Y
 -- Scheme Procedure: ceiling-remainder X Y
 -- C Function: void scm_ceiling_divide (SCM X, SCM Y, SCM *Q, SCM *R)
 -- C Function: SCM scm_ceiling_quotient (X, Y)
 -- C Function: SCM scm_ceiling_remainder (X, Y)
     These procedures accept two real numbers X and Y, where the divisor Y must be non-zero.  ceiling-quotient returns the integer Q and ceiling-remainder returns the real number R such that Q = ceiling(X/Y) and X = Q*Y + R.  ceiling/ returns both Q and R, and is more efficient than computing each separately.  Note that R, if non-zero, will have the opposite sign of Y.

          (ceiling-quotient 123 10) => 13
          (ceiling-remainder 123 10) => -7
          (ceiling/ 123 10) => 13 and -7
          (ceiling/ 123 -10) => -12 and 3
          (ceiling/ -123 10) => -12 and -3
          (ceiling/ -123 -10) => 13 and 7
          (ceiling/ -123.2 -63.5) => 2.0 and 3.8
          (ceiling/ 16/3 -10/7) => -3 and 22/21

 -- Scheme Procedure: truncate/ X Y
 -- Scheme Procedure: truncate-quotient X Y
 -- Scheme Procedure: truncate-remainder X Y
 -- C Function: void scm_truncate_divide (SCM X, SCM Y, SCM *Q, SCM *R)
 -- C Function: SCM scm_truncate_quotient (X, Y)
 -- C Function: SCM scm_truncate_remainder (X, Y)
     These procedures accept two real numbers X and Y, where the divisor Y must be non-zero.  truncate-quotient returns the integer Q and truncate-remainder returns the real number R such that Q is X/Y rounded toward zero, and X = Q*Y + R.  truncate/ returns both Q and R, and is more efficient than computing each separately.  Note that R, if non-zero, will have the same sign as X.

     When X and Y are integers, these operators are equivalent to the R5RS integer-only operators quotient and remainder.

          (truncate-quotient 123 10) => 12
          (truncate-remainder 123 10) => 3
          (truncate/ 123 10) => 12 and 3
          (truncate/ 123 -10) => -12 and 3
          (truncate/ -123 10) => -12 and -3
          (truncate/ -123 -10) => 12 and -3
          (truncate/ -123.2 -63.5) => 1.0 and -59.7
          (truncate/ 16/3 -10/7) => -3 and 22/21

 -- Scheme Procedure: centered/ X Y
 -- Scheme Procedure: centered-quotient X Y
 -- Scheme Procedure: centered-remainder X Y
 -- C Function: void scm_centered_divide (SCM X, SCM Y, SCM *Q, SCM *R)
 -- C Function: SCM scm_centered_quotient (SCM X, SCM Y)
 -- C Function: SCM scm_centered_remainder (SCM X, SCM Y)
     These procedures accept two real numbers X and Y, where the divisor Y must be non-zero.  centered-quotient returns the integer Q and centered-remainder returns the real number R such that X = Q*Y + R and -|Y/2| <= R < |Y/2|.  centered/ returns both Q and R, and is more efficient than computing each separately.

     Note that centered-quotient returns X/Y rounded to the nearest integer.  When X/Y lies exactly half-way between two integers, the tie is broken according to the sign of Y.  If Y > 0, ties are rounded toward positive infinity, otherwise they are rounded toward negative infinity. This is a consequence of the requirement that -|Y/2| <= R < |Y/2|.

     Note that these operators are equivalent to the R6RS operators div0, mod0, and div0-and-mod0.

          (centered-quotient 123 10) => 12
          (centered-remainder 123 10) => 3
          (centered/ 123 10) => 12 and 3
          (centered/ 123 -10) => -12 and 3
          (centered/ -123 10) => -12 and -3
          (centered/ -123 -10) => 12 and -3
          (centered/ 125 10) => 13 and -5
          (centered/ 127 10) => 13 and -3
          (centered/ 135 10) => 14 and -5
          (centered/ -123.2 -63.5) => 2.0 and 3.8
          (centered/ 16/3 -10/7) => -4 and -8/21

 -- Scheme Procedure: round/ X Y
 -- Scheme Procedure: round-quotient X Y
 -- Scheme Procedure: round-remainder X Y
 -- C Function: void scm_round_divide (SCM X, SCM Y, SCM *Q, SCM *R)
 -- C Function: SCM scm_round_quotient (X, Y)
 -- C Function: SCM scm_round_remainder (X, Y)
     These procedures accept two real numbers X and Y, where the divisor Y must be non-zero.  round-quotient returns the integer Q and round-remainder returns the real number R such that X = Q*Y + R and Q is X/Y rounded to the nearest integer, with ties going to the nearest even integer.  round/ returns both Q and R, and is more efficient than computing each separately.

     Note that round/ and centered/ are almost equivalent, but their behavior differs when X/Y lies exactly half-way between two integers.  In this case, round/ chooses the nearest even integer, whereas centered/ chooses in such a way to satisfy the constraint -|Y/2| <= R < |Y/2|, which is stronger than the corresponding constraint for round/, -|Y/2| <= R <= |Y/2|.  In particular, when X and Y are integers, the number of possible remainders returned by centered/ is |Y|, whereas the number of possible remainders returned by round/ is |Y|+1 when Y is even.

          (round-quotient 123 10) => 12
          (round-remainder 123 10) => 3
          (round/ 123 10) => 12 and 3
          (round/ 123 -10) => -12 and 3
          (round/ -123 10) => -12 and -3
          (round/ -123 -10) => 12 and -3
          (round/ 125 10) => 12 and 5
          (round/ 127 10) => 13 and -3
          (round/ 135 10) => 14 and -5
          (round/ -123.2 -63.5) => 2.0 and 3.8
          (round/ 16/3 -10/7) => -4 and -8/21

Scientific Functions
....................

The following procedures accept any kind of number as arguments, including complex numbers.

 -- Scheme Procedure: sqrt z
     Return the square root of Z.  Of the two possible roots (positive and negative), the one with a positive real part is returned, or if that's zero then a positive imaginary part.  Thus,

          (sqrt 9.0)       => 3.0
          (sqrt -9.0)      => 0.0+3.0i
          (sqrt 1.0+1.0i)  => 1.09868411346781+0.455089860562227i
          (sqrt -1.0-1.0i) => 0.455089860562227-1.09868411346781i

 -- Scheme Procedure: expt z1 z2
     Return Z1 raised to the power of Z2.

 -- Scheme Procedure: sin z
     Return the sine of Z.

 -- Scheme Procedure: cos z
     Return the cosine of Z.

 -- Scheme Procedure: tan z
     Return the tangent of Z.

 -- Scheme Procedure: asin z
     Return the arcsine of Z.

 -- Scheme Procedure: acos z
     Return the arccosine of Z.

 -- Scheme Procedure: atan z
 -- Scheme Procedure: atan y x
     Return the arctangent of Z, or of Y/X.

 -- Scheme Procedure: exp z
     Return e to the power of Z, where e is the base of natural logarithms (2.71828...).

 -- Scheme Procedure: log z
     Return the natural logarithm of Z.

 -- Scheme Procedure: log10 z
     Return the base 10 logarithm of Z.

 -- Scheme Procedure: sinh z
     Return the hyperbolic sine of Z.

 -- Scheme Procedure: cosh z
     Return the hyperbolic cosine of Z.

 -- Scheme Procedure: tanh z
     Return the hyperbolic tangent of Z.

 -- Scheme Procedure: asinh z
     Return the hyperbolic arcsine of Z.

 -- Scheme Procedure: acosh z
     Return the hyperbolic arccosine of Z.

 -- Scheme Procedure: atanh z
     Return the hyperbolic arctangent of Z.

Bitwise Operations
..................

For the following bitwise functions, negative numbers are treated as infinite precision twos-complements.  For instance -6 is bits ...111010, with infinitely many ones on the left.  It can be seen that adding 6 (binary 110) to such a bit pattern gives all zeros.

 -- Scheme Procedure: logand n1 n2 ...
 -- C Function: scm_logand (n1, n2)
     Return the bitwise AND of the integer arguments.

          (logand) => -1
          (logand 7) => 7
          (logand #b111 #b011 #b001) => 1

 -- Scheme Procedure: logior n1 n2 ...
 -- C Function: scm_logior (n1, n2)
     Return the bitwise OR of the integer arguments.

          (logior) => 0
          (logior 7) => 7
          (logior #b000 #b001 #b011) => 3

 -- Scheme Procedure: logxor n1 n2 ...
 -- C Function: scm_loxor (n1, n2)
     Return the bitwise XOR of the integer arguments.  A bit is set in the result if it is set in an odd number of arguments.

          (logxor) => 0
          (logxor 7) => 7
          (logxor #b000 #b001 #b011) => 2
          (logxor #b000 #b001 #b011 #b011) => 1

 -- Scheme Procedure: lognot n
 -- C Function: scm_lognot (n)
     Return the integer which is the ones-complement of the integer argument, ie. each 0 bit is changed to 1 and each 1 bit to 0.

          (number->string (lognot #b10000000) 2)
             => "-10000001"
          (number->string (lognot #b0) 2)
             => "-1"

 -- Scheme Procedure: logtest j k
 -- C Function: scm_logtest (j, k)
     Test whether J and K have any 1 bits in common.  This is equivalent to (not (zero? (logand j k))), but without actually calculating the logand, just testing for non-zero.

          (logtest #b0100 #b1011) => #f
          (logtest #b0100 #b0111) => #t

 -- Scheme Procedure: logbit? index j
 -- C Function: scm_logbit_p (index, j)
     Test whether bit number INDEX in J is set.  INDEX starts from 0 for the least significant bit.

          (logbit? 0 #b1101) => #t
          (logbit? 1 #b1101) => #f
          (logbit? 2 #b1101) => #t
          (logbit? 3 #b1101) => #t
          (logbit? 4 #b1101) => #f

 -- Scheme Procedure: ash n count
 -- C Function: scm_ash (n, count)
     Return floor(n * 2^count). N and COUNT must be exact integers.

     With N viewed as an infinite-precision twos-complement integer, ash means a left shift introducing zero bits when COUNT is positive, or a right shift dropping bits when COUNT is negative.  This is an ``arithmetic'' shift.

          (number->string (ash #b1 3) 2)     => "1000"
          (number->string (ash #b1010 -1) 2) => "101"

          ;; -23 is bits ...11101001, -6 is bits ...111010
          (ash -23 -2) => -6

 -- Scheme Procedure: round-ash n count
 -- C Function: scm_round_ash (n, count)
     Return round(n * 2^count). N and COUNT must be exact integers.

     With N viewed as an infinite-precision twos-complement integer, round-ash means a left shift introducing zero bits when COUNT is positive, or a right shift rounding to the nearest integer (with ties going to the nearest even integer) when COUNT is negative.  This is a rounded ``arithmetic'' shift.

          (number->string (round-ash #b1 3) 2)     => \"1000\"
          (number->string (round-ash #b1010 -1) 2) => \"101\"
          (number->string (round-ash #b1010 -2) 2) => \"10\"
          (number->string (round-ash #b1011 -2) 2) => \"11\"
          (number->string (round-ash #b1101 -2) 2) => \"11\"
          (number->string (round-ash #b1110 -2) 2) => \"100\"

 -- Scheme Procedure: logcount n
 -- C Function: scm_logcount (n)
     Return the number of bits in integer N.  If N is positive, the 1-bits in its binary representation are counted. If negative, the 0-bits in its two's-complement binary representation are counted.  If zero, 0 is returned.

          (logcount #b10101010)
             => 4
          (logcount 0)
             => 0
          (logcount -2)
             => 1

 -- Scheme Procedure: integer-length n
 -- C Function: scm_integer_length (n)
     Return the number of bits necessary to represent N.

     For positive N this is how many bits to the most significant one bit.  For negative N it's how many bits to the most significant zero bit in twos complement form.

          (integer-length #b10101010) => 8
          (integer-length #b1111)     => 4
          (integer-length 0)          => 0
          (integer-length -1)         => 0
          (integer-length -256)       => 8
          (integer-length -257)       => 9

 -- Scheme Procedure: integer-expt n k
 -- C Function: scm_integer_expt (n, k)
     Return N raised to the power K.  K must be an exact integer, N can be any number.

     Negative K is supported, and results in 1/n^|k|, 1/n^abs(k) in the usual way.  N^0 is 1, as usual, and that includes 0^0 is 1.

          (integer-expt 2 5)   => 32
          (integer-expt -3 3)  => -27
          (integer-expt 5 -3)  => 1/125
          (integer-expt 0 0)   => 1

 -- Scheme Procedure: bit-extract n start end
 -- C Function: scm_bit_extract (n, start, end)
     Return the integer composed of the START (inclusive) through END (exclusive) bits of N.  The STARTth bit becomes the 0-th bit in the result.

          (number->string (bit-extract #b1101101010 0 4) 2)
             => "1010"
          (number->string (bit-extract #b1101101010 4 9) 2)
             => "10110"

Random Number Generation
........................

Pseudo-random numbers are generated from a random state object, which can be created with seed->random-state or datum->random-state.  An external representation (i.e. one which can written with write and read with read) of a random state object can be obtained via random-state->datum.  The STATE parameter to the various functions below is optional, it defaults to the state object in the *random-state* variable.

 -- Scheme Procedure: copy-random-state [state]
 -- C Function: scm_copy_random_state (state)
     Return a copy of the random state STATE.

 -- Scheme Procedure: random n [state]
 -- C Function: scm_random (n, state)
     Return a number in [0, N).

     Accepts a positive integer or real n and returns a number of the same type between zero (inclusive) and N (exclusive). The values returned have a uniform distribution.

 -- Scheme Procedure: random:exp [state]
 -- C Function: scm_random_exp (state)
     Return an inexact real in an exponential distribution with mean 1.  For an exponential distribution with mean U use (* U (random:exp)).

 -- Scheme Procedure: random:hollow-sphere! vect [state]
 -- C Function: scm_random_hollow_sphere_x (vect, state)
     Fills VECT with inexact real random numbers the sum of whose squares is equal to 1.0.  Thinking of VECT as coordinates in space of dimension N = (vector-length VECT), the coordinates are uniformly distributed over the surface of the unit n-sphere.

 -- Scheme Procedure: random:normal [state]
 -- C Function: scm_random_normal (state)
     Return an inexact real in a normal distribution.  The distribution used has mean 0 and standard deviation 1.  For a normal distribution with mean M and standard deviation D use (+ M (* D (random:normal))).

 -- Scheme Procedure: random:normal-vector! vect [state]
 -- C Function: scm_random_normal_vector_x (vect, state)
     Fills VECT with inexact real random numbers that are independent and standard normally distributed (i.e., with mean 0 and variance 1).

 -- Scheme Procedure: random:solid-sphere! vect [state]
 -- C Function: scm_random_solid_sphere_x (vect, state)
     Fills VECT with inexact real random numbers the sum of whose squares is less than 1.0.  Thinking of VECT as coordinates in space of dimension N = (vector-length VECT), the coordinates are uniformly distributed within the unit N-sphere.

 -- Scheme Procedure: random:uniform [state]
 -- C Function: scm_random_uniform (state)
     Return a uniformly distributed inexact real random number in [0,1).

 -- Scheme Procedure: seed->random-state seed
 -- C Function: scm_seed_to_random_state (seed)
     Return a new random state using SEED.

 -- Scheme Procedure: datum->random-state datum
 -- C Function: scm_datum_to_random_state (datum)
     Return a new random state from DATUM, which should have been obtained by random-state->datum.

 -- Scheme Procedure: random-state->datum state
 -- C Function: scm_random_state_to_datum (state)
     Return a datum representation of STATE that may be written out and read back with the Scheme reader.

 -- Scheme Procedure: random-state-from-platform
 -- C Function: scm_random_state_from_platform ()
     Construct a new random state seeded from a platform-specific source of entropy, appropriate for use in non-security-critical applications. Currently /dev/urandom is tried first, or else the seed is based on the time, date, process ID, an address from a freshly allocated heap cell, an address from the local stack frame, and a high-resolution timer if available.

 -- Variable: *random-state*
     The global random state used by the above functions when the STATE parameter is not given.

Note that the initial value of *random-state* is the same every time Guile starts up.  Therefore, if you don't pass a STATE parameter to the above procedures, and you don't set *random-state* to (seed->random-state your-seed), where your-seed is something that isn't the same every time, you'll get the same sequence of ``random'' numbers on every run.

For example, unless the relevant source code has changed, (map random (cdr (iota 30))), if the first use of random numbers since Guile started up, will always give:

     (map random (cdr (iota 19)))
     =>
     (0 1 1 2 2 2 1 2 6 7 10 0 5 3 12 5 5 12)

To seed the random state in a sensible way for non-security-critical applications, do this during initialization of your program:

     (set! *random-state* (random-state-from-platform))

Characters
----------

In Scheme, there is a data type to describe a single character.

Defining what exactly a character is can be more complicated than it seems.  Guile follows the advice of R6RS and uses The Unicode Standard to help define what a character is.  So, for Guile, a character is anything in the Unicode Character Database.

The Unicode Character Database is basically a table of characters indexed using integers called 'code points'.  Valid code points are in the ranges 0 to #xD7FF inclusive or #xE000 to #x10FFFF inclusive, which is about 1.1 million code points.

Any code point that has been assigned to a character or that has otherwise been given a meaning by Unicode is called a 'designated code point'.  Most of the designated code points, about 200,000 of them, indicate characters, accents or other combining marks that modify other characters, symbols, whitespace, and control characters.  Some are not characters but indicators that suggest how to format or display neighboring characters.

If a code point is not a designated code point -- if it has not been assigned to a character by The Unicode Standard -- it is a 'reserved code point', meaning that they are reserved for future use.  Most of the code points, about 800,000, are 'reserved code points'.

By convention, a Unicode code point is written as ``U+XXXX'' where ``XXXX'' is a hexadecimal number.  Please note that this convenient notation is not valid code.  Guile does not interpret ``U+XXXX'' as a character.

In Scheme, a character literal is written as #\NAME where NAME is the name of the character that you want.  Printable characters have their usual single character name; for example, #\a is a lower case a.

Some of the code points are 'combining characters' that are not meant to be printed by themselves but are instead meant to modify the appearance of the previous character.  For combining characters, an alternate form of the character literal is #\ followed by U+25CC (a small, dotted circle), followed by the combining character. This allows the combining character to be drawn on the circle, not on the backslash of #\.

Many of the non-printing characters, such as whitespace characters and control characters, also have names.

The most commonly used non-printing characters have long character names, described in the table below.

Character Name   Codepoint
#\nul   U+0000
#\alarm   u+0007
#\backspace   U+0008
#\tab   U+0009
#\linefeed   U+000A
#\newline   U+000A
#\vtab   U+000B
#\page   U+000C
#\return   U+000D
#\esc   U+001B
#\space   U+0020
#\delete   U+007F

There are also short names for all of the ``C0 control characters'' (those with code points below 32).  The following table lists the short name for each character.

0 = #\nul
1 = #\soh
2 = #\stx
3 = #\etx
4 = #\eot
5 = #\enq
6 = #\ack
7 = #\bel
8 = #\bs
9 = #\ht
10 = #\lf
11 = #\vt
12 = #\ff
13 = #\cr
14 = #\so
15 = #\si
16 = #\dle
17 = #\dc1
18 = #\dc2
19 = #\dc3
20 = #\dc4
21 = #\nak
22 = #\syn
23 = #\etb
24 = #\can
25 = #\em
26 = #\sub
27 = #\esc
28 = #\fs
29 = #\gs
30 = #\rs
31 = #\us
32 = #\sp

The short name for the ``delete'' character (code point U+007F) is #\del.

The R7RS name for the ``escape'' character (code point U+001B) is #\escape.

There are also a few alternative names left over for compatibility with previous versions of Guile.

Alternate   Standard
#\nl   #\newline
#\np   #\page
#\null   #\nul

Characters may also be written using their code point values.  They can be written with as an octal number, such as #\10 for #\bs or #\177 for #\del.

If one prefers hex to octal, there is an additional syntax for character escapes: #\xHHHH -- the letter 'x' followed by a hexadecimal number of one to eight digits.

 -- Scheme Procedure: char? x
 -- C Function: scm_char_p (x)
     Return #t if X is a character, else #f.

Fundamentally, the character comparison operations below are numeric comparisons of the character's code points.

 -- Scheme Procedure: char=? x y
     Return #t if code point of X is equal to the code point of Y, else #f.

 -- Scheme Procedure: char<? x y
     Return #t if the code point of X is less than the code point of Y, else #f.

 -- Scheme Procedure: char<=? x y
     Return #t if the code point of X is less than or equal to the code point of Y, else #f.

 -- Scheme Procedure: char>? x y
     Return #t if the code point of X is greater than the code point of Y, else #f.

 -- Scheme Procedure: char>=? x y
     Return #t if the code point of X is greater than or equal to the code point of Y, else #f.

Case-insensitive character comparisons use Unicode case folding.  In case folding comparisons, if a character is lowercase and has an uppercase form that can be expressed as a single character, it is converted to uppercase before comparison.  All other characters undergo no conversion before the comparison occurs.  This includes the German sharp S (Eszett) which is not uppercased before conversion because its uppercase form has two characters.  Unicode case folding is language independent: it uses rules that are generally true, but, it cannot cover all cases for all languages.

 -- Scheme Procedure: char-ci=? x y
     Return #t if the case-folded code point of X is the same as the case-folded code point of Y, else #f.

 -- Scheme Procedure: char-ci<? x y
     Return #t if the case-folded code point of X is less than the case-folded code point of Y, else #f.

 -- Scheme Procedure: char-ci<=? x y
     Return #t if the case-folded code point of X is less than or equal to the case-folded code point of Y, else #f.

 -- Scheme Procedure: char-ci>? x y
     Return #t if the case-folded code point of X is greater than the case-folded code point of Y, else #f.

 -- Scheme Procedure: char-ci>=? x y
     Return #t if the case-folded code point of X is greater than or equal to the case-folded code point of Y, else #f.

 -- Scheme Procedure: char-alphabetic? chr
 -- C Function: scm_char_alphabetic_p (chr)
     Return #t if CHR is alphabetic, else #f.

 -- Scheme Procedure: char-numeric? chr
 -- C Function: scm_char_numeric_p (chr)
     Return #t if CHR is numeric, else #f.

 -- Scheme Procedure: char-whitespace? chr
 -- C Function: scm_char_whitespace_p (chr)
     Return #t if CHR is whitespace, else #f.

 -- Scheme Procedure: char-upper-case? chr
 -- C Function: scm_char_upper_case_p (chr)
     Return #t if CHR is uppercase, else #f.

 -- Scheme Procedure: char-lower-case? chr
 -- C Function: scm_char_lower_case_p (chr)
     Return #t if CHR is lowercase, else #f.

 -- Scheme Procedure: char-is-both? chr
 -- C Function: scm_char_is_both_p (chr)
     Return #t if CHR is either uppercase or lowercase, else #f.

 -- Scheme Procedure: char-general-category chr
 -- C Function: scm_char_general_category (chr)
     Return a symbol giving the two-letter name of the Unicode general category assigned to CHR or #f if no named category is assigned.  The following table provides a list of category names along with their meanings.

     Lu
     Uppercase letter
     Pf
     Final quote punctuation
     Ll
     Lowercase letter
     Po
     Other punctuation
     Lt
     Titlecase letter
     Sm
     Math symbol
     Lm
     Modifier letter
     Sc
     Currency symbol
     Lo
     Other letter
     Sk
     Modifier symbol
     Mn
     Non-spacing mark
     So
     Other symbol
     Mc
     Combining spacing mark
     Zs
     Space separator
     Me
     Enclosing mark
     Zl
     Line separator
     Nd
     Decimal digit number
     Zp
     Paragraph separator
     Nl
     Letter number
     Cc
     Control
     No
     Other number
     Cf
     Format
     Pc
     Connector punctuation
     Cs
     Surrogate
     Pd
     Dash punctuation
     Co
     Private use
     Ps
     Open punctuation
     Cn
     Unassigned
     Pe
     Close punctuation

     Pi
     Initial quote punctuation

 -- Scheme Procedure: char->integer chr
 -- C Function: scm_char_to_integer (chr)
     Return the code point of CHR.

 -- Scheme Procedure: integer->char n
 -- C Function: scm_integer_to_char (n)
     Return the character that has code point N.  The integer N must be a valid code point.  Valid code points are in the ranges 0 to #xD7FF inclusive or #xE000 to #x10FFFF inclusive.

 -- Scheme Procedure: char-upcase chr
 -- C Function: scm_char_upcase (chr)
     Return the uppercase character version of CHR.

 -- Scheme Procedure: char-downcase chr
 -- C Function: scm_char_downcase (chr)
     Return the lowercase character version of CHR.

 -- Scheme Procedure: char-titlecase chr
 -- C Function: scm_char_titlecase (chr)
     Return the titlecase character version of CHR if one exists; otherwise return the uppercase version.

     For most characters these will be the same, but the Unicode Standard includes certain digraph compatibility characters, such as U+01F3 ``dz'', for which the uppercase and titlecase characters are different (U+01F1 ``DZ'' and U+01F2 ``Dz'' in this case, respectively).

 -- C Function: scm_t_wchar scm_c_upcase (scm_t_wchar C)
 -- C Function: scm_t_wchar scm_c_downcase (scm_t_wchar C)
 -- C Function: scm_t_wchar scm_c_titlecase (scm_t_wchar C)

     These C functions take an integer representation of a Unicode codepoint and return the codepoint corresponding to its uppercase, lowercase, and titlecase forms respectively.  The type scm_t_wchar is a signed, 32-bit integer.

Character Sets
--------------

The features described in this section correspond directly to SRFI-14.

The data type charset implements sets of characters (see Characters).  Because the internal representation of character sets is not visible to the user, a lot of procedures for handling them are provided.

Character sets can be created, extended, tested for the membership of a characters and be compared to other character sets.

Character Set Predicates/Comparison
...................................

Use these procedures for testing whether an object is a character set, or whether several character sets are equal or subsets of each other. char-set-hash can be used for calculating a hash value, maybe for usage in fast lookup procedures.

 -- Scheme Procedure: char-set? obj
 -- C Function: scm_char_set_p (obj)
     Return #t if OBJ is a character set, #f otherwise.

 -- Scheme Procedure: char-set= char_set ...
 -- C Function: scm_char_set_eq (char_sets)
     Return #t if all given character sets are equal.

 -- Scheme Procedure: char-set<= char_set ...
 -- C Function: scm_char_set_leq (char_sets)
     Return #t if every character set CHAR_SETi is a subset of character set CHAR_SETi+1.

 -- Scheme Procedure: char-set-hash cs [bound]
 -- C Function: scm_char_set_hash (cs, bound)
     Compute a hash value for the character set CS.  If BOUND is given and non-zero, it restricts the returned value to the range 0 ... BOUND - 1.

Iterating Over Character Sets
.............................

Character set cursors are a means for iterating over the members of a character sets.  After creating a character set cursor with char-set-cursor, a cursor can be dereferenced with char-set-ref, advanced to the next member with char-set-cursor-next.  Whether a cursor has passed past the last element of the set can be checked with end-of-char-set?.

Additionally, mapping and (un-)folding procedures for character sets are provided.

 -- Scheme Procedure: char-set-cursor cs
 -- C Function: scm_char_set_cursor (cs)
     Return a cursor into the character set CS.

 -- Scheme Procedure: char-set-ref cs cursor
 -- C Function: scm_char_set_ref (cs, cursor)
     Return the character at the current cursor position CURSOR in the character set CS.  It is an error to pass a cursor for which end-of-char-set? returns true.

 -- Scheme Procedure: char-set-cursor-next cs cursor
 -- C Function: scm_char_set_cursor_next (cs, cursor)
     Advance the character set cursor CURSOR to the next character in the character set CS.  It is an error if the cursor given satisfies end-of-char-set?.

 -- Scheme Procedure: end-of-char-set? cursor
 -- C Function: scm_end_of_char_set_p (cursor)
     Return #t if CURSOR has reached the end of a character set, #f otherwise.

 -- Scheme Procedure: char-set-fold kons knil cs
 -- C Function: scm_char_set_fold (kons, knil, cs)
     Fold the procedure KONS over the character set CS, initializing it with KNIL.

 -- Scheme Procedure: char-set-unfold p f g seed [base_cs]
 -- C Function: scm_char_set_unfold (p, f, g, seed, base_cs)
     This is a fundamental constructor for character sets.

       * G is used to generate a series of ``seed'' values from the initial seed: SEED, (G SEED), (G^2 SEED), (G^3 SEED), ...

       * P tells us when to stop -- when it returns true when applied to one of the seed values.

       * F maps each seed value to a character. These characters are added to the base character set BASE_CS to form the result; BASE_CS defaults to the empty set.

 -- Scheme Procedure: char-set-unfold! p f g seed base_cs
 -- C Function: scm_char_set_unfold_x (p, f, g, seed, base_cs)
     This is a fundamental constructor for character sets.

       * G is used to generate a series of ``seed'' values from the initial seed: SEED, (G SEED), (G^2 SEED), (G^3 SEED), ...

       * P tells us when to stop -- when it returns true when applied to one of the seed values.

       * F maps each seed value to a character. These characters are added to the base character set BASE_CS to form the result; BASE_CS defaults to the empty set.

 -- Scheme Procedure: char-set-for-each proc cs
 -- C Function: scm_char_set_for_each (proc, cs)
     Apply PROC to every character in the character set CS.  The return value is not specified.

 -- Scheme Procedure: char-set-map proc cs
 -- C Function: scm_char_set_map (proc, cs)
     Map the procedure PROC over every character in CS. PROC must be a character -> character procedure.

Creating Character Sets
.......................

New character sets are produced with these procedures.

 -- Scheme Procedure: char-set-copy cs
 -- C Function: scm_char_set_copy (cs)
     Return a newly allocated character set containing all characters in CS.

 -- Scheme Procedure: char-set chr ...
 -- C Function: scm_char_set (chrs)
     Return a character set containing all given characters.

 -- Scheme Procedure: list->char-set list [base_cs]
 -- C Function: scm_list_to_char_set (list, base_cs)
     Convert the character list LIST to a character set.  If the character set BASE_CS is given, the character in this set are also included in the result.

 -- Scheme Procedure: list->char-set! list base_cs
 -- C Function: scm_list_to_char_set_x (list, base_cs)
     Convert the character list LIST to a character set.  The characters are added to BASE_CS and BASE_CS is returned.

 -- Scheme Procedure: string->char-set str [base_cs]
 -- C Function: scm_string_to_char_set (str, base_cs)
     Convert the string STR to a character set.  If the character set BASE_CS is given, the characters in this set are also included in the result.

 -- Scheme Procedure: string->char-set! str base_cs
 -- C Function: scm_string_to_char_set_x (str, base_cs)
     Convert the string STR to a character set.  The characters from the string are added to BASE_CS, and BASE_CS is returned.

 -- Scheme Procedure: char-set-filter pred cs [base_cs]
 -- C Function: scm_char_set_filter (pred, cs, base_cs)
     Return a character set containing every character from CS so that it satisfies PRED.  If provided, the characters from BASE_CS are added to the result.

 -- Scheme Procedure: char-set-filter! pred cs base_cs
 -- C Function: scm_char_set_filter_x (pred, cs, base_cs)
     Return a character set containing every character from CS so that it satisfies PRED.  The characters are added to BASE_CS and BASE_CS is returned.

 -- Scheme Procedure: ucs-range->char-set lower upper [error [base_cs]]
 -- C Function: scm_ucs_range_to_char_set (lower, upper, error, base_cs)
     Return a character set containing all characters whose character codes lie in the half-open range [LOWER,UPPER).

     If ERROR is a true value, an error is signalled if the specified range contains characters which are not contained in the implemented character range.  If ERROR is #f, these characters are silently left out of the resulting character set.

     The characters in BASE_CS are added to the result, if given.

 -- Scheme Procedure: ucs-range->char-set! lower upper error base_cs
 -- C Function: scm_ucs_range_to_char_set_x (lower, upper, error, base_cs)
     Return a character set containing all characters whose character codes lie in the half-open range [LOWER,UPPER).

     If ERROR is a true value, an error is signalled if the specified range contains characters which are not contained in the implemented character range.  If ERROR is #f, these characters are silently left out of the resulting character set.

     The characters are added to BASE_CS and BASE_CS is returned.

 -- Scheme Procedure: ->char-set x
 -- C Function: scm_to_char_set (x)
     Coerces x into a char-set. X may be a string, character or char-set. A string is converted to the set of its constituent characters; a character is converted to a singleton set; a char-set is returned as-is.

Querying Character Sets
.......................

Access the elements and other information of a character set with these procedures.

 -- Scheme Procedure: %char-set-dump cs
     Returns an association list containing debugging information for CS. The association list has the following entries.

     char-set
          The char-set itself

     len
          The number of groups of contiguous code points the char-set contains

     ranges
          A list of lists where each sublist is a range of code points and their associated characters

     The return value of this function cannot be relied upon to be consistent between versions of Guile and should not be used in code.

 -- Scheme Procedure: char-set-size cs
 -- C Function: scm_char_set_size (cs)
     Return the number of elements in character set CS.

 -- Scheme Procedure: char-set-count pred cs
 -- C Function: scm_char_set_count (pred, cs)
     Return the number of the elements int the character set CS which satisfy the predicate PRED.

 -- Scheme Procedure: char-set->list cs
 -- C Function: scm_char_set_to_list (cs)
     Return a list containing the elements of the character set CS.

 -- Scheme Procedure: char-set->string cs
 -- C Function: scm_char_set_to_string (cs)
     Return a string containing the elements of the character set CS.  The order in which the characters are placed in the string is not defined.

 -- Scheme Procedure: char-set-contains? cs ch
 -- C Function: scm_char_set_contains_p (cs, ch)
     Return #t if the character CH is contained in the character set CS, or #f otherwise.

 -- Scheme Procedure: char-set-every pred cs
 -- C Function: scm_char_set_every (pred, cs)
     Return a true value if every character in the character set CS satisfies the predicate PRED.

 -- Scheme Procedure: char-set-any pred cs
 -- C Function: scm_char_set_any (pred, cs)
     Return a true value if any character in the character set CS satisfies the predicate PRED.

Character-Set Algebra
.....................

Character sets can be manipulated with the common set algebra operation, such as union, complement, intersection etc.  All of these procedures provide side-effecting variants, which modify their character set argument(s).

 -- Scheme Procedure: char-set-adjoin cs chr ...
 -- C Function: scm_char_set_adjoin (cs, chrs)
     Add all character arguments to the first argument, which must be a character set.

 -- Scheme Procedure: char-set-delete cs chr ...
 -- C Function: scm_char_set_delete (cs, chrs)
     Delete all character arguments from the first argument, which must be a character set.

 -- Scheme Procedure: char-set-adjoin! cs chr ...
 -- C Function: scm_char_set_adjoin_x (cs, chrs)
     Add all character arguments to the first argument, which must be a character set.

 -- Scheme Procedure: char-set-delete! cs chr ...
 -- C Function: scm_char_set_delete_x (cs, chrs)
     Delete all character arguments from the first argument, which must be a character set.

 -- Scheme Procedure: char-set-complement cs
 -- C Function: scm_char_set_complement (cs)
     Return the complement of the character set CS.

Note that the complement of a character set is likely to contain many reserved code points (code points that are not associated with characters).  It may be helpful to modify the output of char-set-complement by computing its intersection with the set of designated code points, char-set:designated.

 -- Scheme Procedure: char-set-union cs ...
 -- C Function: scm_char_set_union (char_sets)
     Return the union of all argument character sets.

 -- Scheme Procedure: char-set-intersection cs ...
 -- C Function: scm_char_set_intersection (char_sets)
     Return the intersection of all argument character sets.

 -- Scheme Procedure: char-set-difference cs1 cs ...
 -- C Function: scm_char_set_difference (cs1, char_sets)
     Return the difference of all argument character sets.

 -- Scheme Procedure: char-set-xor cs ...
 -- C Function: scm_char_set_xor (char_sets)
     Return the exclusive-or of all argument character sets.

 -- Scheme Procedure: char-set-diff+intersection cs1 cs ...
 -- C Function: scm_char_set_diff_plus_intersection (cs1, char_sets)
     Return the difference and the intersection of all argument character sets.

 -- Scheme Procedure: char-set-complement! cs
 -- C Function: scm_char_set_complement_x (cs)
     Return the complement of the character set CS.

 -- Scheme Procedure: char-set-union! cs1 cs ...
 -- C Function: scm_char_set_union_x (cs1, char_sets)
     Return the union of all argument character sets.

 -- Scheme Procedure: char-set-intersection! cs1 cs ...
 -- C Function: scm_char_set_intersection_x (cs1, char_sets)
     Return the intersection of all argument character sets.

 -- Scheme Procedure: char-set-difference! cs1 cs ...
 -- C Function: scm_char_set_difference_x (cs1, char_sets)
     Return the difference of all argument character sets.

 -- Scheme Procedure: char-set-xor! cs1 cs ...
 -- C Function: scm_char_set_xor_x (cs1, char_sets)
     Return the exclusive-or of all argument character sets.

 -- Scheme Procedure: char-set-diff+intersection! cs1 cs2 cs ...
 -- C Function: scm_char_set_diff_plus_intersection_x (cs1, cs2, char_sets)
     Return the difference and the intersection of all argument character sets.

Standard Character Sets
.......................

In order to make the use of the character set data type and procedures useful, several predefined character set variables exist.

These character sets are locale independent and are not recomputed upon a setlocale call.  They contain characters from the whole range of Unicode code points. For instance, char-set:letter contains about 100,000 characters.

 -- Scheme Variable: char-set:lower-case
 -- C Variable: scm_char_set_lower_case
     All lower-case characters.

 -- Scheme Variable: char-set:upper-case
 -- C Variable: scm_char_set_upper_case
     All upper-case characters.

 -- Scheme Variable: char-set:title-case
 -- C Variable: scm_char_set_title_case
     All single characters that function as if they were an upper-case letter followed by a lower-case letter.

 -- Scheme Variable: char-set:letter
 -- C Variable: scm_char_set_letter
     All letters.  This includes char-set:lower-case, char-set:upper-case, char-set:title-case, and many letters that have no case at all.  For example, Chinese and Japanese characters typically have no concept of case.

 -- Scheme Variable: char-set:digit
 -- C Variable: scm_char_set_digit
     All digits.

 -- Scheme Variable: char-set:letter+digit
 -- C Variable: scm_char_set_letter_and_digit
     The union of char-set:letter and char-set:digit.

 -- Scheme Variable: char-set:graphic
 -- C Variable: scm_char_set_graphic
     All characters which would put ink on the paper.

 -- Scheme Variable: char-set:printing
 -- C Variable: scm_char_set_printing
     The union of char-set:graphic and char-set:whitespace.

 -- Scheme Variable: char-set:whitespace
 -- C Variable: scm_char_set_whitespace
     All whitespace characters.

 -- Scheme Variable: char-set:blank
 -- C Variable: scm_char_set_blank
     All horizontal whitespace characters, which notably includes #\space and #\tab.

 -- Scheme Variable: char-set:iso-control
 -- C Variable: scm_char_set_iso_control
     The ISO control characters are the C0 control characters (U+0000 to U+001F), delete (U+007F), and the C1 control characters (U+0080 to U+009F).

 -- Scheme Variable: char-set:punctuation
 -- C Variable: scm_char_set_punctuation
     All punctuation characters, such as the characters !"#%&'()*,-./:;?@[\\]_{}

 -- Scheme Variable: char-set:symbol
 -- C Variable: scm_char_set_symbol
     All symbol characters, such as the characters $+<=>^`|~.

 -- Scheme Variable: char-set:hex-digit
 -- C Variable: scm_char_set_hex_digit
     The hexadecimal digits 0123456789abcdefABCDEF.

 -- Scheme Variable: char-set:ascii
 -- C Variable: scm_char_set_ascii
     All ASCII characters.

 -- Scheme Variable: char-set:empty
 -- C Variable: scm_char_set_empty
     The empty character set.

 -- Scheme Variable: char-set:designated
 -- C Variable: scm_char_set_designated
     This character set contains all designated code points.  This includes all the code points to which Unicode has assigned a character or other meaning.

 -- Scheme Variable: char-set:full
 -- C Variable: scm_char_set_full
     This character set contains all possible code points.  This includes both designated and reserved code points.

Strings
-------

Strings are fixed-length sequences of characters.  They can be created by calling constructor procedures, but they can also literally get entered at the REPL or in Scheme source files.

Strings always carry the information about how many characters they are composed of with them, so there is no special end-of-string character, like in C.  That means that Scheme strings can contain any character, even the #\nul character \0.

To use strings efficiently, you need to know a bit about how Guile implements them.  In Guile, a string consists of two parts, a head and the actual memory where the characters are stored.  When a string (or a substring of it) is copied, only a new head gets created, the memory is usually not copied.  The two heads start out pointing to the same memory.

When one of these two strings is modified, as with string-set!, their common memory does get copied so that each string has its own memory and modifying one does not accidentally modify the other as well. Thus, Guile's strings are `copy on write'; the actual copying of their memory is delayed until one string is written to.

This implementation makes functions like substring very efficient in the common case that no modifications are done to the involved strings.

If you do know that your strings are getting modified right away, you can use substring/copy instead of substring.  This function performs the copy immediately at the time of creation.  This is more efficient, especially in a multi-threaded program.  Also, substring/copy can avoid the problem that a short substring holds on to the memory of a very large original string that could otherwise be recycled.

If you want to avoid the copy altogether, so that modifications of one string show up in the other, you can use substring/shared.  The strings created by this procedure are called mutation sharing substrings since the substring and the original string share modifications to each other.

If you want to prevent modifications, use substring/read-only.

Guile provides all procedures of SRFI-13 and a few more.

String Read Syntax
..................

The read syntax for strings is an arbitrarily long sequence of characters enclosed in double quotes (").

Backslash is an escape character and can be used to insert the following special characters.  \" and \\ are R5RS standard, \| is R7RS standard, the next seven are R6RS standard --- notice they follow C syntax --- and the remaining four are Guile extensions.

\\
     Backslash character.

\"
     Double quote character (an unescaped " is otherwise the end of the string).

\|
     Vertical bar character.

\a
     Bell character (ASCII 7).

\f
     Formfeed character (ASCII 12).

\n
     Newline character (ASCII 10).

\r
     Carriage return character (ASCII 13).

\t
     Tab character (ASCII 9).

\v
     Vertical tab character (ASCII 11).

\b
     Backspace character (ASCII 8).

\0
     NUL character (ASCII 0).

\ followed by newline (ASCII 10)
     Nothing.  This way if \ is the last character in a line, the string will continue with the first character from the next line, without a line break.

     If the hungry-eol-escapes reader option is enabled, which is not the case by default, leading whitespace on the next line is discarded.

          "foo\
            bar"
          => "foo  bar"
          (read-enable 'hungry-eol-escapes)
          "foo\
            bar"
          => "foobar"

\xHH
     Character code given by two hexadecimal digits.  For example \x7f for an ASCII DEL (127).

\uHHHH
     Character code given by four hexadecimal digits.  For example \u0100 for a capital A with macron (U+0100).

\UHHHHHH
     Character code given by six hexadecimal digits.  For example \U010402.

The following are examples of string literals:

     "foo"
     "bar plonk"
     "Hello World"
     "\"Hi\", he said."

The three escape sequences \xHH, \uHHHH and \UHHHHHH were chosen to not break compatibility with code written for previous versions of Guile.  The R6RS specification suggests a different, incompatible syntax for hex escapes: \xHHHH; -- a character code followed by one to eight hexadecimal digits terminated with a semicolon.  If this escape format is desired instead, it can be enabled with the reader option r6rs-hex-escapes.

     (read-enable 'r6rs-hex-escapes)

For more on reader options, see Scheme Read.

String Predicates
.................

The following procedures can be used to check whether a given string fulfills some specified property.

 -- Scheme Procedure: string? obj
 -- C Function: scm_string_p (obj)
     Return #t if OBJ is a string, else #f.

 -- C Function: int scm_is_string (SCM obj)
     Returns 1 if OBJ is a string, 0 otherwise.

 -- Scheme Procedure: string-null? str
 -- C Function: scm_string_null_p (str)
     Return #t if STR's length is zero, and #f otherwise.

          (string-null? "")  => #t
          y                    => "foo"
          (string-null? y)     => #f

 -- Scheme Procedure: string-any char_pred s [start [end]]
 -- C Function: scm_string_any (char_pred, s, start, end)
     Check if CHAR_PRED is true for any character in string S.

     CHAR_PRED can be a character to check for any equal to that, or a character set (see Character Sets) to check for any in that set, or a predicate procedure to call.

     For a procedure, calls (CHAR_PRED c) are made successively on the characters from START to END.  If CHAR_PRED returns true (ie. non-#f), string-any stops and that return value is the return from string-any.  The call on the last character (ie. at END-1), if that point is reached, is a tail call.

     If there are no characters in S (ie. START equals END) then the return is #f.

 -- Scheme Procedure: string-every char_pred s [start [end]]
 -- C Function: scm_string_every (char_pred, s, start, end)
     Check if CHAR_PRED is true for every character in string S.

     CHAR_PRED can be a character to check for every character equal to that, or a character set (see Character Sets) to check for every character being in that set, or a predicate procedure to call.

     For a procedure, calls (CHAR_PRED c) are made successively on the characters from START to END.  If CHAR_PRED returns #f, string-every stops and returns #f.  The call on the last character (ie. at END-1), if that point is reached, is a tail call and the return from that call is the return from string-every.

     If there are no characters in S (ie. START equals END) then the return is #t.

String Constructors
...................

The string constructor procedures create new string objects, possibly initializing them with some specified character data.  See also String Selection, for ways to create strings from existing strings.

 -- Scheme Procedure: string char...
     Return a newly allocated string made from the given character arguments.

          (string #\x #\y #\z) => "xyz"
          (string)             => ""

 -- Scheme Procedure: list->string lst
 -- C Function: scm_string (lst)
     Return a newly allocated string made from a list of characters.

          (list->string '(#\a #\b #\c)) => "abc"

 -- Scheme Procedure: reverse-list->string lst
 -- C Function: scm_reverse_list_to_string (lst)
     Return a newly allocated string made from a list of characters, in reverse order.

          (reverse-list->string '(#\a #\B #\c)) => "cBa"

 -- Scheme Procedure: make-string k [chr]
 -- C Function: scm_make_string (k, chr)
     Return a newly allocated string of length K.  If CHR is given, then all elements of the string are initialized to CHR, otherwise the contents of the string are unspecified.

 -- C Function: SCM scm_c_make_string (size_t len, SCM chr)
     Like scm_make_string, but expects the length as a size_t.

 -- Scheme Procedure: string-tabulate proc len
 -- C Function: scm_string_tabulate (proc, len)
     PROC is an integer->char procedure.  Construct a string of size LEN by applying PROC to each index to produce the corresponding string element.  The order in which PROC is applied to the indices is not specified.

 -- Scheme Procedure: string-join ls [delimiter [grammar]]
 -- C Function: scm_string_join (ls, delimiter, grammar)
     Append the string in the string list LS, using the string DELIMITER as a delimiter between the elements of LS. GRAMMAR is a symbol which specifies how the delimiter is placed between the strings, and defaults to the symbol infix.

     infix
          Insert the separator between list elements.  An empty string will produce an empty list.

     strict-infix
          Like infix, but will raise an error if given the empty list.

     suffix
          Insert the separator after every list element.

     prefix
          Insert the separator before each list element.

List/String conversion
......................

When processing strings, it is often convenient to first convert them into a list representation by using the procedure string->list, work with the resulting list, and then convert it back into a string. These procedures are useful for similar tasks.

 -- Scheme Procedure: string->list str [start [end]]
 -- C Function: scm_substring_to_list (str, start, end)
 -- C Function: scm_string_to_list (str)
     Convert the string STR into a list of characters.

 -- Scheme Procedure: string-split str char_pred
 -- C Function: scm_string_split (str, char_pred)
     Split the string STR into a list of substrings delimited by appearances of characters that

       * equal CHAR_PRED, if it is a character,

       * satisfy the predicate CHAR_PRED, if it is a procedure,

       * are in the set CHAR_PRED, if it is a character set.

     Note that an empty substring between separator characters will result in an empty string in the result list.

          (string-split "root:x:0:0:root:/root:/bin/bash" #\:)
          =>
          ("root" "x" "0" "0" "root" "/root" "/bin/bash")

          (string-split "::" #\:)
          =>
          ("" "" "")

          (string-split "" #\:)
          =>
          ("")

String Selection
................

Portions of strings can be extracted by these procedures. string-ref delivers individual characters whereas substring can be used to extract substrings from longer strings.

 -- Scheme Procedure: string-length string
 -- C Function: scm_string_length (string)
     Return the number of characters in STRING.

 -- C Function: size_t scm_c_string_length (SCM str)
     Return the number of characters in STR as a size_t.

 -- Scheme Procedure: string-ref str k
 -- C Function: scm_string_ref (str, k)
     Return character K of STR using zero-origin indexing. K must be a valid index of STR.

 -- C Function: SCM scm_c_string_ref (SCM str, size_t k)
     Return character K of STR using zero-origin indexing. K must be a valid index of STR.

 -- Scheme Procedure: string-copy str [start [end]]
 -- C Function: scm_substring_copy (str, start, end)
 -- C Function: scm_string_copy (str)
     Return a copy of the given string STR.

     The returned string shares storage with STR initially, but it is copied as soon as one of the two strings is modified.

 -- Scheme Procedure: substring str start [end]
 -- C Function: scm_substring (str, start, end)
     Return a new string formed from the characters of STR beginning with index START (inclusive) and ending with index END (exclusive). STR must be a string, START and END must be exact integers satisfying:

     0 <= START <= END <= (string-length STR).

     The returned string shares storage with STR initially, but it is copied as soon as one of the two strings is modified.

 -- Scheme Procedure: substring/shared str start [end]
 -- C Function: scm_substring_shared (str, start, end)
     Like substring, but the strings continue to share their storage even if they are modified.  Thus, modifications to STR show up in the new string, and vice versa.

 -- Scheme Procedure: substring/copy str start [end]
 -- C Function: scm_substring_copy (str, start, end)
     Like substring, but the storage for the new string is copied immediately.

 -- Scheme Procedure: substring/read-only str start [end]
 -- C Function: scm_substring_read_only (str, start, end)
     Like substring, but the resulting string can not be modified.

 -- C Function: SCM scm_c_substring (SCM str, size_t start, size_t end)
 -- C Function: SCM scm_c_substring_shared (SCM str, size_t start, size_t end)
 -- C Function: SCM scm_c_substring_copy (SCM str, size_t start, size_t end)
 -- C Function: SCM scm_c_substring_read_only (SCM str, size_t start, size_t end)
     Like scm_substring, etc. but the bounds are given as a size_t.

 -- Scheme Procedure: string-take s n
 -- C Function: scm_string_take (s, n)
     Return the N first characters of S.

 -- Scheme Procedure: string-drop s n
 -- C Function: scm_string_drop (s, n)
     Return all but the first N characters of S.

 -- Scheme Procedure: string-take-right s n
 -- C Function: scm_string_take_right (s, n)
     Return the N last characters of S.

 -- Scheme Procedure: string-drop-right s n
 -- C Function: scm_string_drop_right (s, n)
     Return all but the last N characters of S.

 -- Scheme Procedure: string-pad s len [chr [start [end]]]
 -- Scheme Procedure: string-pad-right s len [chr [start [end]]]
 -- C Function: scm_string_pad (s, len, chr, start, end)
 -- C Function: scm_string_pad_right (s, len, chr, start, end)
     Take characters START to END from the string S and either pad with CHR or truncate them to give LEN characters.

     string-pad pads or truncates on the left, so for example

          (string-pad "x" 3)     => "  x"
          (string-pad "abcde" 3) => "cde"

     string-pad-right pads or truncates on the right, so for example

          (string-pad-right "x" 3)     => "x  "
          (string-pad-right "abcde" 3) => "abc"

 -- Scheme Procedure: string-trim s [char_pred [start [end]]]
 -- Scheme Procedure: string-trim-right s [char_pred [start [end]]]
 -- Scheme Procedure: string-trim-both s [char_pred [start [end]]]
 -- C Function: scm_string_trim (s, char_pred, start, end)
 -- C Function: scm_string_trim_right (s, char_pred, start, end)
 -- C Function: scm_string_trim_both (s, char_pred, start, end)
     Trim occurrences of CHAR_PRED from the ends of S.

     string-trim trims CHAR_PRED characters from the left (start) of the string, string-trim-right trims them from the right (end) of the string, string-trim-both trims from both ends.

     CHAR_PRED can be a character, a character set, or a predicate procedure to call on each character.  If CHAR_PRED is not given the default is whitespace as per char-set:whitespace (see Standard Character Sets).

          (string-trim " x ")              => "x "
          (string-trim-right "banana" #\a) => "banan"
          (string-trim-both ".,xy:;" char-set:punctuation)
                            => "xy"
          (string-trim-both "xyzzy" (lambda (c)
                                       (or (eqv? c #\x)
                                           (eqv? c #\y))))
                            => "zz"

String Modification
...................

These procedures are for modifying strings in-place.  This means that the result of the operation is not a new string; instead, the original string's memory representation is modified.

 -- Scheme Procedure: string-set! str k chr
 -- C Function: scm_string_set_x (str, k, chr)
     Store CHR in element K of STR and return an unspecified value. K must be a valid index of STR.

 -- C Function: void scm_c_string_set_x (SCM str, size_t k, SCM chr)
     Like scm_string_set_x, but the index is given as a size_t.

 -- Scheme Procedure: string-fill! str chr [start [end]]
 -- C Function: scm_substring_fill_x (str, chr, start, end)
 -- C Function: scm_string_fill_x (str, chr)
     Stores CHR in every element of the given STR and returns an unspecified value.

 -- Scheme Procedure: substring-fill! str start end fill
 -- C Function: scm_substring_fill_x (str, start, end, fill)
     Change every character in STR between START and END to FILL.

          (define y (string-copy "abcdefg"))
          (substring-fill! y 1 3 #\r)
          y
          => "arrdefg"

 -- Scheme Procedure: substring-move! str1 start1 end1 str2 start2
 -- C Function: scm_substring_move_x (str1, start1, end1, str2, start2)
     Copy the substring of STR1 bounded by START1 and END1 into STR2 beginning at position START2. STR1 and STR2 can be the same string.

 -- Scheme Procedure: string-copy! target tstart s [start [end]]
 -- C Function: scm_string_copy_x (target, tstart, s, start, end)
     Copy the sequence of characters from index range [START, END) in string S to string TARGET, beginning at index TSTART.  The characters are copied left-to-right or right-to-left as needed -- the copy is guaranteed to work, even if TARGET and S are the same string.  It is an error if the copy operation runs off the end of the target string.

String Comparison
.................

The procedures in this section are similar to the character ordering predicates (see Characters), but are defined on character sequences.

The first set is specified in R5RS and has names that end in ?. The second set is specified in SRFI-13 and the names have not ending ?.

The predicates ending in -ci ignore the character case when comparing strings.  For now, case-insensitive comparison is done using the R5RS rules, where every lower-case character that has a single character upper-case form is converted to uppercase before comparison.  See the (ice-9 i18n) module, for locale-dependent string comparison.

 -- Scheme Procedure: string=? s1 s2 s3 ...
     Lexicographic equality predicate; return #t if all strings are the same length and contain the same characters in the same positions, otherwise return #f.

     The procedure string-ci=? treats upper and lower case letters as though they were the same character, but string=? treats upper and lower case as distinct characters.

 -- Scheme Procedure: string<? s1 s2 s3 ...
     Lexicographic ordering predicate; return #t if, for every pair of consecutive string arguments STR_I and STR_I+1, STR_I is lexicographically less than STR_I+1.

 -- Scheme Procedure: string<=? s1 s2 s3 ...
     Lexicographic ordering predicate; return #t if, for every pair of consecutive string arguments STR_I and STR_I+1, STR_I is lexicographically less than or equal to STR_I+1.

 -- Scheme Procedure: string>? s1 s2 s3 ...
     Lexicographic ordering predicate; return #t if, for every pair of consecutive string arguments STR_I and STR_I+1, STR_I is lexicographically greater than STR_I+1.

 -- Scheme Procedure: string>=? s1 s2 s3 ...
     Lexicographic ordering predicate; return #t if, for every pair of consecutive string arguments STR_I and STR_I+1, STR_I is lexicographically greater than or equal to STR_I+1.

 -- Scheme Procedure: string-ci=? s1 s2 s3 ...
     Case-insensitive string equality predicate; return #t if all strings are the same length and their component characters match (ignoring case) at each position; otherwise return #f.

 -- Scheme Procedure: string-ci<? s1 s2 s3 ...
     Case insensitive lexicographic ordering predicate; return #t if, for every pair of consecutive string arguments STR_I and STR_I+1, STR_I is lexicographically less than STR_I+1 regardless of case.

 -- Scheme Procedure: string-ci<=? s1 s2 s3 ...
     Case insensitive lexicographic ordering predicate; return #t if, for every pair of consecutive string arguments STR_I and STR_I+1, STR_I is lexicographically less than or equal to STR_I+1 regardless of case.

 -- Scheme Procedure: string-ci>? s1 s2 s3 ...
     Case insensitive lexicographic ordering predicate; return #t if, for every pair of consecutive string arguments STR_I and STR_I+1, STR_I is lexicographically greater than STR_I+1 regardless of case.

 -- Scheme Procedure: string-ci>=? s1 s2 s3 ...
     Case insensitive lexicographic ordering predicate; return #t if, for every pair of consecutive string arguments STR_I and STR_I+1, STR_I is lexicographically greater than or equal to STR_I+1 regardless of case.

 -- Scheme Procedure: string-compare s1 s2 proc_lt proc_eq proc_gt [start1 [end1 [start2 [end2]]]]
 -- C Function: scm_string_compare (s1, s2, proc_lt, proc_eq, proc_gt, start1, end1, start2, end2)
     Apply PROC_LT, PROC_EQ, PROC_GT to the mismatch index, depending upon whether S1 is less than, equal to, or greater than S2.  The mismatch index is the largest index I such that for every 0 <= J < I, S1[J] = S2[J] -- that is, I is the first position that does not match.

 -- Scheme Procedure: string-compare-ci s1 s2 proc_lt proc_eq proc_gt [start1 [end1 [start2 [end2]]]]
 -- C Function: scm_string_compare_ci (s1, s2, proc_lt, proc_eq, proc_gt, start1, end1, start2, end2)
     Apply PROC_LT, PROC_EQ, PROC_GT to the mismatch index, depending upon whether S1 is less than, equal to, or greater than S2.  The mismatch index is the largest index I such that for every 0 <= J < I, S1[J] = S2[J] -- that is, I is the first position where the lowercased letters do not match.

 -- Scheme Procedure: string= s1 s2 [start1 [end1 [start2 [end2]]]]
 -- C Function: scm_string_eq (s1, s2, start1, end1, start2, end2)
     Return #f if S1 and S2 are not equal, a true value otherwise.

 -- Scheme Procedure: string<> s1 s2 [start1 [end1 [start2 [end2]]]]
 -- C Function: scm_string_neq (s1, s2, start1, end1, start2, end2)
     Return #f if S1 and S2 are equal, a true value otherwise.

 -- Scheme Procedure: string< s1 s2 [start1 [end1 [start2 [end2]]]]
 -- C Function: scm_string_lt (s1, s2, start1, end1, start2, end2)
     Return #f if S1 is greater or equal to S2, a true value otherwise.

 -- Scheme Procedure: string> s1 s2 [start1 [end1 [start2 [end2]]]]
 -- C Function: scm_string_gt (s1, s2, start1, end1, start2, end2)
     Return #f if S1 is less or equal to S2, a true value otherwise.

 -- Scheme Procedure: string<= s1 s2 [start1 [end1 [start2 [end2]]]]
 -- C Function: scm_string_le (s1, s2, start1, end1, start2, end2)
     Return #f if S1 is greater to S2, a true value otherwise.

 -- Scheme Procedure: string>= s1 s2 [start1 [end1 [start2 [end2]]]]
 -- C Function: scm_string_ge (s1, s2, start1, end1, start2, end2)
     Return #f if S1 is less to S2, a true value otherwise.

 -- Scheme Procedure: string-ci= s1 s2 [start1 [end1 [start2 [end2]]]]
 -- C Function: scm_string_ci_eq (s1, s2, start1, end1, start2, end2)
     Return #f if S1 and S2 are not equal, a true value otherwise.  The character comparison is done case-insensitively.

 -- Scheme Procedure: string-ci<> s1 s2 [start1 [end1 [start2 [end2]]]]
 -- C Function: scm_string_ci_neq (s1, s2, start1, end1, start2, end2)
     Return #f if S1 and S2 are equal, a true value otherwise.  The character comparison is done case-insensitively.

 -- Scheme Procedure: string-ci< s1 s2 [start1 [end1 [start2 [end2]]]]
 -- C Function: scm_string_ci_lt (s1, s2, start1, end1, start2, end2)
     Return #f if S1 is greater or equal to S2, a true value otherwise.  The character comparison is done case-insensitively.

 -- Scheme Procedure: string-ci> s1 s2 [start1 [end1 [start2 [end2]]]]
 -- C Function: scm_string_ci_gt (s1, s2, start1, end1, start2, end2)
     Return #f if S1 is less or equal to S2, a true value otherwise.  The character comparison is done case-insensitively.

 -- Scheme Procedure: string-ci<= s1 s2 [start1 [end1 [start2 [end2]]]]
 -- C Function: scm_string_ci_le (s1, s2, start1, end1, start2, end2)
     Return #f if S1 is greater to S2, a true value otherwise.  The character comparison is done case-insensitively.

 -- Scheme Procedure: string-ci>= s1 s2 [start1 [end1 [start2 [end2]]]]
 -- C Function: scm_string_ci_ge (s1, s2, start1, end1, start2, end2)
     Return #f if S1 is less to S2, a true value otherwise.  The character comparison is done case-insensitively.

 -- Scheme Procedure: string-hash s [bound [start [end]]]
 -- C Function: scm_substring_hash (s, bound, start, end)
     Compute a hash value for S.  The optional argument BOUND is a non-negative exact integer specifying the range of the hash function. A positive value restricts the return value to the range [0,bound).

 -- Scheme Procedure: string-hash-ci s [bound [start [end]]]
 -- C Function: scm_substring_hash_ci (s, bound, start, end)
     Compute a hash value for S.  The optional argument BOUND is a non-negative exact integer specifying the range of the hash function. A positive value restricts the return value to the range [0,bound).

Because the same visual appearance of an abstract Unicode character can be obtained via multiple sequences of Unicode characters, even the case-insensitive string comparison functions described above may return #f when presented with strings containing different representations of the same character.  For example, the Unicode character ``LATIN SMALL LETTER S WITH DOT BELOW AND DOT ABOVE'' can be represented with a single character (U+1E69) or by the character ``LATIN SMALL LETTER S'' (U+0073) followed by the combining marks ``COMBINING DOT BELOW'' (U+0323) and ``COMBINING DOT ABOVE'' (U+0307).

For this reason, it is often desirable to ensure that the strings to be compared are using a mutually consistent representation for every character.  The Unicode standard defines two methods of normalizing the contents of strings: Decomposition, which breaks composite characters into a set of constituent characters with an ordering defined by the Unicode Standard; and composition, which performs the converse.

There are two decomposition operations.  ``Canonical decomposition'' produces character sequences that share the same visual appearance as the original characters, while ``compatibility decomposition'' produces ones whose visual appearances may differ from the originals but which represent the same abstract character.

These operations are encapsulated in the following set of normalization forms:

NFD
     Characters are decomposed to their canonical forms.

NFKD
     Characters are decomposed to their compatibility forms.

NFC
     Characters are decomposed to their canonical forms, then composed.

NFKC
     Characters are decomposed to their compatibility forms, then composed.

The functions below put their arguments into one of the forms described above.

 -- Scheme Procedure: string-normalize-nfd s
 -- C Function: scm_string_normalize_nfd (s)
     Return the NFD normalized form of S.

 -- Scheme Procedure: string-normalize-nfkd s
 -- C Function: scm_string_normalize_nfkd (s)
     Return the NFKD normalized form of S.

 -- Scheme Procedure: string-normalize-nfc s
 -- C Function: scm_string_normalize_nfc (s)
     Return the NFC normalized form of S.

 -- Scheme Procedure: string-normalize-nfkc s
 -- C Function: scm_string_normalize_nfkc (s)
     Return the NFKC normalized form of S.

String Searching
................

 -- Scheme Procedure: string-index s char_pred [start [end]]
 -- C Function: scm_string_index (s, char_pred, start, end)
     Search through the string S from left to right, returning the index of the first occurrence of a character which

       * equals CHAR_PRED, if it is character,

       * satisfies the predicate CHAR_PRED, if it is a procedure,

       * is in the set CHAR_PRED, if it is a character set.

     Return #f if no match is found.

 -- Scheme Procedure: string-rindex s char_pred [start [end]]
 -- C Function: scm_string_rindex (s, char_pred, start, end)
     Search through the string S from right to left, returning the index of the last occurrence of a character which

       * equals CHAR_PRED, if it is character,

       * satisfies the predicate CHAR_PRED, if it is a procedure,

       * is in the set if CHAR_PRED is a character set.

     Return #f if no match is found.

 -- Scheme Procedure: string-prefix-length s1 s2 [start1 [end1 [start2 [end2]]]]
 -- C Function: scm_string_prefix_length (s1, s2, start1, end1, start2, end2)
     Return the length of the longest common prefix of the two strings.

 -- Scheme Procedure: string-prefix-length-ci s1 s2 [start1 [end1 [start2 [end2]]]]
 -- C Function: scm_string_prefix_length_ci (s1, s2, start1, end1, start2, end2)
     Return the length of the longest common prefix of the two strings, ignoring character case.

 -- Scheme Procedure: string-suffix-length s1 s2 [start1 [end1 [start2 [end2]]]]
 -- C Function: scm_string_suffix_length (s1, s2, start1, end1, start2, end2)
     Return the length of the longest common suffix of the two strings.

 -- Scheme Procedure: string-suffix-length-ci s1 s2 [start1 [end1 [start2 [end2]]]]
 -- C Function: scm_string_suffix_length_ci (s1, s2, start1, end1, start2, end2)
     Return the length of the longest common suffix of the two strings, ignoring character case.

 -- Scheme Procedure: string-prefix? s1 s2 [start1 [end1 [start2 [end2]]]]
 -- C Function: scm_string_prefix_p (s1, s2, start1, end1, start2, end2)
     Is S1 a prefix of S2?

 -- Scheme Procedure: string-prefix-ci? s1 s2 [start1 [end1 [start2 [end2]]]]
 -- C Function: scm_string_prefix_ci_p (s1, s2, start1, end1, start2, end2)
     Is S1 a prefix of S2, ignoring character case?

 -- Scheme Procedure: string-suffix? s1 s2 [start1 [end1 [start2 [end2]]]]
 -- C Function: scm_string_suffix_p (s1, s2, start1, end1, start2, end2)
     Is S1 a suffix of S2?

 -- Scheme Procedure: string-suffix-ci? s1 s2 [start1 [end1 [start2 [end2]]]]
 -- C Function: scm_string_suffix_ci_p (s1, s2, start1, end1, start2, end2)
     Is S1 a suffix of S2, ignoring character case?

 -- Scheme Procedure: string-index-right s char_pred [start [end]]
 -- C Function: scm_string_index_right (s, char_pred, start, end)
     Search through the string S from right to left, returning the index of the last occurrence of a character which

       * equals CHAR_PRED, if it is character,

       * satisfies the predicate CHAR_PRED, if it is a procedure,

       * is in the set if CHAR_PRED is a character set.

     Return #f if no match is found.

 -- Scheme Procedure: string-skip s char_pred [start [end]]
 -- C Function: scm_string_skip (s, char_pred, start, end)
     Search through the string S from left to right, returning the index of the first occurrence of a character which

       * does not equal CHAR_PRED, if it is character,

       * does not satisfy the predicate CHAR_PRED, if it is a procedure,

       * is not in the set if CHAR_PRED is a character set.

 -- Scheme Procedure: string-skip-right s char_pred [start [end]]
 -- C Function: scm_string_skip_right (s, char_pred, start, end)
     Search through the string S from right to left, returning the index of the last occurrence of a character which

       * does not equal CHAR_PRED, if it is character,

       * does not satisfy the predicate CHAR_PRED, if it is a procedure,

       * is not in the set if CHAR_PRED is a character set.

 -- Scheme Procedure: string-count s char_pred [start [end]]
 -- C Function: scm_string_count (s, char_pred, start, end)
     Return the count of the number of characters in the string S which

       * equals CHAR_PRED, if it is character,

       * satisfies the predicate CHAR_PRED, if it is a procedure.

       * is in the set CHAR_PRED, if it is a character set.

 -- Scheme Procedure: string-contains s1 s2 [start1 [end1 [start2 [end2]]]]
 -- C Function: scm_string_contains (s1, s2, start1, end1, start2, end2)
     Does string S1 contain string S2?  Return the index in S1 where S2 occurs as a substring, or false. The optional start/end indices restrict the operation to the indicated substrings.

 -- Scheme Procedure: string-contains-ci s1 s2 [start1 [end1 [start2 [end2]]]]
 -- C Function: scm_string_contains_ci (s1, s2, start1, end1, start2, end2)
     Does string S1 contain string S2?  Return the index in S1 where S2 occurs as a substring, or false. The optional start/end indices restrict the operation to the indicated substrings.  Character comparison is done case-insensitively.

Alphabetic Case Mapping
.......................

These are procedures for mapping strings to their upper- or lower-case equivalents, respectively, or for capitalizing strings.

They use the basic case mapping rules for Unicode characters.  No special language or context rules are considered.  The resulting strings are guaranteed to be the same length as the input strings.

See the (ice-9 i18n) module, for locale-dependent case conversions.

 -- Scheme Procedure: string-upcase str [start [end]]
 -- C Function: scm_substring_upcase (str, start, end)
 -- C Function: scm_string_upcase (str)
     Upcase every character in str.

 -- Scheme Procedure: string-upcase! str [start [end]]
 -- C Function: scm_substring_upcase_x (str, start, end)
 -- C Function: scm_string_upcase_x (str)
     Destructively upcase every character in str.

          (string-upcase! y)
          => "ARRDEFG"
          y
          => "ARRDEFG"

 -- Scheme Procedure: string-downcase str [start [end]]
 -- C Function: scm_substring_downcase (str, start, end)
 -- C Function: scm_string_downcase (str)
     Downcase every character in STR.

 -- Scheme Procedure: string-downcase! str [start [end]]
 -- C Function: scm_substring_downcase_x (str, start, end)
 -- C Function: scm_string_downcase_x (str)
     Destructively downcase every character in STR.

          y
          => "ARRDEFG"
          (string-downcase! y)
          => "arrdefg"
          y
          => "arrdefg"

 -- Scheme Procedure: string-capitalize str
 -- C Function: scm_string_capitalize (str)
     Return a freshly allocated string with the characters in STR, where the first character of every word is capitalized.

 -- Scheme Procedure: string-capitalize! str
 -- C Function: scm_string_capitalize_x (str)
     Upcase the first character of every word in STR destructively and return STR.

          y                      => "hello world"
          (string-capitalize! y) => "Hello World"
          y                      => "Hello World"

 -- Scheme Procedure: string-titlecase str [start [end]]
 -- C Function: scm_string_titlecase (str, start, end)
     Titlecase every first character in a word in STR.

 -- Scheme Procedure: string-titlecase! str [start [end]]
 -- C Function: scm_string_titlecase_x (str, start, end)
     Destructively titlecase every first character in a word in STR.

Reversing and Appending Strings
...............................

 -- Scheme Procedure: string-reverse str [start [end]]
 -- C Function: scm_string_reverse (str, start, end)
     Reverse the string STR.  The optional arguments START and END delimit the region of STR to operate on.

 -- Scheme Procedure: string-reverse! str [start [end]]
 -- C Function: scm_string_reverse_x (str, start, end)
     Reverse the string STR in-place.  The optional arguments START and END delimit the region of STR to operate on.  The return value is unspecified.

 -- Scheme Procedure: string-append arg ...
 -- C Function: scm_string_append (args)
     Return a newly allocated string whose characters form the concatenation of the given strings, ARG ....

          (let ((h "hello "))
            (string-append h "world"))
          => "hello world"

 -- Scheme Procedure: string-append/shared arg ...
 -- C Function: scm_string_append_shared (args)
     Like string-append, but the result may share memory with the argument strings.

 -- Scheme Procedure: string-concatenate ls
 -- C Function: scm_string_concatenate (ls)
     Append the elements (which must be strings) of LS together into a single string.  Guaranteed to return a freshly allocated string.

 -- Scheme Procedure: string-concatenate-reverse ls [final_string [end]]
 -- C Function: scm_string_concatenate_reverse (ls, final_string, end)
     Without optional arguments, this procedure is equivalent to

          (string-concatenate (reverse ls))

     If the optional argument FINAL_STRING is specified, it is consed onto the beginning to LS before performing the list-reverse and string-concatenate operations.  If END is given, only the characters of FINAL_STRING up to index END are used.

     Guaranteed to return a freshly allocated string.

 -- Scheme Procedure: string-concatenate/shared ls
 -- C Function: scm_string_concatenate_shared (ls)
     Like string-concatenate, but the result may share memory with the strings in the list LS.

 -- Scheme Procedure: string-concatenate-reverse/shared ls [final_string [end]]
 -- C Function: scm_string_concatenate_reverse_shared (ls, final_string, end)
     Like string-concatenate-reverse, but the result may share memory with the strings in the LS arguments.

Mapping, Folding, and Unfolding
...............................

 -- Scheme Procedure: string-map proc s [start [end]]
 -- C Function: scm_string_map (proc, s, start, end)
     PROC is a char->char procedure, it is mapped over S.  The order in which the procedure is applied to the string elements is not specified.

 -- Scheme Procedure: string-map! proc s [start [end]]
 -- C Function: scm_string_map_x (proc, s, start, end)
     PROC is a char->char procedure, it is mapped over S.  The order in which the procedure is applied to the string elements is not specified.  The string S is modified in-place, the return value is not specified.

 -- Scheme Procedure: string-for-each proc s [start [end]]
 -- C Function: scm_string_for_each (proc, s, start, end)
     PROC is mapped over S in left-to-right order.  The return value is not specified.

 -- Scheme Procedure: string-for-each-index proc s [start [end]]
 -- C Function: scm_string_for_each_index (proc, s, start, end)
     Call (PROC i) for each index i in S, from left to right.

     For example, to change characters to alternately upper and lower case,

          (define str (string-copy "studly"))
          (string-for-each-index
              (lambda (i)
                (string-set! str i
                  ((if (even? i) char-upcase char-downcase)
                   (string-ref str i))))
              str)
          str => "StUdLy"

 -- Scheme Procedure: string-fold kons knil s [start [end]]
 -- C Function: scm_string_fold (kons, knil, s, start, end)
     Fold KONS over the characters of S, with KNIL as the terminating element, from left to right.  KONS must expect two arguments: The actual character and the last result of KONS' application.

 -- Scheme Procedure: string-fold-right kons knil s [start [end]]
 -- C Function: scm_string_fold_right (kons, knil, s, start, end)
     Fold KONS over the characters of S, with KNIL as the terminating element, from right to left.  KONS must expect two arguments: The actual character and the last result of KONS' application.

 -- Scheme Procedure: string-unfold p f g seed [base [make_final]]
 -- C Function: scm_string_unfold (p, f, g, seed, base, make_final)

       * G is used to generate a series of seed values from the initial SEED: SEED, (G SEED), (G^2 SEED), (G^3 SEED), ...

       * P tells us when to stop -- when it returns true when applied to one of these seed values.

       * F maps each seed value to the corresponding character in the result string.  These chars are assembled into the string in a left-to-right order.

       * BASE is the optional initial/leftmost portion of the constructed string; it default to the empty string.

       * MAKE_FINAL is applied to the terminal seed value (on which P returns true) to produce the final/rightmost portion of the constructed string. The default is nothing extra.

 -- Scheme Procedure: string-unfold-right p f g seed [base [make_final]]
 -- C Function: scm_string_unfold_right (p, f, g, seed, base, make_final)

       * G is used to generate a series of seed values from the initial SEED: SEED, (G SEED), (G^2 SEED), (G^3 SEED), ...

       * P tells us when to stop -- when it returns true when applied to one of these seed values.

       * F maps each seed value to the corresponding character in the result string.  These chars are assembled into the string in a right-to-left order.

       * BASE is the optional initial/rightmost portion of the constructed string; it default to the empty string.

       * MAKE_FINAL is applied to the terminal seed value (on which P returns true) to produce the final/leftmost portion of the constructed string. It defaults to (lambda (x) ).

Miscellaneous String Operations
...............................

 -- Scheme Procedure: xsubstring s from [to [start [end]]]
 -- C Function: scm_xsubstring (s, from, to, start, end)
     This is the extended substring procedure that implements replicated copying of a substring of some string.

     S is a string, START and END are optional arguments that demarcate a substring of S, defaulting to 0 and the length of S.  Replicate this substring up and down index space, in both the positive and negative directions. xsubstring returns the substring of this string beginning at index FROM, and ending at TO, which defaults to FROM + (END - START).

 -- Scheme Procedure: string-xcopy! target tstart s sfrom [sto [start [end]]]
 -- C Function: scm_string_xcopy_x (target, tstart, s, sfrom, sto, start, end)
     Exactly the same as xsubstring, but the extracted text is written into the string TARGET starting at index TSTART.  The operation is not defined if (eq? TARGET S) or these arguments share storage -- you cannot copy a string on top of itself.

 -- Scheme Procedure: string-replace s1 s2 [start1 [end1 [start2 [end2]]]]
 -- C Function: scm_string_replace (s1, s2, start1, end1, start2, end2)
     Return the string S1, but with the characters START1 ... END1 replaced by the characters START2 ... END2 from S2.

 -- Scheme Procedure: string-tokenize s [token_set [start [end]]]
 -- C Function: scm_string_tokenize (s, token_set, start, end)
     Split the string S into a list of substrings, where each substring is a maximal non-empty contiguous sequence of characters from the character set TOKEN_SET, which defaults to char-set:graphic. If START or END indices are provided, they restrict string-tokenize to operating on the indicated substring of S.

 -- Scheme Procedure: string-filter char_pred s [start [end]]
 -- C Function: scm_string_filter (char_pred, s, start, end)
     Filter the string S, retaining only those characters which satisfy CHAR_PRED.

     If CHAR_PRED is a procedure, it is applied to each character as a predicate, if it is a character, it is tested for equality and if it is a character set, it is tested for membership.

 -- Scheme Procedure: string-delete char_pred s [start [end]]
 -- C Function: scm_string_delete (char_pred, s, start, end)
     Delete characters satisfying CHAR_PRED from S.

     If CHAR_PRED is a procedure, it is applied to each character as a predicate, if it is a character, it is tested for equality and if it is a character set, it is tested for membership.

Representing Strings as Bytes
.............................

Out in the cold world outside of Guile, not all strings are treated in the same way.  Out there there are only bytes, and there are many ways of representing a strings (sequences of characters) as binary data (sequences of bytes).

As a user, usually you don't have to think about this very much.  When you type on your keyboard, your system encodes your keystrokes as bytes according to the locale that you have configured on your computer. Guile uses the locale to decode those bytes back into characters -- hopefully the same characters that you typed in.

All is not so clear when dealing with a system with multiple users, such as a web server.  Your web server might get a request from one user for data encoded in the ISO-8859-1 character set, and then another request from a different user for UTF-8 data.

Guile provides an iconv module for converting between strings and sequences of bytes.  See Bytevectors, for more on how Guile represents raw byte sequences.  This module gets its name from the common UNIX command of the same name.

Note that often it is sufficient to just read and write strings from ports instead of using these functions.  To do this, specify the port encoding using set-port-encoding!.  See Ports, for more on ports and character encodings.

Unlike the rest of the procedures in this section, you have to load the iconv module before having access to these procedures:

     (use-modules (ice-9 iconv))

 -- Scheme Procedure: string->bytevector string encoding [conversion-strategy]
     Encode STRING as a sequence of bytes.

     The string will be encoded in the character set specified by the ENCODING string.  If the string has characters that cannot be represented in the encoding, by default this procedure raises an encoding-error.  Pass a CONVERSION-STRATEGY argument to specify other behaviors.

     The return value is a bytevector.  See Bytevectors, for more on bytevectors.  See Ports, for more on character encodings and conversion strategies.

 -- Scheme Procedure: bytevector->string bytevector encoding [conversion-strategy]
     Decode BYTEVECTOR into a string.

     The bytes will be decoded from the character set by the ENCODING string.  If the bytes do not form a valid encoding, by default this procedure raises an decoding-error.  As with string->bytevector, pass the optional CONVERSION-STRATEGY argument to modify this behavior.  See Ports, for more on character encodings and conversion strategies.

 -- Scheme Procedure: call-with-output-encoded-string encoding proc [conversion-strategy]
     Like call-with-output-string, but instead of returning a string, returns a encoding of the string according to ENCODING, as a bytevector.  This procedure can be more efficient than collecting a string and then converting it via string->bytevector.

Conversion to/from C
....................

When creating a Scheme string from a C string or when converting a Scheme string to a C string, the concept of character encoding becomes important.

In C, a string is just a sequence of bytes, and the character encoding describes the relation between these bytes and the actual characters that make up the string.  For Scheme strings, character encoding is not an issue (most of the time), since in Scheme you usually treat strings as character sequences, not byte sequences.

Converting to C and converting from C each have their own challenges.

When converting from C to Scheme, it is important that the sequence of bytes in the C string be valid with respect to its encoding.  ASCII strings, for example, can't have any bytes greater than 127.  An ASCII byte greater than 127 is considered ill-formed and cannot be converted into a Scheme character.

Problems can occur in the reverse operation as well.  Not all character encodings can hold all possible Scheme characters.  Some encodings, like ASCII for example, can only describe a small subset of all possible characters.  So, when converting to C, one must first decide what to do with Scheme characters that can't be represented in the C string.

Converting a Scheme string to a C string will often allocate fresh memory to hold the result.  You must take care that this memory is properly freed eventually.  In many cases, this can be achieved by using scm_dynwind_free inside an appropriate dynwind context, see Dynamic Wind.

 -- C Function: SCM scm_from_locale_string (const char *str)
 -- C Function: SCM scm_from_locale_stringn (const char *str, size_t len)
     Creates a new Scheme string that has the same contents as STR when interpreted in the character encoding of the current locale.

     For scm_from_locale_string, STR must be null-terminated.

     For scm_from_locale_stringn, LEN specifies the length of STR in bytes, and STR does not need to be null-terminated. If LEN is (size_t)-1, then STR does need to be null-terminated and the real length will be found with strlen.

     If the C string is ill-formed, an error will be raised.

     Note that these functions should not be used to convert C string constants, because there is no guarantee that the current locale will match that of the execution character set, used for string and character constants.  Most modern C compilers use UTF-8 by default, so to convert C string constants we recommend scm_from_utf8_string.

 -- C Function: SCM scm_take_locale_string (char *str)
 -- C Function: SCM scm_take_locale_stringn (char *str, size_t len)
     Like scm_from_locale_string and scm_from_locale_stringn, respectively, but also frees STR with free eventually. Thus, you can use this function when you would free STR anyway immediately after creating the Scheme string.  In certain cases, Guile can then use STR directly as its internal representation.

 -- C Function: char * scm_to_locale_string (SCM str)
 -- C Function: char * scm_to_locale_stringn (SCM str, size_t *lenp)
     Returns a C string with the same contents as STR in the character encoding of the current locale.  The C string must be freed with free eventually, maybe by using scm_dynwind_free, see Dynamic Wind.

     For scm_to_locale_string, the returned string is null-terminated and an error is signalled when STR contains #\nul characters.

     For scm_to_locale_stringn and LENP not NULL, STR might contain #\nul characters and the length of the returned string in bytes is stored in *LENP.  The returned string will not be null-terminated in this case.  If LENP is NULL, scm_to_locale_stringn behaves like scm_to_locale_string.

     If a character in STR cannot be represented in the character encoding of the current locale, the default port conversion strategy is used.  See Ports, for more on conversion strategies.

     If the conversion strategy is error, an error will be raised.  If it is substitute, a replacement character, such as a question mark, will be inserted in its place.  If it is escape, a hex escape will be inserted in its place.

 -- C Function: size_t scm_to_locale_stringbuf (SCM str, char *buf, size_t max_len)
     Puts STR as a C string in the current locale encoding into the memory pointed to by BUF.  The buffer at BUF has room for MAX_LEN bytes and scm_to_local_stringbuf will never store more than that.  No terminating '\0' will be stored.

     The return value of scm_to_locale_stringbuf is the number of bytes that are needed for all of STR, regardless of whether BUF was large enough to hold them.  Thus, when the return value is larger than MAX_LEN, only MAX_LEN bytes have been stored and you probably need to try again with a larger buffer.

For most situations, string conversion should occur using the current locale, such as with the functions above.  But there may be cases where one wants to convert strings from a character encoding other than the locale's character encoding.  For these cases, the lower-level functions scm_to_stringn and scm_from_stringn are provided.  These functions should seldom be necessary if one is properly using locales.

 -- C Type: scm_t_string_failed_conversion_handler
     This is an enumerated type that can take one of three values: SCM_FAILED_CONVERSION_ERROR, SCM_FAILED_CONVERSION_QUESTION_MARK, and SCM_FAILED_CONVERSION_ESCAPE_SEQUENCE.  They are used to indicate a strategy for handling characters that cannot be converted to or from a given character encoding.  SCM_FAILED_CONVERSION_ERROR indicates that a conversion should throw an error if some characters cannot be converted.  SCM_FAILED_CONVERSION_QUESTION_MARK indicates that a conversion should replace unconvertable characters with the question mark character.  And, SCM_FAILED_CONVERSION_ESCAPE_SEQUENCE requests that a conversion should replace an unconvertable character with an escape sequence.

     While all three strategies apply when converting Scheme strings to C, only SCM_FAILED_CONVERSION_ERROR and SCM_FAILED_CONVERSION_QUESTION_MARK can be used when converting C strings to Scheme.

 -- C Function: char *scm_to_stringn (SCM str, size_t *lenp, const char *encoding, scm_t_string_failed_conversion_handler handler)
     This function returns a newly allocated C string from the Guile string STR.  The length of the returned string in bytes will be returned in LENP.  The character encoding of the C string is passed as the ASCII, null-terminated C string ENCODING.  The HANDLER parameter gives a strategy for dealing with characters that cannot be converted into ENCODING.

     If LENP is NULL, this function will return a null-terminated C string.  It will throw an error if the string contains a null character.

     The Scheme interface to this function is string->bytevector, from the ice-9 iconv module.  See Representing Strings as Bytes.

 -- C Function: SCM scm_from_stringn (const char *str, size_t len, const char *encoding, scm_t_string_failed_conversion_handler handler)
     This function returns a scheme string from the C string STR.  The length in bytes of the C string is input as LEN.  The encoding of the C string is passed as the ASCII, null-terminated C string encoding. The HANDLER parameters suggests a strategy for dealing with unconvertable characters.

     The Scheme interface to this function is bytevector->string. See Representing Strings as Bytes.

The following conversion functions are provided as a convenience for the most commonly used encodings.

 -- C Function: SCM scm_from_latin1_string (const char *str)
 -- C Function: SCM scm_from_utf8_string (const char *str)
 -- C Function: SCM scm_from_utf32_string (const scm_t_wchar *str)
     Return a scheme string from the null-terminated C string STR, which is ISO-8859-1-, UTF-8-, or UTF-32-encoded.  These functions should be used to convert hard-coded C string constants into Scheme strings.

 -- C Function: SCM scm_from_latin1_stringn (const char *str, size_t len)
 -- C Function: SCM scm_from_utf8_stringn (const char *str, size_t len)
 -- C Function: SCM scm_from_utf32_stringn (const scm_t_wchar *str, size_t len)
     Return a scheme string from C string STR, which is ISO-8859-1-, UTF-8-, or UTF-32-encoded, of length LEN.  LEN is the number of bytes pointed to by STR for scm_from_latin1_stringn and scm_from_utf8_stringn; it is the number of elements (code points) in STR in the case of scm_from_utf32_stringn.

 -- C function: char *scm_to_latin1_stringn (SCM str, size_t *lenp)
 -- C function: char *scm_to_utf8_stringn (SCM str, size_t *lenp)
 -- C function: scm_t_wchar *scm_to_utf32_stringn (SCM str, size_t *lenp)
     Return a newly allocated, ISO-8859-1-, UTF-8-, or UTF-32-encoded C string from Scheme string STR.  An error is thrown when STR cannot be converted to the specified encoding.  If LENP is NULL, the returned C string will be null terminated, and an error will be thrown if the C string would otherwise contain null characters.  If LENP is not NULL, the string is not null terminated, and the length of the returned string is returned in LENP.  The length returned is the number of bytes for scm_to_latin1_stringn and scm_to_utf8_stringn; it is the number of elements (code points) for scm_to_utf32_stringn.

String Internals
................

Guile stores each string in memory as a contiguous array of Unicode code points along with an associated set of attributes.  If all of the code points of a string have an integer range between 0 and 255 inclusive, the code point array is stored as one byte per code point: it is stored as an ISO-8859-1 (aka Latin-1) string.  If any of the code points of the string has an integer value greater that 255, the code point array is stored as four bytes per code point: it is stored as a UTF-32 string.

Conversion between the one-byte-per-code-point and four-bytes-per-code-point representations happens automatically as necessary.

No API is provided to set the internal representation of strings; however, there are pair of procedures available to query it.  These are debugging procedures.  Using them in production code is discouraged, since the details of Guile's internal representation of strings may change from release to release.

 -- Scheme Procedure: string-bytes-per-char str
 -- C Function: scm_string_bytes_per_char (str)
     Return the number of bytes used to encode a Unicode code point in string STR.  The result is one or four.

 -- Scheme Procedure: %string-dump str
 -- C Function: scm_sys_string_dump (str)
     Returns an association list containing debugging information for STR. The association list has the following entries.

     string
          The string itself.

     start
          The start index of the string into its stringbuf

     length
          The length of the string

     shared
          If this string is a substring, it returns its parent string.  Otherwise, it returns #f

     read-only
          #t if the string is read-only

     stringbuf-chars
          A new string containing this string's stringbuf's characters

     stringbuf-length
          The number of characters in this stringbuf

     stringbuf-shared
          #t if this stringbuf is shared

     stringbuf-wide
          #t if this stringbuf's characters are stored in a 32-bit buffer, or #f if they are stored in an 8-bit buffer

Bytevectors
-----------

A bytevector is a raw bit string.  The (rnrs bytevectors) module provides the programming interface specified by the Revised^6 Report on the Algorithmic Language Scheme (R6RS) (http://www.r6rs.org/).  It contains procedures to manipulate bytevectors and interpret their contents in a number of ways: bytevector contents can be accessed as signed or unsigned integer of various sizes and endianness, as IEEE-754 floating point numbers, or as strings.  It is a useful tool to encode and decode binary data.

The R6RS (Section 4.3.4) specifies an external representation for bytevectors, whereby the octets (integers in the range 0--255) contained in the bytevector are represented as a list prefixed by #vu8:

     #vu8(1 53 204)

denotes a 3-byte bytevector containing the octets 1, 53, and 204.  Like string literals, booleans, etc., bytevectors are ``self-quoting'', i.e., they do not need to be quoted:

     #vu8(1 53 204)
     => #vu8(1 53 204)

Bytevectors can be used with the binary input/output primitives of the R6RS (see R6RS I/O Ports).

Endianness
..........

Some of the following procedures take an ENDIANNESS parameter. The endianness is defined as the order of bytes in multi-byte numbers: numbers encoded in big endian have their most significant bytes written first, whereas numbers encoded in little endian have their least significant bytes first (Big-endian and little-endian are the most common ``endiannesses'', but others do exist. For instance, the GNU MP library allows word order to be specified independently of byte order (see Integer Import and Export (The GNU Multiple Precision Arithmetic Library Manual)).).

Little-endian is the native endianness of the IA32 architecture and its derivatives, while big-endian is native to SPARC and PowerPC, among others. The native-endianness procedure returns the native endianness of the machine it runs on.

 -- Scheme Procedure: native-endianness
 -- C Function: scm_native_endianness ()
     Return a value denoting the native endianness of the host machine.

 -- Scheme Macro: endianness symbol
     Return an object denoting the endianness specified by SYMBOL.  If SYMBOL is neither big nor little then an error is raised at expand-time.

 -- C Variable: scm_endianness_big
 -- C Variable: scm_endianness_little
     The objects denoting big- and little-endianness, respectively.

Manipulating Bytevectors
........................

Bytevectors can be created, copied, and analyzed with the following procedures and C functions.

 -- Scheme Procedure: make-bytevector len [fill]
 -- C Function: scm_make_bytevector (len, fill)
 -- C Function: scm_c_make_bytevector (size_t len)
     Return a new bytevector of LEN bytes.  Optionally, if FILL is given, fill it with FILL; FILL must be in the range [-128,255].

 -- Scheme Procedure: bytevector? obj
 -- C Function: scm_bytevector_p (obj)
     Return true if OBJ is a bytevector.

 -- C Function: int scm_is_bytevector (SCM obj)
     Equivalent to scm_is_true (scm_bytevector_p (obj)).

 -- Scheme Procedure: bytevector-length bv
 -- C Function: scm_bytevector_length (bv)
     Return the length in bytes of bytevector BV.

 -- C Function: size_t scm_c_bytevector_length (SCM bv)
     Likewise, return the length in bytes of bytevector BV.

 -- Scheme Procedure: bytevector=? bv1 bv2
 -- C Function: scm_bytevector_eq_p (bv1, bv2)
     Return is BV1 equals to BV2---i.e., if they have the same length and contents.

 -- Scheme Procedure: bytevector-fill! bv fill
 -- C Function: scm_bytevector_fill_x (bv, fill)
     Fill bytevector BV with FILL, a byte.

 -- Scheme Procedure: bytevector-copy! source source-start target target-start len
 -- C Function: scm_bytevector_copy_x (source, source_start, target, target_start, len)
     Copy LEN bytes from SOURCE into TARGET, starting reading from SOURCE-START (a positive index within SOURCE) and start writing at TARGET-START.  It is permitted for the SOURCE and TARGET regions to overlap.

 -- Scheme Procedure: bytevector-copy bv
 -- C Function: scm_bytevector_copy (bv)
     Return a newly allocated copy of BV.

 -- C Function: scm_t_uint8 scm_c_bytevector_ref (SCM bv, size_t index)
     Return the byte at INDEX in bytevector BV.

 -- C Function: void scm_c_bytevector_set_x (SCM bv, size_t index, scm_t_uint8 value)
     Set the byte at INDEX in BV to VALUE.

Low-level C macros are available.  They do not perform any type-checking; as such they should be used with care.

 -- C Macro: size_t SCM_BYTEVECTOR_LENGTH (bv)
     Return the length in bytes of bytevector BV.

 -- C Macro: signed char * SCM_BYTEVECTOR_CONTENTS (bv)
     Return a pointer to the contents of bytevector BV.

Interpreting Bytevector Contents as Integers
............................................

The contents of a bytevector can be interpreted as a sequence of integers of any given size, sign, and endianness.

     (let ((bv (make-bytevector 4)))
       (bytevector-u8-set! bv 0 #x12)
       (bytevector-u8-set! bv 1 #x34)
       (bytevector-u8-set! bv 2 #x56)
       (bytevector-u8-set! bv 3 #x78)

       (map (lambda (number)
              (number->string number 16))
            (list (bytevector-u8-ref bv 0)
                  (bytevector-u16-ref bv 0 (endianness big))
                  (bytevector-u32-ref bv 0 (endianness little)))))

     => ("12" "1234" "78563412")

The most generic procedures to interpret bytevector contents as integers are described below.

 -- Scheme Procedure: bytevector-uint-ref bv index endianness size
 -- C Function: scm_bytevector_uint_ref (bv, index, endianness, size)
     Return the SIZE-byte long unsigned integer at index INDEX in BV, decoded according to ENDIANNESS.

 -- Scheme Procedure: bytevector-sint-ref bv index endianness size
 -- C Function: scm_bytevector_sint_ref (bv, index, endianness, size)
     Return the SIZE-byte long signed integer at index INDEX in BV, decoded according to ENDIANNESS.

 -- Scheme Procedure: bytevector-uint-set! bv index value endianness size
 -- C Function: scm_bytevector_uint_set_x (bv, index, value, endianness, size)
     Set the SIZE-byte long unsigned integer at INDEX to VALUE, encoded according to ENDIANNESS.

 -- Scheme Procedure: bytevector-sint-set! bv index value endianness size
 -- C Function: scm_bytevector_sint_set_x (bv, index, value, endianness, size)
     Set the SIZE-byte long signed integer at INDEX to VALUE, encoded according to ENDIANNESS.

The following procedures are similar to the ones above, but specialized to a given integer size:

 -- Scheme Procedure: bytevector-u8-ref bv index
 -- Scheme Procedure: bytevector-s8-ref bv index
 -- Scheme Procedure: bytevector-u16-ref bv index endianness
 -- Scheme Procedure: bytevector-s16-ref bv index endianness
 -- Scheme Procedure: bytevector-u32-ref bv index endianness
 -- Scheme Procedure: bytevector-s32-ref bv index endianness
 -- Scheme Procedure: bytevector-u64-ref bv index endianness
 -- Scheme Procedure: bytevector-s64-ref bv index endianness
 -- C Function: scm_bytevector_u8_ref (bv, index)
 -- C Function: scm_bytevector_s8_ref (bv, index)
 -- C Function: scm_bytevector_u16_ref (bv, index, endianness)
 -- C Function: scm_bytevector_s16_ref (bv, index, endianness)
 -- C Function: scm_bytevector_u32_ref (bv, index, endianness)
 -- C Function: scm_bytevector_s32_ref (bv, index, endianness)
 -- C Function: scm_bytevector_u64_ref (bv, index, endianness)
 -- C Function: scm_bytevector_s64_ref (bv, index, endianness)
     Return the unsigned N-bit (signed) integer (where N is 8, 16, 32 or 64) from BV at INDEX, decoded according to ENDIANNESS.

 -- Scheme Procedure: bytevector-u8-set! bv index value
 -- Scheme Procedure: bytevector-s8-set! bv index value
 -- Scheme Procedure: bytevector-u16-set! bv index value endianness
 -- Scheme Procedure: bytevector-s16-set! bv index value endianness
 -- Scheme Procedure: bytevector-u32-set! bv index value endianness
 -- Scheme Procedure: bytevector-s32-set! bv index value endianness
 -- Scheme Procedure: bytevector-u64-set! bv index value endianness
 -- Scheme Procedure: bytevector-s64-set! bv index value endianness
 -- C Function: scm_bytevector_u8_set_x (bv, index, value)
 -- C Function: scm_bytevector_s8_set_x (bv, index, value)
 -- C Function: scm_bytevector_u16_set_x (bv, index, value, endianness)
 -- C Function: scm_bytevector_s16_set_x (bv, index, value, endianness)
 -- C Function: scm_bytevector_u32_set_x (bv, index, value, endianness)
 -- C Function: scm_bytevector_s32_set_x (bv, index, value, endianness)
 -- C Function: scm_bytevector_u64_set_x (bv, index, value, endianness)
 -- C Function: scm_bytevector_s64_set_x (bv, index, value, endianness)
     Store VALUE as an N-bit (signed) integer (where N is 8, 16, 32 or 64) in BV at INDEX, encoded according to ENDIANNESS.

Finally, a variant specialized for the host's endianness is available for each of these functions (with the exception of the u8 accessors, for obvious reasons):

 -- Scheme Procedure: bytevector-u16-native-ref bv index
 -- Scheme Procedure: bytevector-s16-native-ref bv index
 -- Scheme Procedure: bytevector-u32-native-ref bv index
 -- Scheme Procedure: bytevector-s32-native-ref bv index
 -- Scheme Procedure: bytevector-u64-native-ref bv index
 -- Scheme Procedure: bytevector-s64-native-ref bv index
 -- C Function: scm_bytevector_u16_native_ref (bv, index)
 -- C Function: scm_bytevector_s16_native_ref (bv, index)
 -- C Function: scm_bytevector_u32_native_ref (bv, index)
 -- C Function: scm_bytevector_s32_native_ref (bv, index)
 -- C Function: scm_bytevector_u64_native_ref (bv, index)
 -- C Function: scm_bytevector_s64_native_ref (bv, index)
     Return the unsigned N-bit (signed) integer (where N is 8, 16, 32 or 64) from BV at INDEX, decoded according to the host's native endianness.

 -- Scheme Procedure: bytevector-u16-native-set! bv index value
 -- Scheme Procedure: bytevector-s16-native-set! bv index value
 -- Scheme Procedure: bytevector-u32-native-set! bv index value
 -- Scheme Procedure: bytevector-s32-native-set! bv index value
 -- Scheme Procedure: bytevector-u64-native-set! bv index value
 -- Scheme Procedure: bytevector-s64-native-set! bv index value
 -- C Function: scm_bytevector_u16_native_set_x (bv, index, value)
 -- C Function: scm_bytevector_s16_native_set_x (bv, index, value)
 -- C Function: scm_bytevector_u32_native_set_x (bv, index, value)
 -- C Function: scm_bytevector_s32_native_set_x (bv, index, value)
 -- C Function: scm_bytevector_u64_native_set_x (bv, index, value)
 -- C Function: scm_bytevector_s64_native_set_x (bv, index, value)
     Store VALUE as an N-bit (signed) integer (where N is 8, 16, 32 or 64) in BV at INDEX, encoded according to the host's native endianness.

Converting Bytevectors to/from Integer Lists
............................................

Bytevector contents can readily be converted to/from lists of signed or unsigned integers:

     (bytevector->sint-list (u8-list->bytevector (make-list 4 255))
                            (endianness little) 2)
     => (-1 -1)

 -- Scheme Procedure: bytevector->u8-list bv
 -- C Function: scm_bytevector_to_u8_list (bv)
     Return a newly allocated list of unsigned 8-bit integers from the contents of BV.

 -- Scheme Procedure: u8-list->bytevector lst
 -- C Function: scm_u8_list_to_bytevector (lst)
     Return a newly allocated bytevector consisting of the unsigned 8-bit integers listed in LST.

 -- Scheme Procedure: bytevector->uint-list bv endianness size
 -- C Function: scm_bytevector_to_uint_list (bv, endianness, size)
     Return a list of unsigned integers of SIZE bytes representing the contents of BV, decoded according to ENDIANNESS.

 -- Scheme Procedure: bytevector->sint-list bv endianness size
 -- C Function: scm_bytevector_to_sint_list (bv, endianness, size)
     Return a list of signed integers of SIZE bytes representing the contents of BV, decoded according to ENDIANNESS.

 -- Scheme Procedure: uint-list->bytevector lst endianness size
 -- C Function: scm_uint_list_to_bytevector (lst, endianness, size)
     Return a new bytevector containing the unsigned integers listed in LST and encoded on SIZE bytes according to ENDIANNESS.

 -- Scheme Procedure: sint-list->bytevector lst endianness size
 -- C Function: scm_sint_list_to_bytevector (lst, endianness, size)
     Return a new bytevector containing the signed integers listed in LST and encoded on SIZE bytes according to ENDIANNESS.

Interpreting Bytevector Contents as Floating Point Numbers
..........................................................

Bytevector contents can also be accessed as IEEE-754 single- or double-precision floating point numbers (respectively 32 and 64-bit long) using the procedures described here.

 -- Scheme Procedure: bytevector-ieee-single-ref bv index endianness
 -- Scheme Procedure: bytevector-ieee-double-ref bv index endianness
 -- C Function: scm_bytevector_ieee_single_ref (bv, index, endianness)
 -- C Function: scm_bytevector_ieee_double_ref (bv, index, endianness)
     Return the IEEE-754 single-precision floating point number from BV at INDEX according to ENDIANNESS.

 -- Scheme Procedure: bytevector-ieee-single-set! bv index value endianness
 -- Scheme Procedure: bytevector-ieee-double-set! bv index value endianness
 -- C Function: scm_bytevector_ieee_single_set_x (bv, index, value, endianness)
 -- C Function: scm_bytevector_ieee_double_set_x (bv, index, value, endianness)
     Store real number VALUE in BV at INDEX according to ENDIANNESS.

Specialized procedures are also available:

 -- Scheme Procedure: bytevector-ieee-single-native-ref bv index
 -- Scheme Procedure: bytevector-ieee-double-native-ref bv index
 -- C Function: scm_bytevector_ieee_single_native_ref (bv, index)
 -- C Function: scm_bytevector_ieee_double_native_ref (bv, index)
     Return the IEEE-754 single-precision floating point number from BV at INDEX according to the host's native endianness.

 -- Scheme Procedure: bytevector-ieee-single-native-set! bv index value
 -- Scheme Procedure: bytevector-ieee-double-native-set! bv index value
 -- C Function: scm_bytevector_ieee_single_native_set_x (bv, index, value)
 -- C Function: scm_bytevector_ieee_double_native_set_x (bv, index, value)
     Store real number VALUE in BV at INDEX according to the host's native endianness.

Interpreting Bytevector Contents as Unicode Strings
...................................................

Bytevector contents can also be interpreted as Unicode strings encoded in one of the most commonly available encoding formats. See Representing Strings as Bytes, for a more generic interface.

     (utf8->string (u8-list->bytevector '(99 97 102 101)))
     => "cafe"

     (string->utf8 "café") ;; SMALL LATIN LETTER E WITH ACUTE ACCENT
     => #vu8(99 97 102 195 169)

 -- Scheme Procedure: string->utf8 str
 -- Scheme Procedure: string->utf16 str [endianness]
 -- Scheme Procedure: string->utf32 str [endianness]
 -- C Function: scm_string_to_utf8 (str)
 -- C Function: scm_string_to_utf16 (str, endianness)
 -- C Function: scm_string_to_utf32 (str, endianness)
     Return a newly allocated bytevector that contains the UTF-8, UTF-16, or UTF-32 (aka. UCS-4) encoding of STR.  For UTF-16 and UTF-32, ENDIANNESS should be the symbol big or little; when omitted, it defaults to big endian.

 -- Scheme Procedure: utf8->string utf
 -- Scheme Procedure: utf16->string utf [endianness]
 -- Scheme Procedure: utf32->string utf [endianness]
 -- C Function: scm_utf8_to_string (utf)
 -- C Function: scm_utf16_to_string (utf, endianness)
 -- C Function: scm_utf32_to_string (utf, endianness)
     Return a newly allocated string that contains from the UTF-8-, UTF-16-, or UTF-32-decoded contents of bytevector UTF.  For UTF-16 and UTF-32, ENDIANNESS should be the symbol big or little; when omitted, it defaults to big endian.

Accessing Bytevectors with the Array API
........................................

As an extension to the R6RS, Guile allows bytevectors to be manipulated with the array procedures (see Arrays).  When using these APIs, bytes are accessed one at a time as 8-bit unsigned integers:

     (define bv #vu8(0 1 2 3))

     (array? bv)
     => #t

     (array-rank bv)
     => 1

     (array-ref bv 2)
     => 2

     ;; Note the different argument order on array-set!.
     (array-set! bv 77 2)
     (array-ref bv 2)
     => 77

     (array-type bv)
     => vu8

Accessing Bytevectors with the SRFI-4 API
.........................................

Bytevectors may also be accessed with the SRFI-4 API. See SRFI-4 and Bytevectors, for more information.

Symbols
-------

Symbols in Scheme are widely used in three ways: as items of discrete data, as lookup keys for alists and hash tables, and to denote variable references.

A symbol is similar to a string in that it is defined by a sequence of characters.  The sequence of characters is known as the symbol's name.  In the usual case --- that is, where the symbol's name doesn't include any characters that could be confused with other elements of Scheme syntax --- a symbol is written in a Scheme program by writing the sequence of characters that make up the name, without any quotation marks or other special syntax.  For example, the symbol whose name is ``multiply-by-2'' is written, simply:

     multiply-by-2

Notice how this differs from a string with contents ``multiply-by-2'', which is written with double quotation marks, like this:

     "multiply-by-2"

Looking beyond how they are written, symbols are different from strings in two important respects.

The first important difference is uniqueness.  If the same-looking string is read twice from two different places in a program, the result is two different string objects whose contents just happen to be the same.  If, on the other hand, the same-looking symbol is read twice from two different places in a program, the result is the same symbol object both times.

Given two read symbols, you can use eq? to test whether they are the same (that is, have the same name).  eq? is the most efficient comparison operator in Scheme, and comparing two symbols like this is as fast as comparing, for example, two numbers.  Given two strings, on the other hand, you must use equal? or string=?, which are much slower comparison operators, to determine whether the strings have the same contents.

     (define sym1 (quote hello))
     (define sym2 (quote hello))
     (eq? sym1 sym2) => #t

     (define str1 "hello")
     (define str2 "hello")
     (eq? str1 str2) => #f
     (equal? str1 str2) => #t

The second important difference is that symbols, unlike strings, are not self-evaluating.  This is why we need the (quote ...)s in the example above: (quote hello) evaluates to the symbol named "hello" itself, whereas an unquoted hello is read as the symbol named "hello" and evaluated as a variable reference ... about which more below (see Symbol Variables).

Symbols as Discrete Data
........................

Numbers and symbols are similar to the extent that they both lend themselves to eq? comparison.  But symbols are more descriptive than numbers, because a symbol's name can be used directly to describe the concept for which that symbol stands.

For example, imagine that you need to represent some colours in a computer program.  Using numbers, you would have to choose arbitrarily some mapping between numbers and colours, and then take care to use that mapping consistently:

     ;; 1=red, 2=green, 3=purple

     (if (eq? (colour-of car) 1)
         ...)

You can make the mapping more explicit and the code more readable by defining constants:

     (define red 1)
     (define green 2)
     (define purple 3)

     (if (eq? (colour-of car) red)
         ...)

But the simplest and clearest approach is not to use numbers at all, but symbols whose names specify the colours that they refer to:

     (if (eq? (colour-of car) 'red)
         ...)

The descriptive advantages of symbols over numbers increase as the set of concepts that you want to describe grows.  Suppose that a car object can have other properties as well, such as whether it has or uses:

  * automatic or manual transmission

  * leaded or unleaded fuel

  * power steering (or not).

Then a car's combined property set could be naturally represented and manipulated as a list of symbols:

     (properties-of car1)
     =>
     (red manual unleaded power-steering)

     (if (memq 'power-steering (properties-of car1))
         (display "Unfit people can drive this car.\n")
         (display "You'll need strong arms to drive this car!\n"))
     -|
     Unfit people can drive this car.

Remember, the fundamental property of symbols that we are relying on here is that an occurrence of 'red in one part of a program is an indistinguishable symbol from an occurrence of 'red in another part of a program; this means that symbols can usefully be compared using eq?.  At the same time, symbols have naturally descriptive names.  This combination of efficiency and descriptive power makes them ideal for use as discrete data.

Symbols as Lookup Keys
......................

Given their efficiency and descriptive power, it is natural to use symbols as the keys in an association list or hash table.

To illustrate this, consider a more structured representation of the car properties example from the preceding subsection.  Rather than mixing all the properties up together in a flat list, we could use an association list like this:

     (define car1-properties '((colour . red)
                               (transmission . manual)
                               (fuel . unleaded)
                               (steering . power-assisted)))

Notice how this structure is more explicit and extensible than the flat list.  For example it makes clear that manual refers to the transmission rather than, say, the windows or the locking of the car. It also allows further properties to use the same symbols among their possible values without becoming ambiguous:

     (define car1-properties '((colour . red)
                               (transmission . manual)
                               (fuel . unleaded)
                               (steering . power-assisted)
                               (seat-colour . red)
                               (locking . manual)))

With a representation like this, it is easy to use the efficient assq-XXX family of procedures (see Association Lists) to extract or change individual pieces of information:

     (assq-ref car1-properties 'fuel) => unleaded
     (assq-ref car1-properties 'transmission) => manual

     (assq-set! car1-properties 'seat-colour 'black)
     =>
     ((colour . red)
      (transmission . manual)
      (fuel . unleaded)
      (steering . power-assisted)
      (seat-colour . black)
      (locking . manual)))

Hash tables also have keys, and exactly the same arguments apply to the use of symbols in hash tables as in association lists.  The hash value that Guile uses to decide where to add a symbol-keyed entry to a hash table can be obtained by calling the symbol-hash procedure:

 -- Scheme Procedure: symbol-hash symbol
 -- C Function: scm_symbol_hash (symbol)
     Return a hash value for SYMBOL.

See Hash Tables for information about hash tables in general, and for why you might choose to use a hash table rather than an association list.

Symbols as Denoting Variables
.............................

When an unquoted symbol in a Scheme program is evaluated, it is interpreted as a variable reference, and the result of the evaluation is the appropriate variable's value.

For example, when the expression (string-length "abcd") is read and evaluated, the sequence of characters string-length is read as the symbol whose name is "string-length".  This symbol is associated with a variable whose value is the procedure that implements string length calculation.  Therefore evaluation of the string-length symbol results in that procedure.

The details of the connection between an unquoted symbol and the variable to which it refers are explained elsewhere.  See Binding Constructs, for how associations between symbols and variables are created, and Modules, for how those associations are affected by Guile's module system.

Operations Related to Symbols
.............................

Given any Scheme value, you can determine whether it is a symbol using the symbol? primitive:

 -- Scheme Procedure: symbol? obj
 -- C Function: scm_symbol_p (obj)
     Return #t if OBJ is a symbol, otherwise return #f.

 -- C Function: int scm_is_symbol (SCM val)
     Equivalent to scm_is_true (scm_symbol_p (val)).

Once you know that you have a symbol, you can obtain its name as a string by calling symbol->string.  Note that Guile differs by default from R5RS on the details of symbol->string as regards case-sensitivity:

 -- Scheme Procedure: symbol->string s
 -- C Function: scm_symbol_to_string (s)
     Return the name of symbol S as a string.  By default, Guile reads symbols case-sensitively, so the string returned will have the same case variation as the sequence of characters that caused S to be created.

     If Guile is set to read symbols case-insensitively (as specified by R5RS), and S comes into being as part of a literal expression (see Literal expressions (The Revised^5 Report on Scheme)) or by a call to the read or string-ci->symbol procedures, Guile converts any alphabetic characters in the symbol's name to lower case before creating the symbol object, so the string returned here will be in lower case.

     If S was created by string->symbol, the case of characters in the string returned will be the same as that in the string that was passed to string->symbol, regardless of Guile's case-sensitivity setting at the time S was created.

     It is an error to apply mutation procedures like string-set! to strings returned by this procedure.

Most symbols are created by writing them literally in code.  However it is also possible to create symbols programmatically using the following procedures:

 -- Scheme Procedure: symbol char...
     Return a newly allocated symbol made from the given character arguments.

          (symbol #\x #\y #\z) => xyz

 -- Scheme Procedure: list->symbol lst
     Return a newly allocated symbol made from a list of characters.

          (list->symbol '(#\a #\b #\c)) => abc

 -- Scheme Procedure: symbol-append arg ...
     Return a newly allocated symbol whose characters form the concatenation of the given symbols, ARG ....

          (let ((h 'hello))
            (symbol-append h 'world))
          => helloworld

 -- Scheme Procedure: string->symbol string
 -- C Function: scm_string_to_symbol (string)
     Return the symbol whose name is STRING.  This procedure can create symbols with names containing special characters or letters in the non-standard case, but it is usually a bad idea to create such symbols because in some implementations of Scheme they cannot be read as themselves.

 -- Scheme Procedure: string-ci->symbol str
 -- C Function: scm_string_ci_to_symbol (str)
     Return the symbol whose name is STR.  If Guile is currently reading symbols case-insensitively, STR is converted to lowercase before the returned symbol is looked up or created.

The following examples illustrate Guile's detailed behaviour as regards the case-sensitivity of symbols:

     (read-enable 'case-insensitive)   ; R5RS compliant behaviour

     (symbol->string 'flying-fish)    => "flying-fish"
     (symbol->string 'Martin)         => "martin"
     (symbol->string
        (string->symbol "Malvina"))   => "Malvina"

     (eq? 'mISSISSIppi 'mississippi)  => #t
     (string->symbol "mISSISSIppi")   => mISSISSIppi
     (eq? 'bitBlt (string->symbol "bitBlt")) => #f
     (eq? 'LolliPop
       (string->symbol (symbol->string 'LolliPop))) => #t
     (string=? "K. Harper, M.D."
       (symbol->string
         (string->symbol "K. Harper, M.D."))) => #t

     (read-disable 'case-insensitive)   ; Guile default behaviour

     (symbol->string 'flying-fish)    => "flying-fish"
     (symbol->string 'Martin)         => "Martin"
     (symbol->string
        (string->symbol "Malvina"))   => "Malvina"

     (eq? 'mISSISSIppi 'mississippi)  => #f
     (string->symbol "mISSISSIppi")   => mISSISSIppi
     (eq? 'bitBlt (string->symbol "bitBlt")) => #t
     (eq? 'LolliPop
       (string->symbol (symbol->string 'LolliPop))) => #t
     (string=? "K. Harper, M.D."
       (symbol->string
         (string->symbol "K. Harper, M.D."))) => #t

From C, there are lower level functions that construct a Scheme symbol from a C string in the current locale encoding.

When you want to do more from C, you should convert between symbols and strings using scm_symbol_to_string and scm_string_to_symbol and work with the strings.

 -- C Function: SCM scm_from_latin1_symbol (const char *name)
 -- C Function: SCM scm_from_utf8_symbol (const char *name)
     Construct and return a Scheme symbol whose name is specified by the null-terminated C string NAME.  These are appropriate when the C string is hard-coded in the source code.

 -- C Function: SCM scm_from_locale_symbol (const char *name)
 -- C Function: SCM scm_from_locale_symboln (const char *name, size_t len)
     Construct and return a Scheme symbol whose name is specified by NAME.  For scm_from_locale_symbol, NAME must be null terminated; for scm_from_locale_symboln the length of NAME is specified explicitly by LEN.

     Note that these functions should not be used when NAME is a C string constant, because there is no guarantee that the current locale will match that of the execution character set, used for string and character constants.  Most modern C compilers use UTF-8 by default, so in such cases we recommend scm_from_utf8_symbol.

 -- C Function: SCM scm_take_locale_symbol (char *str)
 -- C Function: SCM scm_take_locale_symboln (char *str, size_t len)
     Like scm_from_locale_symbol and scm_from_locale_symboln, respectively, but also frees STR with free eventually. Thus, you can use this function when you would free STR anyway immediately after creating the Scheme string.  In certain cases, Guile can then use STR directly as its internal representation.

The size of a symbol can also be obtained from C:

 -- C Function: size_t scm_c_symbol_length (SCM sym)
     Return the number of characters in SYM.

Finally, some applications, especially those that generate new Scheme code dynamically, need to generate symbols for use in the generated code.  The gensym primitive meets this need:

 -- Scheme Procedure: gensym [prefix]
 -- C Function: scm_gensym (prefix)
     Create a new symbol with a name constructed from a prefix and a counter value.  The string PREFIX can be specified as an optional argument.  Default prefix is  g.  The counter is increased by 1 at each call.  There is no provision for resetting the counter.

The symbols generated by gensym are likely to be unique, since their names begin with a space and it is only otherwise possible to generate such symbols if a programmer goes out of their way to do so.  Uniqueness can be guaranteed by instead using uninterned symbols (see Symbol Uninterned), though they can't be usefully written out and read back in.

Function Slots and Property Lists
.................................

In traditional Lisp dialects, symbols are often understood as having three kinds of value at once:

  * a variable value, which is used when the symbol appears in code in a variable reference context

  * a function value, which is used when the symbol appears in code in a function name position (i.e. as the first element in an unquoted list)

  * a property list value, which is used when the symbol is given as the first argument to Lisp's put or get functions.

Although Scheme (as one of its simplifications with respect to Lisp) does away with the distinction between variable and function namespaces, Guile currently retains some elements of the traditional structure in case they turn out to be useful when implementing translators for other languages, in particular Emacs Lisp.

Specifically, Guile symbols have two extra slots, one for a symbol's property list, and one for its ``function value.''  The following procedures are provided to access these slots.

 -- Scheme Procedure: symbol-fref symbol
 -- C Function: scm_symbol_fref (symbol)
     Return the contents of SYMBOL's function slot.

 -- Scheme Procedure: symbol-fset! symbol value
 -- C Function: scm_symbol_fset_x (symbol, value)
     Set the contents of SYMBOL's function slot to VALUE.

 -- Scheme Procedure: symbol-pref symbol
 -- C Function: scm_symbol_pref (symbol)
     Return the property list currently associated with SYMBOL.

 -- Scheme Procedure: symbol-pset! symbol value
 -- C Function: scm_symbol_pset_x (symbol, value)
     Set SYMBOL's property list to VALUE.

 -- Scheme Procedure: symbol-property sym prop
     From SYM's property list, return the value for property PROP.  The assumption is that SYM's property list is an association list whose keys are distinguished from each other using equal?; PROP should be one of the keys in that list.  If the property list has no entry for PROP, symbol-property returns #f.

 -- Scheme Procedure: set-symbol-property! sym prop val
     In SYM's property list, set the value for property PROP to VAL, or add a new entry for PROP, with value VAL, if none already exists.  For the structure of the property list, see symbol-property.

 -- Scheme Procedure: symbol-property-remove! sym prop
     From SYM's property list, remove the entry for property PROP, if there is one.  For the structure of the property list, see symbol-property.

Support for these extra slots may be removed in a future release, and it is probably better to avoid using them.  For a more modern and Schemely approach to properties, see Object Properties.

Extended Read Syntax for Symbols
................................

The read syntax for a symbol is a sequence of letters, digits, and extended alphabetic characters, beginning with a character that cannot begin a number.  In addition, the special cases of +, -, and ... are read as symbols even though numbers can begin with +, - or ..

Extended alphabetic characters may be used within identifiers as if they were letters.  The set of extended alphabetic characters is:

     ! $ % & * + - . / : < = > ? @ ^ _ ~

In addition to the standard read syntax defined above (which is taken from R5RS (see Formal syntax (The Revised^5 Report on Scheme))), Guile provides an extended symbol read syntax that allows the inclusion of unusual characters such as space characters, newlines and parentheses.  If (for whatever reason) you need to write a symbol containing characters not mentioned above, you can do so as follows.

  * Begin the symbol with the characters #{,

  * write the characters of the symbol and

  * finish the symbol with the characters }#.

Here are a few examples of this form of read syntax.  The first symbol needs to use extended syntax because it contains a space character, the second because it contains a line break, and the last because it looks like a number.

     #{foo bar}#

     #{what
     ever}#

     #{4242}#

Although Guile provides this extended read syntax for symbols, widespread usage of it is discouraged because it is not portable and not very readable.

Alternatively, if you enable the r7rs-symbols read option (see Scheme Read), you can write arbitrary symbols using the same notation used for strings, except delimited by vertical bars instead of double quotes.

     |foo bar|
     |\x3BB; is a greek lambda|
     |\| is a vertical bar|

Note that there's also an r7rs-symbols print option (see Scheme Write).  To enable the use of this notation, evaluate one or both of the following expressions:

     (read-enable  'r7rs-symbols)
     (print-enable 'r7rs-symbols)

Uninterned Symbols
..................

What makes symbols useful is that they are automatically kept unique. There are no two symbols that are distinct objects but have the same name.  But of course, there is no rule without exception.  In addition to the normal symbols that have been discussed up to now, you can also create special uninterned symbols that behave slightly differently.

To understand what is different about them and why they might be useful, we look at how normal symbols are actually kept unique.

Whenever Guile wants to find the symbol with a specific name, for example during read or when executing string->symbol, it first looks into a table of all existing symbols to find out whether a symbol with the given name already exists.  When this is the case, Guile just returns that symbol.  When not, a new symbol with the name is created and entered into the table so that it can be found later.

Sometimes you might want to create a symbol that is guaranteed `fresh', i.e. a symbol that did not exist previously.  You might also want to somehow guarantee that no one else will ever unintentionally stumble across your symbol in the future.  These properties of a symbol are often needed when generating code during macro expansion.  When introducing new temporary variables, you want to guarantee that they don't conflict with variables in other people's code.

The simplest way to arrange for this is to create a new symbol but not enter it into the global table of all symbols.  That way, no one will ever get access to your symbol by chance.  Symbols that are not in the table are called uninterned.  Of course, symbols that are in the table are called interned.

You create new uninterned symbols with the function make-symbol. You can test whether a symbol is interned or not with symbol-interned?.

Uninterned symbols break the rule that the name of a symbol uniquely identifies the symbol object.  Because of this, they can not be written out and read back in like interned symbols.  Currently, Guile has no support for reading uninterned symbols.  Note that the function gensym does not return uninterned symbols for this reason.

 -- Scheme Procedure: make-symbol name
 -- C Function: scm_make_symbol (name)
     Return a new uninterned symbol with the name NAME.  The returned symbol is guaranteed to be unique and future calls to string->symbol will not return it.

 -- Scheme Procedure: symbol-interned? symbol
 -- C Function: scm_symbol_interned_p (symbol)
     Return #t if SYMBOL is interned, otherwise return #f.

For example:

     (define foo-1 (string->symbol "foo"))
     (define foo-2 (string->symbol "foo"))
     (define foo-3 (make-symbol "foo"))
     (define foo-4 (make-symbol "foo"))

     (eq? foo-1 foo-2)
     => #t
     ; Two interned symbols with the same name are the same object,

     (eq? foo-1 foo-3)
     => #f
     ; but a call to make-symbol with the same name returns a
     ; distinct object.

     (eq? foo-3 foo-4)
     => #f
     ; A call to make-symbol always returns a new object, even for
     ; the same name.

     foo-3
     => #<uninterned-symbol foo 8085290>
     ; Uninterned symbols print differently from interned symbols,

     (symbol? foo-3)
     => #t
     ; but they are still symbols,

     (symbol-interned? foo-3)
     => #f
     ; just not interned.

Keywords
--------

Keywords are self-evaluating objects with a convenient read syntax that makes them easy to type.

Guile's keyword support conforms to R5RS, and adds a (switchable) read syntax extension to permit keywords to begin with : as well as #:, or to end with :.

Why Use Keywords?
.................

Keywords are useful in contexts where a program or procedure wants to be able to accept a large number of optional arguments without making its interface unmanageable.

To illustrate this, consider a hypothetical make-window procedure, which creates a new window on the screen for drawing into using some graphical toolkit.  There are many parameters that the caller might like to specify, but which could also be sensibly defaulted, for example:

  * color depth -- Default: the color depth for the screen

  * background color -- Default: white

  * width -- Default: 600

  * height -- Default: 400

If make-window did not use keywords, the caller would have to pass in a value for each possible argument, remembering the correct argument order and using a special value to indicate the default value for that argument:

     (make-window 'default              ;; Color depth
                  'default              ;; Background color
                  800                   ;; Width
                  100                   ;; Height
                  ...)                  ;; More make-window arguments

With keywords, on the other hand, defaulted arguments are omitted, and non-default arguments are clearly tagged by the appropriate keyword.  As a result, the invocation becomes much clearer:

     (make-window #:width 800 #:height 100)

On the other hand, for a simpler procedure with few arguments, the use of keywords would be a hindrance rather than a help.  The primitive procedure cons, for example, would not be improved if it had to be invoked as

     (cons #:car x #:cdr y)

So the decision whether to use keywords or not is purely pragmatic: use them if they will clarify the procedure invocation at point of call.

Coding With Keywords
....................

If a procedure wants to support keywords, it should take a rest argument and then use whatever means is convenient to extract keywords and their corresponding arguments from the contents of that rest argument.

The following example illustrates the principle: the code for make-window uses a helper procedure called get-keyword-value to extract individual keyword arguments from the rest argument.

     (define (get-keyword-value args keyword default)
       (let ((kv (memq keyword args)))
         (if (and kv (>= (length kv) 2))
             (cadr kv)
             default)))

     (define (make-window . args)
       (let ((depth  (get-keyword-value args #:depth  screen-depth))
             (bg     (get-keyword-value args #:bg     "white"))
             (width  (get-keyword-value args #:width  800))
             (height (get-keyword-value args #:height 100))
             ...)
         ...))

But you don't need to write get-keyword-value.  The (ice-9 optargs) module provides a set of powerful macros that you can use to implement keyword-supporting procedures like this:

     (use-modules (ice-9 optargs))

     (define (make-window . args)
       (let-keywords args #f ((depth  screen-depth)
                              (bg     "white")
                              (width  800)
                              (height 100))
         ...))

Or, even more economically, like this:

     (use-modules (ice-9 optargs))

     (define* (make-window #:key (depth  screen-depth)
                                 (bg     "white")
                                 (width  800)
                                 (height 100))
       ...)

For further details on let-keywords, define* and other facilities provided by the (ice-9 optargs) module, see Optional Arguments.

To handle keyword arguments from procedures implemented in C, use scm_c_bind_keyword_arguments (see Keyword Procedures).

Keyword Read Syntax
...................

Guile, by default, only recognizes a keyword syntax that is compatible with R5RS.  A token of the form #:NAME, where NAME has the same syntax as a Scheme symbol (see Symbol Read Syntax), is the external representation of the keyword named NAME.  Keyword objects print using this syntax as well, so values containing keyword objects can be read back into Guile.  When used in an expression, keywords are self-quoting objects.

If the keyword read option is set to 'prefix, Guile also recognizes the alternative read syntax :NAME.  Otherwise, tokens of the form :NAME are read as symbols, as required by R5RS.

If the keyword read option is set to 'postfix, Guile recognizes the SRFI-88 read syntax NAME: (see SRFI-88). Otherwise, tokens of this form are read as symbols.

To enable and disable the alternative non-R5RS keyword syntax, you use the read-set! procedure documented Scheme Read.  Note that the prefix and postfix syntax are mutually exclusive.

     (read-set! keywords 'prefix)

     #:type
     =>
     #:type

     :type
     =>
     #:type

     (read-set! keywords 'postfix)

     type:
     =>
     #:type

     :type
     =>
     :type

     (read-set! keywords #f)

     #:type
     =>
     #:type

     :type
     -|
     ERROR: In expression :type:
     ERROR: Unbound variable: :type
     ABORT: (unbound-variable)

Keyword Procedures
..................

 -- Scheme Procedure: keyword? obj
 -- C Function: scm_keyword_p (obj)
     Return #t if the argument OBJ is a keyword, else #f.

 -- Scheme Procedure: keyword->symbol keyword
 -- C Function: scm_keyword_to_symbol (keyword)
     Return the symbol with the same name as KEYWORD.

 -- Scheme Procedure: symbol->keyword symbol
 -- C Function: scm_symbol_to_keyword (symbol)
     Return the keyword with the same name as SYMBOL.

 -- C Function: int scm_is_keyword (SCM obj)
     Equivalent to scm_is_true (scm_keyword_p (OBJ)).

 -- C Function: SCM scm_from_locale_keyword (const char *name)
 -- C Function: SCM scm_from_locale_keywordn (const char *name, size_t len)
     Equivalent to scm_symbol_to_keyword (scm_from_locale_symbol (NAME)) and scm_symbol_to_keyword (scm_from_locale_symboln (NAME, LEN)), respectively.

     Note that these functions should not be used when NAME is a C string constant, because there is no guarantee that the current locale will match that of the execution character set, used for string and character constants.  Most modern C compilers use UTF-8 by default, so in such cases we recommend scm_from_utf8_keyword.

 -- C Function: SCM scm_from_latin1_keyword (const char *name)
 -- C Function: SCM scm_from_utf8_keyword (const char *name)
     Equivalent to scm_symbol_to_keyword (scm_from_latin1_symbol (NAME)) and scm_symbol_to_keyword (scm_from_utf8_symbol (NAME)), respectively.

 -- C Function: void scm_c_bind_keyword_arguments (const char *subr,
     SCM rest, scm_t_keyword_arguments_flags flags,
     SCM keyword1, SCM *argp1,
     ...,
     SCM keywordN, SCM *argpN,
     SCM_UNDEFINED)

     Extract the specified keyword arguments from REST, which is not modified.  If the keyword argument KEYWORD1 is present in REST with an associated value, that value is stored in the variable pointed to by ARGP1, otherwise the variable is left unchanged.  Similarly for the other keywords and argument pointers up to KEYWORDN and ARGPN.  The argument list to scm_c_bind_keyword_arguments must be terminated by SCM_UNDEFINED.

     Note that since the variables pointed to by ARGP1 through ARGPN are left unchanged if the associated keyword argument is not present, they should be initialized to their default values before calling scm_c_bind_keyword_arguments.  Alternatively, you can initialize them to SCM_UNDEFINED before the call, and then use SCM_UNBNDP after the call to see which ones were provided.

     If an unrecognized keyword argument is present in REST and FLAGS does not contain SCM_ALLOW_OTHER_KEYS, or if non-keyword arguments are present and FLAGS does not contain SCM_ALLOW_NON_KEYWORD_ARGUMENTS, an exception is raised. SUBR should be the name of the procedure receiving the keyword arguments, for purposes of error reporting.

     For example:

          SCM k_delimiter;
          SCM k_grammar;
          SCM sym_infix;

          SCM my_string_join (SCM strings, SCM rest)
          {
            SCM delimiter = SCM_UNDEFINED;
            SCM grammar   = sym_infix;

            scm_c_bind_keyword_arguments ("my-string-join", rest, 0,
                                          k_delimiter, &delimiter,
                                          k_grammar, &grammar,
                                          SCM_UNDEFINED);

            if (SCM_UNBNDP (delimiter))
              delimiter = scm_from_utf8_string (" ");

            return scm_string_join (strings, delimiter, grammar);
          }

          void my_init ()
          {
            k_delimiter = scm_from_utf8_keyword ("delimiter");
            k_grammar   = scm_from_utf8_keyword ("grammar");
            sym_infix   = scm_from_utf8_symbol  ("infix");
            scm_c_define_gsubr ("my-string-join", 1, 0, 1, my_string_join);
          }

``Functionality-Centric'' Data Types
------------------------------------

Procedures and macros are documented in their own sections: see Procedures and Macros.

Variable objects are documented as part of the description of Guile's module system: see Variables.

Asyncs, dynamic roots and fluids are described in the section on scheduling: see Scheduling.

Hooks are documented in the section on general utility functions: see Hooks.

Ports are described in the section on I/O: see Input and Output.

Regular expressions are described in their own section: see Regular Expressions.