Chapter 3

Basic Data Extensions

3.1  Void and Undefined

MzScheme returns the unique void value -- printed as #<void> -- for expressions that have unspecified results in R5RS. The procedure void takes any number of arguments and returns void:

• (void v ···) returns void.

• (void? v) returns #t if v is void, #f otherwise.

Variables bound by letrec-values that are accessible but not yet initialized are bound to the unique undefined value, printed as #<undefined>.

3.2  Booleans

Unless otherwise specified, two instances of a particular MzScheme data type are equal? only when they are eq?. Two values are eqv? only when they are either eq?, both +nan.0, or both = and have the same exactness and sign. (The inexact numbers 0.0 and -0.0 are not eqv?, although they are =.)

The andmap and ormap procedures apply a test procedure to the elements of a list, returning immediately when the result for testing the entire list is determined. The arguments to andmap and ormap are the same as for map, but a single boolean value is returned as the result, rather than a list:

• (andmap proc list ···¹) applies proc to elements of the lists from the first elements to the last, returning #f as soon as any application returns #f. If no application of proc returns #f, then the result of the last application of proc is returned. If the lists are empty, then #t is returned.

• (ormap proc list ···¹) applies proc to elements of the lists from the first elements to the last. If any application returns a value other than #f, that value is immediately returned as the result of the ormap application. If all applications of proc return #f, then the result is #f. If the lists are empty, then #f is returned.

Examples:

(andmap positive? '(1 2 3)) ; => #t
(ormap eq? '(a b c) '(a b c)) ; => #t
(andmap positive? '(1 2 a)) ; => raises exn:fail:contract
(ormap positive? '(1 2 a)) ; => #t
(andmap positive? '(1 -2 a)) ; => #f
(andmap + '(1 2 3) '(4 5 6)) ; => 9
(ormap + '(1 2 3) '(4 5 6)) ; => 5

3.3  Numbers

A number in MzScheme is one of the following:

• a fixnum exact integer (30 bits⁴ plus a sign bit)

• a bignum exact integer (cannot be represented in a fixnum)

• a fraction exact rational (represented by two exact integers)

• a flonum inexact rational (double-precision floating-point number)

• a complex number; either the real and imaginary parts are both exact or inexact, or the number has an exact zero real part and an inexact imaginary part; a complex number with an inexact zero imaginary part is a real number

MzScheme extends the number syntax of R5RS in three ways:

• All input radixes (#b, #o, #d, and #x) allow ``decimal'' numbers that contain a period or exponent marker. For example, #b1.1 is equivalent to 1.5. In hexadecimal numbers, e and d always stand for a hexadecimal digit, not an exponent marker.

• The mantissa of a number with an exponent marker can be expressed as a fraction. For example, 1/2e3 is equivalent to 500.0, and 1/2e2+1/2e4i is equivalent to 50.0+5000.0i.

• The following are inexact numerical constants: +inf.0 (infinity), -inf.0 (negative infinity), +nan.0 (not a number), and -nan.0 (same as +nan.0). These names can also be used within complex constants, as in -inf.0+inf.0i. These names are case-insensitive.

The special inexact numbers +inf.0, -inf.0, and +nan.0 have no exact form. Dividing by an inexact zero returns +inf.0 or -inf.0, depending on the sign of the dividend. The infinities are integers, and they answer #t for both even? and odd?. The +nan.0 value is not an integer and is not = to itself, but +nan.0 is eqv? to itself.⁵ Similarly, (= 0.0 -0.0) is #t, but (eqv? 0.0 -0.0) is #f.

All multi-argument arithmetic procedures operate pairwise on arguments from left to right.

The string->number procedure works on all number representations and exact integer radix values in the range 2 to 16 (inclusive). The number->string procedure accepts all number types and the radix values 2, 8, 10, and 16; however, if an inexact number is provided with a radix other than 10, the exn:fail:contract exception is raised.

The add1 and sub1 procedures work on any number:

• (add1 z) returns z + 1.

• (sub1 z) returns z - 1.

The following procedures work on integers:

• (quotient/remainder n1 n2) returns two values: (quotient n1 n2) and (remainder n1 n2).

• (integer-sqrt n) returns the integer square-root of n. For positive n, the result is the largest positive integer bounded by the (sqrt n). For negative n, the result is (* (integer-sqrt (- n)) 0+i).

• (integer-sqrt/remainder n) returns two values: (integer-sqrt n) and (- n (expt (integer-sqrt n) 2)).

The following procedures work on exact integers in their (semi-infinite) two's complement representation:

• (bitwise-ior n ···¹) returns the bitwise ``inclusive or'' of the ns.

• (bitwise-and n ···¹) returns the bitwise ``and'' of the ns.

• (bitwise-xor n ···¹) returns the bitwise ``exclusive or'' of the ns.

• (bitwise-not n) returns the bitwise ``not'' of n.

• (arithmetic-shift n m) returns the bitwise ``shift'' of n. The integer n is shifted left by m bits; i.e., m new zeros are introduced as rightmost digits. If m is negative, n is shifted right by - m bits; i.e., the rightmost m digits are dropped.

The random procedure generates pseudo-random numbers:

• (random k) returns a random exact integer in the range 0 to k - 1 where k is an exact integer between 1 and 2³¹ - 1, inclusive. The number is provided by the current pseudo-random number generator, which maintains an internal state for generating numbers.⁶

• (random) returns a random inexact number between 0 and 1, exclusive, using the current pseudo-random number generator.

• (random-seed k) seeds the current pseudo-random number generator with k, an exact integer between 0 and 2³¹ - 1, inclusive. Seeding a generator sets its internal state deterministically; seeding a generator with a particular number forces it to produce a sequence of pseudo-random numbers that is the same across runs and across platforms.

• (pseudo-random-generator->vector generator) produces a vector that represents the complete internal state of generator. The vector is suitable as an argument to vector->pseudo-random-generator to recreate the generator in its current state (across runs and across platforms).

• (vector->pseudo-random-generator vec) produces a pseudo-random number generator whose internal state corresponds to vec. The vector vec must contain six exact integers; the first three integers must be in the range 0 to 4294967086, inclusive; the last three integers must be in the range 0 to 4294944442, inclusive; at least one of the first three integers must be non-zero; and at least one of the last three integers must be non-zero.

• (current-pseudo-random-generator) returns the current pseudo-random number generator, and (current-pseudo-random-generator generator) sets the current generator to generator. See also section 7.9.1.10.

• (make-pseudo-random-generator) returns a new pseudo-random number generator. The new generator is seeded with a number derived from (current-milliseconds).

• (pseudo-random-generator? v) returns #t if v is a pseudo-random number generator, #f otherwise.

The following procedures convert between Scheme numbers and common machine byte representations:

• (integer-bytes->integer string signed? [big-endian?]) converts the machine-format number encoded in string to an exact integer. The string must contain either 2, 4, or 8 characters. If signed? is true, then the string is decoded as a two's-complement number, otherwise it is decoded as an unsigned integer. If big-endian? is true, then the first character's ASCII value provides the most significant eight bits of the number, otherwise the first character provides the least-significant eight bits, and so on. The default value of big-endian? is the result of system-big-endian?.

• (integer->integer-bytes n size-n signed? [big-endian? to-string]) converts the exact integer n to a machine-format number encoded in a string of length size-n, which must be 2, 4, or 8. If signed? is true, then the number is encoded with two's complement, otherwise it is encoded as an unsigned bit stream. If big-endian? is true, then the most significant eight bits of the number are encoded in the first character of the resulting string, otherwise the least-significant bits are encoded in the first character, and so on. The default value of big-endian? is the result of system-big-endian?.

  If to-string is provided, it must be a mutable string of length size-n; in that case, the encoding of n is written into to-string, and to-string is returned as the result. If to-string is not provided, the result is a newly allocated string.

  If n cannot be encoded in a string of the requested size and format, the exn:fail:contract exception is raised. If to-string is provided and it is not of length size-n, the exn:fail:contract exception is raised.

• (floating-point-bytes->real string [big-endian?]) converts the IEEE floating-point number encoded in string to an inexact real number. The string must contain either 4 or 8 characters. If big-endian? is true, then the first character's ASCII value provides the most significant eight bits of the IEEE representation, otherwise the first character provides the least-significant eight bits, and so on. The default value of big-endian? is the result of system-big-endian?.

• (real->floating-point-bytes x size-n [big-endian? to-string]) converts the real number x to its IEEE representation in a string of length size-n, which must be 4 or 8. If big-endian? is true, then the most significant eight bits of the number are encoded in the first character of the resulting string, otherwise the least-significant bits are encoded in the first character, and so on. The default value of big-endian? is the result of system-big-endian?.

  If to-string is provided, it must be a mutable string of length size-n; in that case, the encoding of n is written into to-string, and to-string is returned as the result. If to-string is not provided, the result is a newly allocated string.

  If to-string is provided and it is not of length size-n, the exn:fail:contract exception is raised.

• (system-big-endian?) returns #t if the native encoding of numbers is big-endian for the machine running MzScheme, #f if the native encoding is little-endian.

3.4  Characters

MzScheme characters range over Unicode scalar values (see section 1.2.1), which includes characters whose values range from #x0 to #x10FFFF, but not including #xD800 to #xDFFF. The procedure char->integer returns a character's code-point number, and integer->char converts a code-point number to a character. If integer->char is given an integer that is either outside #x0 to #x10FFFF or in the excluded range #xD800 to #xDFFF, the exn:fail:contract exception is raised.

Character constants include special named characters, such as #\newline, plus octal representations (e.g., #\251), and Unicode-style hexadecimal representations (e.g., #\u03BB). See section 11.2.4 for more information on character constants.

The character comparison procedures char=?, char<?, char-ci=?, etc. take two or more character arguments and check the arguments pairwise (like the numerical comparison procedures). Two characters are eq? whenever they are char=?. The expression (char<? char1 char2) produces the same result as (< (char->integer char1) (char->integer char2)), etc. The case-independent -ci procedures compare characters after case-folding with char-foldcase (described below).

The character predicates produce results consistent with the Unicode database⁷ and (usually) SRFI-14. These procedures are fully portable; their results do not depend on the current platform or locale.

• (char-alphabetic? char) -- returns #t if char's Unicode general category is Lu, Ll, Lt, Lm, or Lo, #f otherwise.

• (char-lower-case? char) -- returns #t if char has the Unicode ``Lowercase'' property.

• (char-upper-case? char) -- returns #t if char has the Unicode ``Uppercase'' property.

• (char-title-case? char) -- returns #t if char's Unicode general category is Lt, #f otherwise.

• (char-numeric? char) -- returns #t if char's Unicode general category is Nd, #f otherwise.

• (char-symbolic? char) -- returns #t if char's Unicode general category is Sm, Sc, Sk, or So, #f otherwise.

• (char-punctuation? char) -- returns #t if char's Unicode general category is Pc, Pd, Ps, Pe, Pi, Pf, or Po, #f otherwise.

• (char-graphic? char) -- returns #t if char's Unicode general category is Mn, Mc, Me, or if one of the following produces #t when applied to char: char-alphabetic?, char-numeric?, char-symbolic?, or char-punctuation?.

• (char-whitespace? char) -- returns #t if char's Unicode general category is Zs, Zl, or Zp, or if char is one of the following: #\tab, #\newline, #\vtab, #\page, or #\return.

• (char-blank? char) -- returns #t if char's Unicode general category is Zs or if char is #\tab. (These correspond to horizontal whitespace.)

• (char-iso-control? char) -- return #t if char is between #\u0000 and #\u001F inclusive or #\u007F and #\u009F inclusive.

Character conversions are also consistent with the 1-to-1 code point mapping defined by Unicode. String procedures (see section 3.5) handle the case where Unicode defines a locale-independent mapping from the code point to a code-point sequence (in addition to the 1-1 mapping on scalar values).

• (char-upcase char) produces a character according to the upcase mapping provided by the Unicode database for char; if char has no upcase mapping, char-upcase produces char.

• (char-downcase char) produces a character according to the downcase mapping provided by the Unicode database for char; if char has no downcase mapping, char-upcase produces char.

• (char-titlecase char) produces a character according to the titlecase mapping provided by the Unicode database for char; if char has no titlecase mapping, char-upcase produces char.

• (char-foldcase char) produces a character according to the case-folding mapping provided by the Unicode database for char.

(make-known-char-range-list) produces a list of three-element lists, where each three-element list represents a set of consecutive code points for which the Unicode standard specifies character properties. Each three-element list contains two integers and a boolean; the first integer is a starting code-point value (inclusive), the second integer is an ending code-point value (inclusive), and the boolean is #t when all characters in the code-point range have identical results for all of the character predicates above. The three-element lists are ordered in the overall result list such that later lists represent larger code-point values, and all three-element lists are separated from every other by at least one code-point value that is not specified by Unicode.

(char-utf-8-length char) produces the same result as (bytes-length (string->bytes/utf-8 (string char))).

3.5  Strings

Since a string consists of a sequence of characters, a string in MzScheme is a Unicode code-point sequence. MzScheme also provides byte strings, as well as functions to convert between byte strings and strings with respect to various encodings, including UTF-8 and the current locale's encoding. See section 1.2 for an overview of Unicode, locales, and encodings, and see section 3.6 for more specific information on byte-string conversions.

A string can be mutable or immutable. When an immutable string is provided to a procedure like string-set!, the exn:fail:contract exception is raised. String constants generated by read are immutable. (string->immutable-string string) returns an immutable string with the same content as string, and it returns string itself if string is immutable. (See also immutable? in section 3.10.)

(substring string start-k [end-k]) returns a mutable string, even if the string argument is immutable. The end-k argument defaults to (string-length string)

(string-copy! dest-string dest-start-k src-string [src-start-k src-end-k]) changes the characters of dest-string from positions dest-start-k (inclusive) to dest-end-k (exclusive) to match the characters in src-string from src-start-k (inclusive). If src-start-k is not provided, it defaults to 0. If src-end-k is not provided, it defaults to (string-length src-string). The strings dest-string and src-string can be the same string, and in that case the destination region can overlap with the source region; the destination characters after the copy match the source characters from before the copy. If any of dest-start-k, src-start-k, or src-end-k are out of range (taking into account the sizes of the strings and the source and destination regions), the exn:fail:contract exception is raised.

When a string is created with make-string without a fill value, it is initialized with the null character (#\nul) in all positions.

The string comparison procedures string=?, string<?, string-ci=?, etc. take two or more string arguments and check the arguments pairwise (like the numerical comparison procedures). String comparisons are performed through pairwise comparison of characters; for the -ci operations, the two strings are first case-folded using string-foldcase (described below). Comparisons using all of these functions are fully portable; the results do not depend on the current platform or locale.

Four string-conversion procedures take into account Unicode's locale-independent conversion rules that map code-point sequences to code-point sequences (instead of simply mapping a 1-to-1 function on code points over the string). In each case, the string produced by the conversion can be longer that the input string.

• (string-upcase string) returns a string whose characters are the upcase conversion of the characters in string.

• (string-downcase string) returns a string whose characters are the downcase conversion of the characters in string.

• (string-titlecase string) returns a string where the first character in each sequence of cased characters in string (ignoring case-ignorable characters) is converted to titlecase, and all other cased characters are downcased.

• (string-foldcase string) returns a string whose characters are the case-fold conversion of the characters in string.

Examples:

(string-upcase "abc!") ; => "ABC!"
(string-upcase "Stra\xDFe") ; => "STRASSE"

(string-downcase "aBC!") ; => "abc!"
(string-downcase "Stra\xDFe") ; => "stra\xDFe"
(string-downcase "\u039A\u0391\u039F\u03A3") ; => "\u03BA\u03b1\u03BF\u03C2"
(string-downcase "\u03A3") ; => "\u03C3"

(string-titlecase "aBC  twO") ; => "Abc  Two"
(string-titlecase "y2k") ; => "Y2K"
(string-titlecase "main stra\xDFe") ; => "Main Stra\xDFe"
(string-titlecase "stra \xDFe") ; => "Stra Sse"

(string-foldcase "aBC!") ; => "abc!"
(string-foldcase "Stra\xDFe") ; => "strasse"
(string-foldcase "\u039A\u0391\u039F\u03A3") ; => "\u03BA\u03b1\u03BF\u03C3"

In addition to the character-based string procedures, MzScheme provides the following locale-sensitive procedures (see also section 1.2.2 and section 7.9.1.11):

• (string-locale=? string1 string2 ···¹)

• (string-locale<? string1 string2 ···¹)

• (string-locale>? string1 string2 ···¹)

• (string-locale-ci=? string1 string2 ···¹)

• (string-locale-ci<? string1 string2 ···¹)

• (string-locale-ci>? string1 string2 ···¹)

• (string-locale-upcase string) -- may produce a string that is longer or shorter than string if the current locale has complex case-folding rules.

• (string-locale-downcase string) -- like string-locale-upcase, may produce a string that is longer or shorter than string

These procedures depend only on the current locale's case-conversion and collation rules, and not on its encoding rules.

3.6  Byte Strings

A byte string is like a string, but it a sequence of bytes instead of characters. A byte is an exact integer between 0 and 255 inclusive; (byte? v) produces #t if v is such an exact integer, #f otherwise. Two bytes strings are equal? if they are bytewise equal, and two byte strings are eqv? only if they are eq?.

MzScheme provides byte-string operations in parallel to the character-string operations:

• (bytes? v)

• (bytes byte ···¹)

• (make-bytes k [byte])

• (bytes-length bytes)

• (bytes-ref bytes k)

• (bytes-set! bytes k byte)

• (bytes-fill! bytes byte)

• (subbytes bytes start-k [end-k])

• (bytes-append bytes ···¹)

• (bytes-copy bytes)

• (bytes-copy! dest-bytes dest-start-k src-bytes [src-start-k src-end-k])

• (bytes->list bytes)

• (list->bytes byte-list)

• (bytes->immutable-bytes bytes)

• (bytes=? bytes1 bytes2 ···¹)

• (bytes<? bytes1 bytes2 ···¹)

• (bytes>? bytes1 bytes2 ···¹)

A byte-string constant is written like a string, but prefixed with # (with no space between # and the opening double-quote). A byte-string constant can contain escape sequences, as in #"\n", just like strings; an exn:fail:read exception is raised if a ``\u'' sequence appears within a byte string and the given hexadecimal value is larger than 255.

Like character strings, byte strings generated by read are immutable, and when an immutable string is provided to a procedure like bytes-set!, the exn:fail:contract exception is raised.

The following procedures convert between byte strings and character strings:

• (bytes->string/utf-8 bytes [err-char start-k end-k]) -- produces a string by decoding the start-k to end-k substring of bytes as a UTF-8 encoding of Unicode code points. If err-char is provided and not #f, then it is used for bytes that fall in the range #o200 to #o377 but are not part of a valid encoding sequence. (This is consistent with reading characters from a port; see section 11.1 for more details.) If err-char is #f or not provided, and if the start-k to end-k substring of bytes is not a valid UTF-8 encoding overall, then the exn:fail:contract exception is raised. If start-k or end-k are not provided, they default to 0 and (bytes-length bytes), respectively.

• (bytes->string/locale bytes [err-char start-k end-k]) -- produces a string by decoding the start-k to end-k substring of bytes using the current locale's encoding (see also section 1.2.2). If err-char is provided and not #f, it is used for each byte in bytes that is not part of a valid encoding; if err-char is #f or not provided, and if the start-k to end-k substring of bytes is not a valid encoding overall, then the exn:fail:contract exception is raised. If start-k or end-k are not provided, they default to 0 and (bytes-length bytes), respectively.

• (bytes->string/latin-1 bytes [err-char start-k end-k]) -- produces a string by decoding the start-k to end-k substring of bytes as a Latin-1 encoding of Unicode code points; i.e., each byte is translated directly to a character using integer->char, so the decoding always succeeds.⁸ The err-char argument is ignored, but for consistency with the other operations, it must be a character or #f if provided. If start-k or end-k are not provided, they default to 0 and (bytes-length bytes), respectively.

• (string->bytes/utf-8 string [err-byte start-k end-k]) -- produces a byte string by ending the start-k to end-k substring of string via UTF-8 (always succeeding). The err-char argument is ignored, but for consistency with the other operations, it must be a byte or #f if provided. If start-k or end-k are not provided, they default to 0 and (string-length string), respectively.

• (string->bytes/locale string [err-byte start-k end-k]) -- produces a string by encoding the start-k to end-k substring of string using the current locale's encoding (see also section 1.2.2). If err-byte is provided and not #f, it is used for each character in string that cannot be encoded for the current locale; if err-byte is #f or not provided, and if the start-k to end-k substring of string cannot be encoded, then the exn:fail:contract exception is raised. If start-k or end-k are not provided, they default to 0 and (string-length string), respectively.

• (string->bytes/latin-1 string [err-byte start-k end-k]) -- produces a string by encoding the start-k to end-k substring of string using Latin-1; i.e., each character is translated directly to a byte using char->integer. If err-byte is provided and not #f, it is used for each character in string whose value is greater than 255;⁹ if err-byte is #f or not provided, and if the start-k to end-k substring of string has a character with a value greater than 255, then the exn:fail:contract exception is raised. If start-k or end-k are not provided, they default to 0 and (string-length string), respectively.

• (string-utf-8-length string [start-k end-k]) returns the length in bytes of the UTF-8 encoding of string's substring from start-k to end-k, but without actually generating the encoded bytes. If start-k is not provided, it defaults to 0, and end-k defaults to (string-length string).

• (bytes-utf-8-length bytes [err-char start-k end-k]) returns the length in characters of the UTF-8 decoding of bytes's substring from start-k to end-k, but without actually generating the decoded characters. If start-k is not provided, it defaults to 0, and end-k defaults to (bytes-length bytes). If err-char is #f and the substring is not a UTF-8 encoding overall, the result is #f. Otherwise, err-char is used to resolve decoding errors as in bytes->string/utf-8.

• (bytes-utf-8-ref bytes [skip-k err-char start-k end-k]) returns the skip-kth character in the UTF-8 decoding of bytes's substring from start-k to end-k, but without actually generating the other decoded characters. If start-k is not provided, it defaults to 0, and end-k defaults to (bytes-length bytes). If the substring is not a UTF-8 encoding up to the skip-kth character (when err-char is #f), or if the substring decoding produces fewer than skip-k characters, the result is #f. If err-char is not #f, it is used to resolve decoding errors as in bytes->string/utf-8.

• (bytes-utf-8-index bytes [skip-k err-char start-k end-k]) returns the offset in bytes into bytes at which the skip-kth character's encoding starts in the UTF-8 decoding of bytes's substring from start-k to end-k (but without actually generating the other decoded characters). If start-k is not provided, it defaults to 0, and end-k defaults to (bytes-length bytes). The result is relative to the start of bytes, not to start-k. If the substring is not a UTF-8 encoding up to the skip-kth character (when err-char is #f), or if the substring decoding produces fewer than skip-k characters, the result is #f. If err-char is not #f, it is used to resolve decoding errors as in bytes->string/utf-8.

A string converter can be used to convert directly from one byte-string encoding of characters to another byte-string encoding.

• (bytes-open-converter from-name-string to-name-string) -- produces a string converter to go from the encoding named by from-name-string to the encoding named by to-name-string. If the requested conversion pair is not available, #f is returned instead of a converter.

  Three encodings are always available in certain positions:

  • "UTF-8" as either from or to -- the UTF-8 encoding.

  • "UTF-8-permissive" as from with "UTF-8" as to -- the UTF-8 encoding where encoding errors are tolerated, producing the same result as (char->integer #\?) for bytes that are not part of a valid encoding sequence. (This handling of invalid sequences is consistent with the interpretation of port bytes streams into characters; see section 11.1.)

  • "" as either from or to -- the current locale's default encoding (see section 1.2.2).

  A newly opened byte converter is registered with the current custodian (see section 9.2), so that the converter is closed when the custodian is shut down. A converter is not registered with a custodian (and does not need to be closed) if it is is one of the guaranteed combinations involving only "UTF-8" and "UTF-8-permissive" under Unix, or if it is any of the guaranteed combinations (including "") under Windows and Mac OS X.

  The set of available encodings and combinations varies by platform, depending on the iconv library that is installed. Under Windows, iconv.dll or libiconv.dll must be in the user's path or the current executable's directory at run time, and iconv.dll or libiconv.dll must link to msvcrt.dll for _errno; otherwise, only the guaranteed combinations are available.

• (bytes-close-converter bytes-converter) -- closes the given converter, so that it can no longer be used with bytes-convert or bytes-convert-end.

• (bytes-convert bytes-converter src-bytes [src-start-k src-end-k dest-bytes dest-start-k dest-end-k]) converts the bytes from src-start-k to src-end-k in src-bytes. If dest-bytes is supplied and not #f, the converted byte are written into dest-bytes from dest-start-k to dest-end-k. If dest-bytes is not supplied or is #f, then a newly allocated byte string holds the conversion results, and the size of the result byte string is no more than (- dest-end-k start-start-k).

  If src-start-k or dest-start-k is not provided, it defaults to 0. If src-end-k is not provided, it defaults to (bytes-length src-bytes. If src-end-k is not provided or is #f, then it defaults to (bytes-length dest-bytes) when dest-bytes is a byte string or to an arbitrarily large integer otherwise.

  The result of bytes-convert is three values:

  • result-bytes or dest-wrote-k -- a byte string if dest-bytes is #f or not provided, or the number of bytes written into dest-bytes otherwise.

  • src-read-k -- the number of bytes successfully converted from src-bytes.

  • 'complete, 'continues, 'aborts, or 'error -- indicates how conversion terminated.

    • 'complete: The entire input was processed, and src-read-k will be equal to (- src-end-k src-start-k).

    • 'continues: Conversion stopped due to the limit on the result size or the space in dest-bytes; in this case, fewer than (- dest-end-k dest-start-k) bytes may be returned if more space is needed to process the next complete encoding sequence in src-bytes.

    • 'aborts: The input stopped part-way through an encoding sequence, and more input bytes are necessary to continue. For example, if the last byte of input is #o303 for a "UTF-8-permissive" decoding, the result is 'aborts, because another byte is needed to determine how to use the #o303 byte.

    • 'error: The bytes starting at (+ src-start-k src-read-k) bytes in src-bytes do not form a legal encoding sequence. This result is never produced for some encodings, where all byte sequences are valid encodings. For example, since "UTF-8-permissive" handles an invalid UTF-8 sequence by dropping characters or generating ``?'', every byte sequence is effectively valid.

  Applying a converter accumulates state in the converter (even when the third result of bytes-convert is 'complete). This state can affect both further processing of input and further generation of output, but only for conversions that involve ``shift sequences'' to change modes within a stream. To terminate an input sequence and reset the converter, use bytes-convert-end.

• (bytes-convert-end bytes-converter [dest-bytes dest-start-k dest-end-k]) -- like bytes-convert, but instead of converting bytes, this procedure generates an ending sequence for the conversion (sometimes called a ``shift sequence''), if any. Few encodings use shift sequences, so this function will succeed with no output for most encodings. In any case, successful output of a (possibly empty) shift sequence resets the converter to its initial state.

  The result of bytes-convert-end is two values:

  • result-bytes or dest-wrote-k -- a byte string if dest-bytes is #f or not provided, or the number of bytes written into dest-bytes otherwise.

  • 'complete or 'continues -- indicates whether conversion completed. If 'complete, then an entire ending sequence was produced. If 'continues, then the conversion could not complete due to the limit on the result size or the space in dest-bytes, and the first result is either an empty byte string or 0.

• (bytes-converter? v) returns #t if v is a byte converter produced by bytes-open-converter, #f otherwise.

• (locale-string-encoding) returns a string for the current locale's encoding (i.e., the encoding normally identified by ""). See also system-language+country in section 15.5.

3.7  Symbols

For information about symbol parsing and printing, see section 11.2.4 and section 11.2.5, respectively.

MzScheme provides two ways of generating an uninterned symbol, i.e., a symbol that is not eq?, eqv?, or equal? to any other symbol, although it may print the same as another symbol:

• (string->uninterned-symbol string) is like (string->symbol string), but the resulting symbol is a new uninterned symbol. Calling string->uninterned-symbol twice with the same string returns two distinct symbols.

• (gensym [symbol/string]) creates an uninterned symbol with an automatically-generated name. The optional symbol/string argument is a prefix symbol or string.

Regular (interned) symbols are only weakly held by the internal symbol table. This weakness can never affect the result of an eq?, eqv?, or equal? test, but a symbol may disappear when placed into a weak box (see section 13.1) used as the key in a weak hash table (see section 3.14), or used as an ephemeron key (see section 13.2).

3.8  Keywords

A symbol-like datum that starts with a hash and colon (``#:'') is parsed as a keyword constant. Keywords behave like symbols -- two keywords are eq? if and only if they print the same -- but they are a distinct set of values.

• (keyword? v) returns #t if v is a keyword, #f otherwise.

• (keyword->string keyword) returns a string for the displayed form of keyword, not including the leading #:.

• (string->keyword string) returns a keyword whose displayed form is the same as that of string, but with a leading #:.

Like symbols, keywords are only weakly held by the internal keyword table; see section 3.7 for more information.

3.9  Vectors

When a vector is created with make-vector without a fill value, it is initialized with 0 in all positions. A vector can be immutable, such as a vector returned by syntax-e, but vectors generated by read are mutable. (See also immutable? in section 3.10.)

(vector->immutable-vector vec) returns an immutable vector with the same content as vec, and it returns vec itself if vec is immutable. (See also immutable? in section 3.10.)

(vector-immutable v ···¹) is like (vector v ···1) except that the resulting vector is immutable. (See also immutable? in section 3.10.)

3.10  Lists

A cons cell can be mutable or immutable. When an immutable cons cell is provided to a procedure like set-cdr!, the exn:fail:contract exception is raised. Cons cells generated by read are always mutable.

The global variable null is bound to the empty list.

(reverse! list) is the same as (reverse list), but list is destructively reversed using set-cdr! (i.e., each cons cell in list is mutated).

(append! list ···¹) is like (append list), but it destructively appends the lists (i.e., except for the last list, the last cons cell of each list is mutated to append the lists; empty lists are essentially dropped).

(list* v ···¹) is similar to (list v ···1) but the last argument is used directly as the cdr of the last pair constructed for the list:

(list* 1 2 3 4) ; => '(1 2 3 . 4)

(cons-immutable v1 v2) returns an immutable pair whose car is v1 and cdr is v2.

(list-immutable v ···¹) is like (list v ···1), but using immutable pairs.

(list*-immutable v ···¹) is like (list* v ···1), but using immutable pairs.

(immutable? v) returns #t if v is an immutable cons cell, string, vector, box, or hash table, #f otherwise.

The list-ref and list-tail procedures accept an improper list as a first argument. If either procedure is applied to an improper list and an index that would require taking the car or cdr of a non-cons-cell, the exn:fail:contract exception is raised.

The member, memv, and memq procedures accept an improper list as a second argument. If the membership search reaches the improper tail, the exn:fail:contract exception is raised.

The assoc, assv, and assq procedures accept an improperly formed association list as a second argument. If the association search reaches an improper list tail or a list element that is not a pair, the exn:fail:contract exception is raised.

3.11  Boxes

MzScheme provides boxes, which are records that have a single field:

• (box v) returns a new mutable box that contains v.

• (box-immutable v) returns a new immutable box that contains v.

• (unbox box) returns the content of box. For any v, (unbox (box v)) returns v.

• (set-box! mutable-box v) sets the content of mutable-box to v.

• (box? v) returns #t if v is a box, #f otherwise.

Two boxes are equal? if the contents of the boxes are equal?.

A box returned by syntax-e (see section 12.2.2) is immutable; if set-box! is applied to such a box, the exn:fail:contract exception is raised. A box produced by read (via #&) is mutable. (See also immutable? in section 3.10.)

3.12  Procedures

See section 4.6 for information on defining new procedure types.

3.12.1  Arity

MzScheme's procedure-arity procedure returns the input arity of a procedure:

• (procedure-arity proc) returns information about the number of arguments accepted by the procedure proc. The result a is either:

  • an exact non-negative integer ==> the procedure always takes exactly a arguments;

  • an arity-at-least¹⁰ instance ==> the procedure takes (arity-at-least-value a) or more arguments; or

  • a list containing integers and arity-at-least instances ==> the procedure takes any number of arguments that can match one of the arities in the list.

• (procedure-arity-includes? proc k) returns #t if the procedure can accept n arguments (where k is an exact, non-negative integer), #f otherwise.

Examples:

(procedure-arity cons) ; => 2
(procedure-arity list) ; => #<struct:arity-at-least>
(arity-at-least? (procedure-arity list)) ; => #t
(arity-at-least-value (procedure-arity list)) ; => 0
(arity-at-least-value (procedure-arity (lambda (x . y) x))) ; => 1
(procedure-arity (case-lambda [(x) 0] [(x y) 1])) ; => '(1 2)
(procedure-arity-includes? cons 2) ; => #t
(procedure-arity-includes? display 3) ; => #f

When compiling a lambda or case-lambda expression, MzScheme looks for a 'method-arity-error property attached to the expression (see section 12.6.2). If it is present with a true value, and if no case of the procedure accepts zero arguments, then the procedure is marked so that an exn:fail:contract:arity exception involving the procedure will hide the first argument, if one was provided. (Hiding the first argument is useful when the procedure implements a method, where the first argument is implicit in the original source). The property affects only the format of exn:fail:contract:arity exceptions, not the result of procedure-arity.

3.12.2  Primitives

A primitive procedure is a built-in procedure that is implemented in low-level language. Not all built-in procedures are primitives, but almost all R5RS procedures are primitives, as are most of the procedures described in this manual.

• (primitive? v) returns #t if v is a primitive procedure or #f otherwise.

• (primitive-result-arity prim-proc) returns the arity of the result of the primitive procedure prim-proc (as opposed to the procedure's input arity as returned by arity; see section 3.12.1). For most primitives, this procedure returns 1, since most primitives return a single value when applied. For information about arity values, see section 3.12.1.

• (primitive-closure? v) returns #t if v is internally implemented as a primitive closure rather than a simple primitive procedure, #f otherwise. This information is intended for use by the mzc compiler.

3.12.3  Procedure Names

See section 6.2.4 for information about the names of primitives, and the names inferred for lambda and case-lambda procedures.

3.13  Promises

The force procedure can only be applied to values returned by delay, and promises are never implicitly forced.

(promise? v) returns #t if v is a promise created by delay, #f otherwise.

3.14  Hash Tables

(make-hash-table [flag-symbol flag-symbol]) creates and returns a new hash table. If provided, each flag-symbol must one of the following:

• 'weak -- creates a hash table with weakly-held keys (see section 13.1).

• 'equal -- creates a hash table that compares keys using equal? instead of eq? (needed, for example, when using strings as keys).

By default, key comparisons use eq?. If the second flag-symbol is redundant, the exn:fail:contract exception is raised.

Two hash tables are equal? if they are created with the same flags, and if they map the same keys to equal? values (where ``same key'' means either eq? or equal?, depending on the way the hash table compares keys).

(make-immutable-hash-table assoc-list [flag-symbol]) creates an immutable hash table. (See also immutable? in section 3.10.) The assoc-list must be a list of pairs, where the car of each pair is a key, and the cdr is the corresponding value. The mappings are added to the table in the order that they appear in assoc-list, so later mappings can hide earlier mappings. If the optional flag-symbol argument is provided, it must be 'equal, and the created hash table compares keys with equal?; otherwise, the created table compares keys with eq?.

(hash-table? v [flag-symbol flag-symbol]) returns #t if v was created by make-hash-table or make-immutable-hash-table with the given flag-symbols (or more), #f otherwise. Each provided flag-symbol must be a distinct flag supported by make-hash-table; if the second flag-symbol is redundant, the exn:fail:contract exception is raised.

(hash-table-put! hash-table key-v v) maps key-v to v in hash-table, overwriting any existing mapping for key-v. If hash-table is immutable, the exn:fail:contract exception is raised.

(hash-table-get hash-table key-v [failure-thunk]) returns the value for key-v in hash-table. If no value is found for key-v, then the result of invoking failure-thunk (a procedure of no arguments) is returned. If failure-thunk is not provided, the exn:fail:contract exception is raised when no value is found for key-v.

(hash-table-remove! hash-table key-v) removes the value mapping for key-v if it exists in hash-table. If hash-table is immutable, the exn:fail:contract exception is raised.

(hash-table-map hash-table proc) applies the procedure proc to each element in hash-table, accumulating the results into a list. The procedure proc must take two arguments: a key and its value. See the caveat below about concurrent modification.

(hash-table-for-each hash-table proc) applies the procedure proc to each element in hash-table (for the side-effects of proc) and returns void. The procedure proc must take two arguments: a key and its value. See the caveat below about concurrent modification.

(hash-table-count hash-table) returns the number of keys mapped by hash-table. If hash-table is not created with 'weak, then the result is computed in constant time and atomically. If hash-table is created with 'weak, see the caveat below about concurrent modification.

(hash-table-copy hash-table) returns a mutable hash table with the same mappings, same key-comparison mode, and same key-holding strength as hash-table.

(eq-hash-code v) returns an exact integer; for any two eq? values, the returned integer is the same. Furthermore, for the result integer k and any other exact integer j, (= k j) implies (eq? k j).

(equal-hash-code v) returns an exact integer; for any two equal? values, the returned integer is the same. Furthermore, for the result integer k and any other exact integer j, (= k j) implies (eq? k j). If v contains a cycle through pairs, vectors, boxes, and inspectable structure fields, then equal-hash-code applied to v will loop indefinitely.

Caveat concerning concurrent modification: A hash table can be manipulated with hash-table-get, hash-table-put!, and hash-table-remove! concurrently by multiple threads, and the operations are protected by a table-specific semaphore as needed. A few caveats apply, however:

• If a thread is terminated while applying hash-table-get, hash-table-put!, or hash-table-remove! to a hash table that uses equal? comparisons, all current and future operations on the hash table block indefinitely.

• The hash-table-map, hash-table-for-each, and hash-table-count procedures do not use the table's semaphore. Consequently, if a hash table is extended with new keys by another thread while a map, for-each, or count is in process, arbitrary key-value pairs can be dropped or duplicated in the map or for-each. Similarly, if a map or for-each procedure itself extends the table, arbitrary key-value pairs can be dropped or duplicated. However, key mappings can be deleted or remapped by any thread with no adverse affects (i.e., the change does not affect a traversal if the key has been seen already, otherwise the traversal skips a deleted key or uses the remapped key's new value).

Caveat concerning mutable keys: If a key into an equal?-based hash table is mutated (e.g., a key string is modified with string-set!), then the hash table's behavior for put and get operations becomes unpredictable.