Characters

Characters are objects that represent printed characters such as letters and digits. All Scheme implementations must support at least the ASCII character repertoire: that is, Unicode characters U+0000 through U+007F. Implementations may support any other Unicode characters they see fit, and may also support non-Unicode characters as well. Except as otherwise specified, the result of applying any of the following procedures to a non-Unicode character is implementation-dependent.

Characters are written using the notation #\{character} or #\{character name} or #\x{hex scalar value}.

The following character names are supported:

#\alarm ; U+0007 #\backspace ; U+0008 #\delete ; U+007F #\escape ; U+001B #\newline ; the linefeed character, U+000A #\null ; the null character, U+0000 #\return ; the return character, U+000D #\space ; the preferred way to write a space #\tab ; the tab character, U+0009

Here are some additional examples:

#\a ; lower case letter #\A ; upper case letter #\( ; left parenthesis #\ ; the space character

Case is significant in #{character}, and in #{character name}, but not in #\xhhex scalar value}. If {character} in #{character} is alphabetic, then any character immediately following {character} cannot be one that can appear in an identifier. This rule resolves the ambiguous case where, for example, the sequence of characters \#\space" could be taken to be either a representation of the space character or a representation of the character \#\s" followed by a representation of the symbol \pace."

Characters written in the #\ notation are self-evaluating. That is, they do not have to be quoted in programs. Some of the procedures that operate on characters ignore the difference between upper and lower case. The procedures that ignore case have "-ci" (for case insensitive) embedded in their names.

(char? obj ) procedure
Returns #t if obj is a character, otherwise returns #f.

(char=? char1 char2 char3 ... ) procedure
(char<? char1 char2 char3 ... ) procedure
(char>? char1 char2 char3 ... ) procedure
(char<=? char1 char2 char3 ... ) procedure
(char>=? char1 char2 char3 ... ) procedure
These procedures return #t if the results of passing their arguments to char->integer are respectively equal, monotonically increasing, monotonically decreasing, monotonically non-decreasing, or monotonically non-increasing. These predicates are required to be transitive.

(char-ci=? char1 char2 char3 ... ) char library procedure
(char-ci<? char1 char2 char3 ... ) char library procedure
(char-ci>? char1 char2 char3 ... ) char library procedure
(char-ci<=? char1 char2 char3 ... ) char library procedure
(char-ci>=? char1 char2 char3 ... ) char library procedure
These procedures are similar to char=? et cetera, but they treat upper case and lower case letters as the same. For example, (char-ci=? #\A #\a) returns #t.

Specifically, these procedures behave as if char-foldcase were applied to their arguments before they were compared.

(char-alphabetic? char ) char library procedure
(char-numeric? char ) char library procedure
(char-whitespace? char ) char library procedure
(char-upper-case? char ) char library procedure
(char-lower-case? char ) char library procedure
These procedures return #t if their arguments are alphabetic, numeric, whitespace, upper case, or lower case characters, respectively, otherwise they return #f.

Specifically, they must return #t when applied to characters with the Unicode properties Alphabetic, Numeric Digit, White Space, Uppercase, and Lowercase respectively, and #f when applied to any other Unicode characters. Note that many Unicode characters are alphabetic but neither upper nor lower case.

(digit-value char ) char library procedure
This procedure returns the numeric value (0 to 9) of its argument if it is a numeric digit (that is, if char-numeric? returns #t), or #f on any other character.
(digit-value #\3) ==> 3 (digit-value #\x0664) ==> 4 (digit-value #\x0AE6) ==> 0 (digit-value #\x0EA6) ==> #f

(char->integer char ) procedure
(integer->char n ) procedure
Given a Unicode character, char->integer returns an exact integer between 0 and #xD7FF or between #xE000 and #x10FFFF which is equal to the Unicode scalar value of that character. Given a non-Unicode character, it returns an exact integer greater than #x10FFFF. This is true independent of whether the implementation uses the Unicode representation internally.

Given an exact integer that is the value returned by a character when char->integer is applied to it, integer->char returns that character.

(char-upcase char ) char library procedure
(char-downcase char ) char library procedure
The char-upcase procedure, given an argument that is the lowercase part of a Unicode casing pair, returns the uppercase member of the pair, provided that both characters are supported by the Scheme implementation. Note that language-sensitive casing pairs are not used. If the argument is not the lowercase member of such a pair, it is returned.

The char-downcase procedure, given an argument that is the uppercase part of a Unicode casing pair, returns the lowercase member of the pair, provided that both characters are supported by the Scheme implementation. Note that language-sensitive casing pairs are not used. If the argument is not the uppercase member of such a pair, it is returned.

husk-scheme online documentation rev 3.19.3 (2016.07.10)