LispPad

Lightweight Scheme Development Tool for macOS

LispKit Char

Characters are objects that represent printed characters such as letters and digits. In LispKit, characters are UTF-16 code units.

Character literals are written using the notation #\character, or #\character-name, or #\xhex-scalar-value. Characters written using this #\ notation are self-evaluating, i.e. they do not have to be quoted.

The following standard character names are supported by LispKit:

  • #\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 space character U+0020
  • #\tab: the tab character U+0009

Here are some examples using the #\ notation:

  • #\m: lowercase letter 'm'
  • #\M: uppercase letter 'M'
  • #\(: left parenthesis ')'
  • #\\: backslash '\'
  • #\: space character ' '
  • #\x03BB: the lambda character 'λ' (equivalent to #\λ)

Case is significant in #\character, and in #\character-name, but not in #\xhex-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.

Some of the procedures that operate on characters ignore the difference between upper case and lower case. The procedures that ignore case have "-ci" (for "case insensitive") embedded in their names.


Predicates

(char? obj)     [procedure]

Returns #t if obj is a character, otherwise returns #f.

(char=? char ...)     [procedure]
(char<? char ...)
(char>? char ...)
(char<=? char ...)
(char>=? char ...)

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 transitive.

(char-ci=? char ...)     [procedure]
(char-ci<? char ...)
(char-ci>? char ...)
(char-ci<=? char ...)
(char-ci>=? char ...)

These procedures are similar to char=? etc., 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)     [procedure]

Procedure char-alphabetic? returns #t if its argument is a alphabetic character, otherwise it returns #f. Note that many Unicode characters are alphabetic but neither upper nor lower case.

(char-numeric? char)     [procedure]

Procedure char-numeric? returns #t if its argument is a numeric character, otherwise it returns #f.

(char-whitespace? char)     [procedure]

Procedure char-whitespace? returns #t if its argument is a whitespace character, otherwise it returns #f.

(char-upper-case? char)     [procedure]

Procedure char-upper-case?? returns #t if its argument is an upper-case character, otherwise it returns #f.

(char-lower-case? char)     [procedure]

Procedure char-lower-case? returns #t if its argument is a lower-case character, otherwise it returns #f.


Transforming characters

(char-upcase char)     [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 LispKit. Note that language-sensitive casing pairs are not used. If the argument is not the lowercase member of such a pair, it is returned.

(char-downcase char)     [procedure]

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 LispKit. If the argument is not the uppercase member of such a pair, it is returned.

(char-foldcase char)     [procedure]

The char-foldcase procedure applies the Unicode simple case-folding algorithm to its argument and returns the result. Note that language-sensitive folding is not used. If the argument is an uppercase letter, the result will be either a lowercase letter or the same as the argument if the lowercase letter does not exist or is not supported by LispKit.


Converting characters

(digit-value char)     [procedure]

Procedure digit-value 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 #\x0EA6)  ⇒  #f

(char->integer char)     [procedure]
(integer->char n)

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.

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