76 lines
2.5 KiB
Racket
76 lines
2.5 KiB
Racket
#lang scribble/doc
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@(require scribble/manual
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scribble/eval
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"guide-utils.ss")
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@title[#:tag "characters"]{Characters}
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A Scheme @deftech{character} corresponds to a Unicode @defterm{scalar
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value}. Roughly, a scalar value is an unsigned integer whose
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representation fits into 21 bits, and that maps to some notion of a
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natural-language character or piece of a character. Technically, a
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scalar value is a simpler notion than the concept called a
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``character'' in the Unicode standard, but it's an approximation that
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works well for many purposes. For example, any accented Roman letter
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can be represented as a scalar value, as can any Chinese character.
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Although each Scheme character corresponds to an integer, the
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character datatype is separate from numbers. The
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@scheme[char->integer] and @scheme[integer->char] procedures convert
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between scalar-value numbers and the corresponding character.
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A printable character normally prints as @litchar["#\\"] followed
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by the represented character. An unprintable character normally prints
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as @litchar{#\u} followed by the scalar value as hexadecimal
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number. A few characters are printed specially; for example, the space
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and linefeed characters print as @scheme[#\space] and
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@scheme[#\newline], respectively.
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@refdetails/gory["parse-character"]{the syntax of characters}
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@examples[
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(integer->char 65)
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(char->integer #\A)
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#\u03BB
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(eval:alts #, @schemevalfont["#\\u03BB"] #\u03BB)
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(integer->char 17)
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(char->integer #\space)
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]
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The @scheme[display] procedure directly writes a character to the
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current output port (see @secref["i/o"]), in contrast to the
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character-constant syntax used to print a character result.
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@examples[
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#\A
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(display #\A)
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]
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Scheme provides several classification and conversion procedures on
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characters. Beware, however, that conversions on some Unicode
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characters work as a human would expect only when they are in a string
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(e.g., upcasing ``@elem["\uDF"]'' or downcasing ``@elem["\u03A3"]'').
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@examples[
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(char-alphabetic? #\A)
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(char-numeric? #\0)
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(char-whitespace? #\newline)
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(char-downcase #\A)
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(char-upcase #\uDF)
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]
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The @scheme[char=?] procedure compares two or more characters, and
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@scheme[char-ci=?] compares characters ignoring case. The
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@scheme[eqv?] and @scheme[equal?] procedures behave the same as
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@scheme[char=?] on characters; use @scheme[char=?] when you want to
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more specifically declare that the values being compared are
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characters.
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@examples[
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(char=? #\a #\A)
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(char-ci=? #\a #\A)
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(eqv? #\a #\A)
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]
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@refdetails["characters"]{characters and character procedures}
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