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Improve documentation of letter-case conversions

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* doc/lispref/nonascii.texi (Character Properties):
* doc/lispref/strings.texi (Case Conversion, Case Tables):
Document that special-casing rules override the case-table
conversions.  (Bug#74155)
This commit is contained in:
Eli Zaretskii 2024-11-01 16:39:39 +02:00
parent 0f9d48e99c
commit f7b85fe986
2 changed files with 39 additions and 13 deletions
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21
doc/lispref/nonascii.texi
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@ -632,8 +632,10 @@ is @code{nil}, which means the character itself.
Corresponds to Unicode language- and context-independent special upper-casing
rules. The value of this property is a string (which may be empty). For
example mapping for U+00DF @sc{latin small letter sharp s} is
@code{"SS"}. For characters with no special mapping, the value is @code{nil}
which means @code{uppercase} property needs to be consulted instead.
@code{"SS"}. This mapping overrides the @code{uppercase} property, and
thus the current case table. For characters with no special mapping,
the value is @code{nil}, which means @code{uppercase} property needs to
be consulted instead.
@item special-lowercase
Corresponds to Unicode language- and context-independent special
@ -641,16 +643,19 @@ lower-casing rules. The value of this property is a string (which may
be empty). For example mapping for U+0130 @sc{latin capital letter i
with dot above} the value is @code{"i\u0307"} (i.e. 2-character string
consisting of @sc{latin small letter i} followed by U+0307
@sc{combining dot above}). For characters with no special mapping,
the value is @code{nil} which means @code{lowercase} property needs to
be consulted instead.
@sc{combining dot above}). This mapping overrides the @code{lowercase}
property, and thus the current case table. For characters with no
special mapping, the value is @code{nil}, which means @code{lowercase}
property needs to be consulted instead.
@item special-titlecase
Corresponds to Unicode unconditional special title-casing rules. The value of
this property is a string (which may be empty). For example mapping for
U+FB01 @sc{latin small ligature fi} the value is @code{"Fi"}. For
characters with no special mapping, the value is @code{nil} which means
@code{titlecase} property needs to be consulted instead.
U+FB01 @sc{latin small ligature fi} the value is @code{"Fi"}. This
mapping overrides the @code{titlecase} property, and thus the current
case table. For characters with no special mapping, the value is
@code{nil}, which means @code{titlecase} property needs to be consulted
instead.
@end table
@defun get-char-code-property char propname

31
doc/lispref/strings.texi
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@ -1591,9 +1591,12 @@ using @code{string} function, before being passed to one of the casing
functions. Of course, no assumptions on the length of the result may
be made.
Mapping for such special cases are taken from
@code{special-uppercase}, @code{special-lowercase} and
@code{special-titlecase} @xref{Character Properties}.
Other characters can also have special case-conversion rules. They
all have non-@code{nil} character properties @code{special-uppercase},
@code{special-lowercase} or @code{special-titlecase} (@pxref{Character
Properties}) defined by the Unicode Standard. These properties define
special case-conversion rules which override the current case table
(@pxref{Case Tables}).
@xref{Text Comparison}, for functions that compare strings; some of
them ignore case differences, or can optionally ignore case differences.
@ -1634,14 +1637,32 @@ correspondence. There may be two different lower case letters with the
same upper case equivalent. In these cases, you need to specify the
maps for both lower case and upper case.
The extra table @var{canonicalize} maps each character to a canonical
Some characters have special case-conversion rules defined for them,
which by default override the current case table. These characters have
non-@code{nil} character properties @code{special-uppercase},
@code{special-lowercase} or @code{special-titlecase} (@pxref{Character
Properties}) defined by the Unicode Standard. An example is U+00DF
LATIN SMALL LETTER SHARP S, @ss{}, which by default up-cases to the
string @code{"SS"}, not to U+1E9E LATIN CAPITAL LETTER SHARP S@. To
force these characters follow the case-table conversions, set the
corresponding Unicode property to @code{nil}:
@example
(upcase "@ss{}")
=> "SS"
(put-char-code-property ?@ss{} 'special-uppercase nil)
(upcase "@ss{}")
=> "ẞ"
@end example
The extra slot @var{canonicalize} of a case table maps each character to a canonical
equivalent; any two characters that are related by case-conversion have
the same canonical equivalent character. For example, since @samp{a}
and @samp{A} are related by case-conversion, they should have the same
canonical equivalent character (which should be either @samp{a} for both
of them, or @samp{A} for both of them).
The extra table @var{equivalences} is a map that cyclically permutes
The extra slot @var{equivalences} is a map that cyclically permutes
each equivalence class (of characters with the same canonical
equivalent). (For ordinary @acronym{ASCII}, this would map @samp{a} into
@samp{A} and @samp{A} into @samp{a}, and likewise for each set of