Subject: bug#38235: string-foldcase bug for trailing sigma

From: John Cowan <[email-masked]>
Date: Sun, 17 Nov 2019 13:13:42 -0500

On Sat, Nov 16, 2019 at 3:42 PM Andy Wingo <[email-masked]> wrote:

> The expected result is "μέλοσ"; see R6RS libraries section 1.2.  However
> instead Guile's result is "μέλος".  Note that although Σ usually
> downcases to σ, at the end of a string it's ς.

More precisely, it downcases to σ if a letter follows and to ς if not
(being at the end of a string is a particular case).  However, this is not
actually always Greekly correct:  the string "ΦΙΛΟΣ." with a period at the
end downcases to "φιλος." if it is the word φίλος 'friend' (without its
proper accent) at the end of a sentence, but as "φιλος." if it is an
abbreviation for φιλοσοφία 'philosophy'.  For this reason, R7RS does not
require mapping to  ς in this situation as R6RS does.

This test shows a
> limitation of defining string-foldcase as simply (string-downcase
> (string-upcase str)).
>

As explained in Unicode section 5.18, the foldcase mappings (in <
https://www.unicode.org/Public/UNIDATA/CaseFolding.txt>, the lines with
status C and F) actually create a set of equivalence classes that are
closed under {upper,lower,title}case mapping, and then choose a single
character to represent each class.  This is usually the unique lowercase
character, but not always: in Cherokee it is the uppercase character, and
in the set {Σ, σ, ς} it is  σ.

On Sun, Nov 17, 2019 at 6:20 AM <[email-masked]> wrote:

Good catch. I think there's even a worse example: dotless
> and dotted I [1]. Here it seems even impossible to do
> up- and downcase correctly without knowing the language
> context.
>

Language-specific case mappings are explicitly out of Scheme's remit: they
have to be performed by specialized libraries.  There is an additional
situation in Lithuanian dictionaries (but not running text): an "i" with a
tone accent is represented as "i" + dot above + accent, like this:  "i̇́".
However, this dot above must be dropped when uppercasing, producing
ordinary "Í".