Re: Character folding in the pretest

[Eli Zaretskii <eliz@gnu.org>]

> From: Marcin Borkowski <mbork@mbork.pl>
> Date: Mon, 08 Feb 2016 15:05:05 +0100
> Cc: ofv@wanadoo.es, lokedhs@gmail.com, emacs-devel@gnu.org
> 
> Just as another datapoint in discussion: for me, searching for "l" and
> finding "ł" seems a bit weird.  (The opposite even more so.)

Which is why neither one happens under character folding.

> BTW, strangely enough, here isearching for "l" does /not/ find "ł", but
> isearching for "a" (with character folding on) finds "ą".  Whatever one
> thinks about char folding, this is clearly a bug.

It's not a bug, it's the feature working as designed: we only fold
characters that have suitable decompositions in the Unicode Character
Database.  So:

  (get-char-code-property ?ą 'decomposition) => (97 808)

but

  (get-char-code-property ?ł 'decomposition) => (322)

IOW, ą is canonically equivalent to the 2-character sequence a ̨ (which
is why searching for a finds that character), while ł has no canonical
decomposition (nor any other decomposition).

This means that the Unicode guys decided that ł should not be
equivalent to any other sequence of characters, and therefore Emacs
doesn't find it unless you search for it literally.

If you want to know why ł doesn't have any decompositions, I suggest
to ask on the Unicode mailing list, I'm sure they had good reasons,
most probably reasons that came from people who are experts in the
Polish language and its intricacies.  We just trust the results.