Character folding issues with isearch (was: bug#22038: 25.1.50; ...)

[Stephen Berman <stephen.berman@gmx.net>]

On Sun, 29 Nov 2015 01:04:58 -0500 Richard Stallman <rms@gnu.org> wrote:
>
>   >							 maybe you are
>   > referring to a more subtle issue, which may be unrelated to my point,
>   > which is simply that I think it should be just as convenient for a user
>   > whose keyboard may lack "ß" or "æ" to match these characters by
>   > searching with "s" or "a" (or "e" or "ae") as it is to match "ff" by
>   > searching with "f".  This is not a language-specific issue AFAICS.
>
> When a character has a standard equivalent character series in a
> certain language, as "ß" does in German, that is a specific powerful
> argument for isearch to treat the two as equivalent.  But this
> argument doesn't apply in a a language-independent way.

I think I conflated two issues better treated separately.  What seems to
me really to be language-independent is being able to match characters
which as written obviously (i.e., whether you know the language or not)
are a combination of characters including the one(s) used in the search
string (whether or not such "compound" characters are Unicode composite
characters); e.g. using "a" (or "e" or "ae") to match "æ" (this includes
not just ligatures but also such composite characters as "℻").  Such
cases I think Emacs should ideally (i.e. WIBNI) handle by default.  In
contrast, I now agree that the case of "ß" really is language-specific,
since it is not obvious (unless you know German or the history of this
character) that it is a "compound" including "s".  Such cases could
ideally be handled by a language-specific setting or, as a workaround,
by customization.

Steve Berman