bug#7668: ispell and dictionary encodings

[Reuben Thomas]

I've just been puzzling my way through ispell.gz's dictionary encoding
code, after switching from aspell to hunspell in order to be able to
treat Unicode curly single quotes as normal intraword punctuation
(which it seems aspell cannot be persuaded to do, but that's another
story).

I noticed a feature of ispell-dictionary-base-alist, which I don't
understand: the last (7th) element of each dictionary definition is
called "Coding System", which seems to be the coding system of the
case character and non-case-character strings, but it is also passed
to the spelling program as the input encoding, which is wrong, since
the input encoding depends on the file to be checked.

I currently use the classic workaround of making up my own dictionary
definition which includes accented characters that I want to be able
to use in words (which is necessary anyway), and which specifies utf-8
as the coding system. This only works because I use utf-8 for all my
text files.

It seems, therefore, that the argument to follow
ispell-encoding8-command (which itself is mis-documented:

Command line option prefix to select UTF-8 if supported, nil otherwise.
If UTF-8 if supported by spellchecker and is selectable from the command line
this variable will contain \"--encoding=\" for aspell and \"-i \" for hunspell,
so UTF-8 or other mime charsets can be selected.  That will be set for hunspell
>=1.1.6 or aspell >= 0.60 in `ispell-check-version'.

It is not just for selecting UTF-8; indeed, that's the irony: in the
default configuration it's used mostly to select 8-bit character sets!
And there are one or two other typos. How about (suitably rewrapped):

Command line option prefix to select coding system if supported, nil otherwise.
If the coding system is selectable from the command line
this variable will contain \"--encoding=\" for aspell and \"-i \" for hunspell,
so that the input encoding can be selected.  That will be set for hunspell
>= 1.1.6 or aspell >= 0.60 in `ispell-check-version'.

Then, the following code in ispell-start-process:

    ;; If we are using recent aspell or hunspell, make sure we use the
right encoding
    ;; for communication. ispell or older aspell/hunspell does not support this
    (if ispell-encoding8-command
        (setq args
              (append args
                      (list
                       (concat ispell-encoding8-command
                               (symbol-name (ispell-get-coding-system)))))))

needs fixing: rather than using ispell-get-coding-system, it should
use a prefix of buffer-file-coding-system (without the suffix that
specifies the line ending).

I'm sure I'm missing things here, but if what I've said above makes
any sense, I'd like to help refine it into a sensible proposal to
improve ispell.el.

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