bug#49982: 27.2; ispell.el fails to find a Hunspell dictionary to use as default despite ispell-dictionary being set

[Eli Zaretskii <eliz@gnu.org>]

> Date: Wed, 11 Aug 2021 00:12:06 +0900
> From:  Kisaragi Hiu via "Bug reports for GNU Emacs,
>  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
> 
> This configuration should be everything that's needed for ispell.el to
> work with Hunspell, regardless of system locale:
> 
>      (setq ispell-program-name (executable-find "hunspell")
>            ispell-dictionary "en_US"))
> 
> However, when system locale (the LANG environment variable) does not 
> have a corresponding Hunspell dictionary, 
> `ispell-find-hunspell-dictionaries` returns the error "Can't find 
> Hunspell dictionary with a .aff affix file", despite ispell-dictionary 
> being set.
> 
> ispell.el relies on Hunspell to load a default and report it, but
> Hunspell just errors out if it can't find a dictionary for the system
> locale. And because ispell.el is trying to get Hunspell's default
> dictionary, it doesn't pass `ispell-dictionary' onto Hunspell.
> 
> This behavior is surprising. If `ispell-dictionary` is non-nil, that
> means the user has already specified their preferred dictionary, and it
> should not matter that Hunspell cannot find the dictionary it would use
> when a preferred dictionary isn't specified.
> 
> It's ispell.el that needs to be fixed here because the user specifies
> their preference in Emacs, and it is its job to communicate that
> preference to Hunspell.
> 
> `ispell-find-hunspell-dictionaries` should pass "-d
> ${ispell-dictionary}" to Hunspell if `ispell-dictionary` is set. This 
> invocation:
> 
>      hunspell -d "en_US" -D /dev/null
> 
> works as expected regardless of the system locale.

Thanks for the report and the analysis.

Frankly, I'm a bit wary of making the proposed change unconditionally.
First, yours is an unusual use case, I think: when Hunspell is
installed, the dictionary that corresponds to the locale is always
installed, because otherwise Hunspell will not work reliably from the
shell command line.  And second, relying on the non-nil value of
ispell-dictionary is fragile: the value could be a remnant from some
previous invocation or from an unsuccessful customization that has
nothing to do with the user's choice or his/her current intent.

Moreover, if you manually set ispell-dictionary, then what would be
the purpose of calling ispell-find-hunspell-dictionaries at all?

So maybe we should add a new user option that would force using the
value of ispell-dictionary right from the start.  That would at least
avoid the risk of breaking somebody else's use case.

I wonder if anyone else has an opinion about this.