Evaluate any non-string, non-special args in apheleia-formatters

[joshforged]

file, input, and output are special cases in apheleia-formatters that
are specially expanded when interpreted by apheleia-format-buffer.
This commit makes it so any other symbols will be evaluated.

[lassik]

I advise against this. This is potentially unsafe if the values come from .dir-locals.el.

[joshforged]

Yeah it's definitely generally unsafe -- but what could actually happen (I actually am not sure)? A user defines some nastiness in their own .dir-locals which gets evaluated? In general the dynamic nature of elisp means that any evaluated configuration code can do nasty things in certain circumstances, so I don't see how this is worse than anything else, as long as it's clear that the formatters alist is code.

[raxod502]

This is perfectly safe. The whole notion of safe file-local variables exists for exactly this reason, and Emacs will refuse to automatically apply a file-local setting for apheleia-formatters unless it is approved by the user. Besides, you could already (setf (alist-get prettier apheleia-formatters) '("sudo" "something-malicious")); this doesn't make things any worse.

The implementation looks fine; could you add a changelog entry and update the docstring?

[mohkale]

Is there anything I can do to help move this along? I'm running into a need for this with shfmt with regards to current indentation (tabs or space-width) and shell type (bash, sh, zsh, etc.)

Since #51 will conflict with this I could merge the functionality onto there. Thoughts?

Edit:

Okay I've implemented it here. Since I've implemented it on top of #51 I'll avoid creating a PR for it until that's merged. My implementation isn't too different from @joshforged except it can smartly handle when the result of an evaluation is a list of strings or nothing. This lets you have conditionals and multi-flag outputs from an evaluation. See the example in the README with shfmt.

[joshforged]

@mohkale's solution looks more comprehensive and I've been busy lately -- let's close this PR and use #51 instead.