Using an expression to generate command

[Compro-Prasad]

When I do

(setf (alist-get 'black apheleia-formatters) '("/path/to/venv/bin/black" "-"))

it works. But I have different projects where I don't want to run black if its not installed in the virtualenv. So, I want to use a function to get the command dynamically per file and disable it where not available:

(defun get-command ()
  (when black-was-found-in-venv
    '("/path/to/venv/bin/black" "-")))

(setf (alist-get 'black apheleia-formatters) 'get-command)

Here, get-command function will return the command based on virtualenv and nil when executable wasn't found there. What is the best way to accomplish this?

[raxod502]

I think you would use something like this:

(setf (alist-get 'black apheleia-formatters) '((get-command)))

If the value is a symbol then Apheleia assumes it is a function that defines an Elisp-based formatter. You instead want a list to use a regular command formatter. If an element of the list is not a string then it is evaluated to get a string or list of strings. In this case the sole list element is the form (get-command). This wasn't documented before; I added documentation in 1bd5f2b.

You could also put the logic in an external shell script and add that to $PATH. Then you can invoke it normally from Apheleia.

[Compro-Prasad]

I get the following error when I do it:

error in process sentinel: apheleia--format-command: Command cannot start with (compro/black)
error in process sentinel: Command cannot start with (compro/black)

Code I am using:

(defun compro/black ()
  (when-let* ((root (pet-virtualenv-root))
              (executable (concat root "/bin/black"))
              (exists (file-exists-p executable)))
    executable))
(setf (alist-get 'black apheleia-formatters) '((compro/black)))

[raxod502]

Odd, I don't know why that error is even in there, although I added it. Fixed in the linked commit.

[Compro-Prasad]

Thanks, that worked. I am currently using:

(defun compro/black ()
  (when-let* ((root (pet-virtualenv-root))
              (executable (concat root "/bin/black"))
              (exists (file-exists-p executable)))
    `(,executable "-")))
(setf (alist-get 'black apheleia-formatters) '((compro/black)))

I am going to close this issue but I will ask for another small feature as its not fixed completely yet. I get the following error when black is not installed in the virtualenv:

error in process sentinel: Wrong type argument: stringp, nil

Instead of showing an error, either fail silently or show a message stating that black was enabled but function returned nil.

[raxod502]

Hmmm. I'm not necessarily sure it would be correct to handle the function returning nil. After all, the interface here is that you have (compro/black) as one of the arguments in the command line - thus it should return a string. What about returning "cat" when there is no formatting to be done? Then it would leave the file as is.

[Compro-Prasad]

What about returning "cat" when there is no formatting to be done?

Thanks! That does solve the problem.

But don't you think using cat is kind of a hack?

If this is okay then documenting it as an option would be a plus.

[raxod502]

I think it's reasonable to request a way that a dynamically generated formatter command could signal that no command is to be run. One reasonable way to implement that might be to have a special symbol noop that, if present in the command string, would cause the command invocation to be skipped.

Let's track that feature, if desired, in a separate issue.