WIP: Add customization point to retrieve next-checkers

[damienrg]

This PR introduce a customization point to retrieve next checkers.

In 0d250c9, the customization point is for a given checker based on new next-checkers-function property.

In 67ce045, the customization point is flycheck-next-checkers-function that is buffer local.

Related to:
#1660
#1762
emacs-lsp/lsp-mode#2329

[CLAassistant]

All committers have signed the CLA.

[wyuenho]

This is an interesting approach. Can you give a couple of examples of how one would use this to say, chain other checkers to lsp-mode's checker? I imagine python, JS and go devs will run into this issue the most.

[yyoncho]

@wyuenho you may wanna check - #1762 (comment)

[damienrg]

Can you give a couple of examples of how one would use this to say, chain other checkers to lsp-mode's checker? I imagine python, JS and go devs will run into this issue the most.

Example with lsp and the major-mode:

(defun my-flycheck-lsp-next-checkers (checker)
  (pcase major-mode
    ('sh-mode
     (pcase checker
       ('lsp '(sh-shellcheck sh-bash))
       ('sh-shellcheck '(sh-bash))
       ))
    ('web-mode '(javascript-eslint))
    )
  )

(defcustom my-lsp-flycheck-next-checkers-function nil
  "Variable to hold the function that returns flycheck next checkers for lsp."
  :type '(choice function (const nil)))

(make-variable-buffer-local 'my-lsp-flycheck-next-checkers-function)

;; in my case I only use this function for lsp checker
(defun my-lsp-flycheck-next-checkers (checker)
  "Function called by flycheck to find next checkers for CHECKER."
  (when (functionp my-lsp-flycheck-next-checkers-function)
    (funcall my-lsp-flycheck-next-checkers-function checker)))

(flycheck-set-next-checkers-function 'lsp #'my-lsp-flycheck-next-checkers)

(add-hook 'lsp-after-initialize-hook
          (lambda()
            (setq my-lsp-flycheck-next-checkers-function #'my-flycheck-lsp-next-checkers)))

[damienrg]

With the previous code to have (lsp foo bar), you would also have to do: (flycheck-set-next-checkers-function 'foo #'your-function) because of the way flycheck works.

With the new code, you should be able to do :

(defun my-flycheck-lsp-next-checkers (checker)
  (pcase major-mode
    ('sh-mode
     (pcase checker
       ('lsp '(sh-shellcheck sh-bash))
       ('sh-shellcheck '(sh-bash))
       ))
    ('web-mode '(javascript-eslint))
    )
  )
  
  (add-hook 'lsp-after-initialize-hook
          (lambda()
            (setq flycheck-next-checkers-function #'my-flycheck-lsp-next-checkers)))

Anyway, I do not think this PR will land and you can use an advice as @yyoncho mentioned.

[wyuenho]

Why won't this land? Buffer local checker chain sounds like a great idea to me.

[cpitclaudel]

Why won't this land? Buffer local checker chain sounds like a great idea to me.

Because none of the maintainers have found the time to review the code, unfortunately :/ I'm on the job market atm and finishing a PhD, so not much time for free software.

Additionally, what this PR does can already be achieved with advice; yyoncho posted sample code for that in his previous comment. With that and the buffer-local obarray options, there's a family of possible design choices that need to be evaluated.

@damienrg , thanks a lot for the PR. Have you had a look at the advice-based solution? Is there a reason to prefer this patch over that? Your example use case would work the same, just with advice-add instead of (setq flycheck-next-checkers-function. WDYT?

[damienrg]

wyuenho:
Why won't this land? Buffer local checker chain sounds like a great idea to me.

I have opened this PR to have feedback about the solution I propose to chain checkers and it is maybe not the right way to do it.

cpitclaudel:
@damienrg , thanks a lot for the PR. Have you had a look at the advice-based solution? Is there a reason to prefer this patch over that? Your example use case would work the same, just with advice-add instead of (setq flycheck-next-checkers-function. WDYT?

Yes it works the same way. The main difference is that advice-based solution means it is not an official feature of flycheck.

From a user point of view, I would like to be able to do either:

1. (setq flycheck-use-checkers '(foo bar baz))
2. (setq flycheck-use-checkers-function #'my-fun) where my-fun returns a list of checkers
3. (flycheck-use-checkers '(foo bar baz)) that automatically creates the correct chain.

With that and the buffer-local obarray options, there's a family of possible design choices that need to be evaluated.

I do not believe that changing properties of a checker is the way to go because the user has to change the properties on all the next checkers or flycheck should provide (flycheck-use-checkers '(foo bar baz)).