Re: Average-user-facing interface for tree-sitter

[Yuan Fu]

> On Oct 14, 2022, at 8:26 PM, Stefan Monnier <monnier@iro.umontreal.ca> wrote:
> 
>> (add-hook 'xxx-mode-hook
>>          (lambda () (when (eq major-mode 'xxx-mode)
>>                       (treesit-mode))))
>> 
>> Which again is a bit awkward.
> 
> Awkward but possible.  Another approach can be:
> 
>    (add-hook 'xxx-mode-hook #'treesit-mode)
>    (add-hook 'yyy-mode-hook (lambda () (treesit-mode -1)))

With the central variable approach, you can do (push ‘(xxx-mode t nil) 
treesit-settings), where t means enable, nil means don’t inherit.

> 
>> So maybe a central variable isn’t that bad an idea.  Could you layout
>> why mapping modes to some information is bad?
> 
> Because it needs to be able to say "for this mode and all its derived
> modes" but also "for this mode only" as well as various combinations,
> and then you need to document how it interacts with the major mode's
> hook, ...

Do you mind elaborate on: What are the other combinations? What are the ways a 
central variable can interact with major mode hooks? 

> Maybe we should devise a way to "centrally" control the value of some
> vars depending on major modes, but if so we should carefully design
> a thing specifically for that, make sure it's sufficiently flexible, and
> then use it for several (any?) variable.
> 
> All the vars I've seen so far which do that do it quite naively, which
> works OK for simple cases but breaks down one way or another when you
> start taking derived modes into account.  Hence my considering it a code
> smell (just like most uses of the `major-mode` variable).

How about a function mode-specific-value that takes any variable with the 
following shape:

((MODE VALUE INHERIT)…)

And returns the right VALUE for the current mode? INHERIT decides whether VALUE 
is inherited by MODE’s derived modes. Something like:

(defun mode-specific-value (central-var)
  (cl-loop for setting in central-var
           for mode = (nth 0 setting)
           for value = (nth 1 setting)
           for inherit = (nth 2 setting)
           if (and (derived-mode-p mode) inherit)
           return value
           finally return
           (if-let ((default (alist-get t central-var))) ; t sets the default 
value for all mode.
               (car default)
             nil)))

This function would handle derived mode ok, but I don’t know what are the other 
problems you are think of, maybe you can tell me what this function falls short 
for. And we can go from there.

Yuan