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

[Stefan Monnier <monnier@iro.umontreal.ca>]

> Here’s my thought (that didn’t go anywhere): since major modes sets
> a plethora of local hooks and variables, only the major mode itself knows
> how to reverse them. The cleanest way is probably to clear all the local
> variables and hooks and re-run the major mode setup, which suggests we
> should let major mode branch on whether to enable tree-sitter during
> initialization. I wonder if minor modes can somehow work with this model?

Re-running is fairly problematic.  Not only because it risks repeating
side effects but also because it starts by killing all buffer-local
vars, so we'd need extra hacks to try and preserve the treesit-mode's
own information (making it permanent-local is one way, but that can
cause further breakage when the user really wants to change to another
mode, so it tends to be hackish).

> It would be also nice to leave room for inclusion of other “backends”
> besides elisp and tree-sitter in the future.

I'm not comfortable with this notion of "backend", because each one of
those "backends" (elisp, treesit, eglot, ...) tends to support
a different set of features, so in practice, I'd expect that in the
common case many major modes will use a mix of those backends.

A simple solution, tho not as elegant as I'd like, is to keep the code
we have (where the major mode sets all vars upfront) but add to the
major mode something like:

   (add-hook 'treesit-mode-hook #'js--treesit-mode-hook nil t)
   (js--treesit-mode-hook)

where `js--treesit-mode-hook` is in charge of removing those settings
that don't apply when `treesit-mode` is enabled` (and to re-instate
them when `treesit-mode` is disabled, which is why I call it right away
in the example above, so we don't duplicate the code between the major
mode's body and the `js--treesit-mode-hook`).

Sample completely untested patch below.

We could try and help write this code by providing a helper function
that relies on some buffer-local var containing a list of vars to be set
(along with their values), a list of hooks to add (and remove), ...
so we don't need to duplicate the list into a "set" and an "unset"
branch like I had to do in the patch.

Note that it's very similar to a "backend" function.  But it's only
meant to choose between "treesit activated" and "treesit not activated".


        Stefan


diff --git a/lisp/progmodes/js.el b/lisp/progmodes/js.el
index 52160fbb5ee..94295da5167 100644
--- a/lisp/progmodes/js.el
+++ b/lisp/progmodes/js.el
@@ -3617,14 +3617,8 @@ js-mode
               (list js--font-lock-keywords nil nil nil nil
                     '(font-lock-syntactic-face-function
                       . js-font-lock-syntactic-face-function)))
-  (setq-local syntax-propertize-function #'js-syntax-propertize)
-  (add-hook 'syntax-propertize-extend-region-functions
-            #'syntax-propertize-multiline 'append 'local)
-  (add-hook 'syntax-propertize-extend-region-functions
-            #'js--syntax-propertize-extend-region 'append 'local)
   (setq-local prettify-symbols-alist js--prettify-symbols-alist)
 
-  (setq-local parse-sexp-ignore-comments t)
   (setq-local which-func-imenu-joiner-function #'js--which-func-joiner)
 
   ;; Comments
@@ -3634,25 +3628,11 @@ js-mode
   (setq-local fill-paragraph-function #'js-fill-paragraph)
   (setq-local normal-auto-fill-function #'js-do-auto-fill)
 
-  ;; Parse cache
-  (add-hook 'before-change-functions #'js--flush-caches t t)
-
-  ;; Frameworks
-  (js--update-quick-match-re)
-
-  ;; Syntax extensions
-  (unless (js-jsx--detect-and-enable)
-    (add-hook 'after-change-functions #'js-jsx--detect-after-change nil t))
-  (js-use-syntactic-mode-name)
-
   ;; Imenu
   (setq imenu-case-fold-search nil)
   (setq imenu-create-index-function #'js--imenu-create-index)
 
   ;; for filling, pretend we're cc-mode
-  (c-foreign-init-lit-pos-cache)
-  (add-hook 'before-change-functions #'c-foreign-truncate-lit-pos-cache nil t)
-  (setq-local comment-line-break-function #'c-indent-new-comment-line)
   (setq-local comment-multi-line t)
   (setq-local electric-indent-chars
 	      (append "{}():;," electric-indent-chars)) ;FIXME: js2-mode adds "[]*".
@@ -3698,7 +3678,51 @@ js-mode
                       "function_declaration"
                       "lexical_declaration")))
   (setq-local treesit-font-lock-settings js--treesit-font-lock-settings)
-  (setq-local treesit-font-lock-feature-list '((minimal) (moderate) (full))))
+  (setq-local treesit-font-lock-feature-list '((minimal) (moderate) (full)))
+
+  (add-hook 'treesit-mode-hook #'js--treesit-mode-hook nil t)
+  (js--treesit-mode-hook))
+
+(defun js--treesit-mode-hook ()
+  (cond
+   (treesit-mode
+    (kill-local-variable 'syntax-propertize-function)
+    (remove-hook 'syntax-propertize-extend-region-functions
+                 #'syntax-propertize-multiline 'local)
+    (remove-hook 'syntax-propertize-extend-region-functions
+                 #'js--syntax-propertize-extend-region 'local)
+    (kill-local-variable 'parse-sexp-ignore-comments)
+    (remove-hook 'before-change-functions #'js--flush-caches t)
+
+    ;; Syntax extensions
+    (remove-hook 'after-change-functions #'js-jsx--detect-after-change t)
+    (js-use-syntactic-mode-name)        ;FIXME?
+
+    (remove-hook 'before-change-functions #'c-foreign-truncate-lit-pos-cache t)
+    (kill-local-variable 'comment-line-break-function)
+    )
+   (t
+    ;; Parse cache
+    (setq-local syntax-propertize-function #'js-syntax-propertize)
+    (add-hook 'syntax-propertize-extend-region-functions
+              #'syntax-propertize-multiline 'append 'local)
+    (add-hook 'syntax-propertize-extend-region-functions
+              #'js--syntax-propertize-extend-region 'append 'local)
+    (setq-local parse-sexp-ignore-comments t)
+    (add-hook 'before-change-functions #'js--flush-caches t t)
+
+    ;; Frameworks
+    (js--update-quick-match-re)
+
+    ;; Syntax extensions
+    (unless (js-jsx--detect-and-enable)
+      (add-hook 'after-change-functions #'js-jsx--detect-after-change nil t))
+    (js-use-syntactic-mode-name)
+
+    (c-foreign-init-lit-pos-cache)
+    (add-hook 'before-change-functions #'c-foreign-truncate-lit-pos-cache nil t)
+    (setq-local comment-line-break-function #'c-indent-new-comment-line)
+    )))
 
 (defvar js-json--treesit-font-lock-settings
   (treesit-font-lock-rules