Move comments down to back end with view toward new info key

Although we keep the racket-hash-lang-module-language-hook added in
the previous commit, for end users, we no longer use a default hook to
set comment-{start end padding}. Instead have the back end provide
those values. The goal is to have a new info key, e.g.
"drracket:comments", for langs to supply this. Meanwhile, any
fallbacks live down there (although a user could still use the hook to
add their own fallback or work-around).

For background on this commit and the previous commit see PR #661.

racket-hash-lang.el

@@ -54,6 +54,12 @@ re-tokenization has progressed sufficiently.")
 (defvar-local racket-hash-lang-module-language nil
   "The symbol for the module language, if any, else nil.")
 
+(defvar racket-hash-lang-module-language-hook nil
+  "Hook run when the module language changes.
+
+Your hook function may do additional customization based on the
+the variable `racket-hash-lang-module-language'.")
+
 ;;;###autoload
 (define-minor-mode racket-hash-lang-mode
   "Use color-lexer, indent, and navigation supplied by a #lang.
@@ -238,8 +244,6 @@ lang's attributes that we care about have changed."
       (widen)
       (racket--hash-lang-remove-text-properties (point-min) (point-max))
       (font-lock-flush (point-min) (point-max))
-      (setq racket-hash-lang-module-language (plist-get plist 'module-language))
-      (run-hooks 'racket-hash-lang-module-language-hook)
       ;; If the lang uses racket-grouping-position, i.e. it uses
       ;; s-expressions, then use racket-mode-syntax-table. That way
       ;; other Emacs features and packages are more likely to work.
@@ -269,7 +273,15 @@ lang's attributes that we care about have changed."
       (setq-local racket-hash-lang-mode-lighter
                   (concat " #lang"
                           (when (plist-get plist 'racket-grouping) "()")
-                          (when (plist-get plist 'range-indenter) "⇉"))))))
+                          (when (plist-get plist 'range-indenter) "⇉")))
+      (pcase-let ((`(,start ,end ,padding) (plist-get plist 'comments)))
+        (setq-local comment-start   start)
+        (setq-local comment-end     end)
+        (setq-local comment-padding padding))
+      ;; Finally run user's module language hooks.
+      (progn
+        (setq racket-hash-lang-module-language (plist-get plist 'module-language))
+        (run-hooks 'racket-hash-lang-module-language-hook)))))
 
 (defun racket--hash-lang-on-changed-tokens (_gen beg end)
   "The back end has processed a change that resulted in new tokens.
@@ -547,31 +559,6 @@ not a complete expression, in which case `newline-and-indent'."
       (racket-repl-submit prefix)
     (newline-and-indent)))
 
-(defun racket-hash-lang-module-language-hook-default ()
-  "A default value for hook variable `racket-hash-lang-module-language-hook'.
-
-Sets the variable `comment-start'. This is probably just a
-work-around until a lang info key for this is defined and
-implemented by various langs?"
-  (setq comment-start
-        (cl-case racket-hash-lang-module-language
-          ((racket
-            racket/base)
-           ";;")
-          ((rhombus)
-           "//")
-          ((scribble/base/lang
-            scribble/manual/lang)
-           "@;"))))
-
-(defvar racket-hash-lang-module-language-hook
-  (list #'racket-hash-lang-module-language-hook-default)
-  "Hook run when the module language changes.
-
-Your hook function may consult the variable
-`racket-hash-lang-module-language' and do additional
-customization based on the specific language.")
-
 (provide 'racket-hash-lang)
 
 ;; racket-hash-lang.el ends here

racket/hash-lang-bridge.rkt

@@ -46,7 +46,8 @@
                             'module-language (lang-info-module-language li)
                             'racket-grouping (lang-info-grouping-position-is-racket? li)
                             'range-indenter  (and (lang-info-range-indenter li) #t)
-                            'submit-predicate (and (lang-info-submit-predicate li) #t))))
+                            'submit-predicate (and (lang-info-submit-predicate li) #t)
+                            'comments (lang-info-comments li))))
       (define/override (on-changed-tokens gen beg end)
         (when (< beg end)
           (async-channel-put hash-lang-notify-channel

racket/hash-lang.rkt

@@ -671,23 +671,39 @@
              racket-grouping-position
              racket-amount-to-indent
              #f
+             #f
              #f))
 
 (define (read-lang-info* in)
   (define info (or (with-handlers ([values (λ _ #f)])
                      (read-language in (λ _ #f)))
                    (λ (_key default) default)))
   (define-values (_line _col end-pos) (port-next-location in))
-  (values (lang-info (info 'module-language default-module-language)
+  (define mod-lang (info 'module-language default-module-language))
+  (values (lang-info mod-lang
                      (info 'color-lexer default-lexer)
                      (info 'drracket:paren-matches default-paren-matches)
                      (info 'drracket:quote-matches default-quote-matches)
                      (info 'drracket:grouping-position racket-grouping-position)
                      (info 'drracket:indentation racket-amount-to-indent)
                      (info 'drracket:range-indentation #f)
-                     (info 'drracket:submit-predicate #f))
+                     (info 'drracket:submit-predicate #f)
+                     (or (info 'comments #f) ;; TODO: drracket:comments ?
+                         (comments-fallback mod-lang)))
           end-pos))
 
+;; Fallback when langs don't support a comments info key.
+(define (comments-fallback mod-lang-sym)
+  (define (root sym) ;e.g. 'racket and 'racket/base => racket
+    (match (and sym (symbol->string sym))
+      [(pregexp "^([^/]+)" (list _ str))
+       (string->symbol str)]
+      [_ #f]))
+  (case (root mod-lang-sym)
+    [(scribble) '("@;" "" " ")]
+    [(rhombus)  '("//" "" " ")]
+    [else       '(";;" "" " ")]))
+
 (define (read-lang-info in)
   (define-values (v _pos) (read-lang-info* in))
   v)

racket/lang-info.rkt

@@ -17,7 +17,8 @@
    grouping-position
    line-indenter
    range-indenter
-   submit-predicate)
+   submit-predicate
+   comments)
   #:transparent #:authentic)
 
 (define racket-grouping-position