Add hook for module language to racket-hash-lang-mode

This is roughly in the same spirit as PR #661, but using the
module-language key supported by new lang's info function.

Also this defines a hook as the means for users to customize. A
default hook function sets comment-start for a few popular
langs (although ultimately comment-start should become a new
"official" lang info key).

racket-hash-lang.el

@@ -34,7 +34,7 @@ until the back end has handled the update commands and also
 re-tokenization has progressed sufficiently.")
 
 (defvar-local racket--hash-lang-changed-vars nil
-  "Where we save spec to restore original values.")
+  "Original values when minor mode was enabled, to restore when disabled.")
 
 (defvar racket-hash-lang-mode-map
   (racket--easy-keymap-define
@@ -51,6 +51,9 @@ re-tokenization has progressed sufficiently.")
 
 (defvar-local racket--hash-lang-submit-predicate-p nil)
 
+(defvar-local racket-hash-lang-module-language nil
+  "The symbol for the module language, if any, else nil.")
+
 ;;;###autoload
 (define-minor-mode racket-hash-lang-mode
   "Use color-lexer, indent, and navigation supplied by a #lang.
@@ -70,6 +73,9 @@ For `racket-repl-mode' buffers, be aware that only input portions
 of the buffer use coloring/indent/navigation from the hash-lang.
 Output portions are treated as whitespace.
 
+Runs the hook variable `racket-hash-lang-module-language-hook'
+when the module language changes.
+
 \\{racket-hash-lang-mode-map}
 "
   :lighter racket-hash-lang-mode-lighter
@@ -225,13 +231,15 @@ not. Intended as a convenience so users needn't set a
 We do /not/ get notified when a new lang uses exactly the same
 attributes as the old one. For example changing from #lang racket
 to #lang racket/base will /not/ notify us, because none of the
-lang's attributes that care about have changed."
+lang's attributes that we care about have changed."
   ;;;(message "racket--hash-lang-on-new-lang %s" plist)
   (with-silent-modifications
     (save-restriction
       (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.
@@ -539,6 +547,31 @@ 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

@@ -43,14 +43,16 @@
                            (list
                             'hash-lang id
                             'lang
+                            '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))))
       (define/override (on-changed-tokens gen beg end)
         (when (< beg end)
           (async-channel-put hash-lang-notify-channel
                              (list 'hash-lang id
-                                   'update gen (add1 beg) (add1 end))))))))
+                                   'update
+                                   gen (add1 beg) (add1 end))))))))
 
 (define (hash-lang . args)
   (unless (class? hash-lang-class-or-error-message)

racket/hash-lang.rkt

@@ -658,12 +658,14 @@
     (define/public (get-regions)
       '((0 end)))))
 
+(define default-module-language #f)
 (define default-lexer (waive-option module-lexer*))
 (define default-paren-matches '((\( \)) (\[ \]) (\{ \})))
 (define default-quote-matches '(#\" #\|))
 
 (define default-lang-info
-  (lang-info default-lexer
+  (lang-info default-module-language
+             default-lexer
              default-paren-matches
              default-quote-matches
              racket-grouping-position
@@ -676,7 +678,8 @@
                      (read-language in (λ _ #f)))
                    (λ (_key default) default)))
   (define-values (_line _col end-pos) (port-next-location in))
-  (values (lang-info (info 'color-lexer default-lexer)
+  (values (lang-info (info 'module-language default-module-language)
+                     (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)

racket/lang-info.rkt

@@ -10,7 +10,8 @@
 ;; require it normally and not need to do more dynamic-requires.
 
 (struct lang-info
-  (lexer
+  (module-language
+   lexer
    paren-matches
    quote-matches
    grouping-position