Ligatures with the JetBrains Mono font

Question

After installing the JetBrains Mono Font and setting it as my default face. I am wandering how to enable Ligatures in Emacs.

I tried a solution based on this answer that show how to do this with Fira Code, with what I think are the required modifications I am speaking about:

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EDIT: @Piquan's answer, suggested to me that I wasn't clear about what are the required modifications to the fira-code-mode, so here they are:

• Search and replace from fira-code to jetbrains-mono
• Change the code points from #Xe100 . #Xe16f to #X10001 . #X1009c
• Update the jetbrains-ligature-mode--ligatures constant to reflect the actual character map of the JetBrains Mono font as I see them (using FontForge).
• Since the #X10001 . #X1009c contains some diacritics I didn't wish to be part of the resulting prettify-symbols-alist, I put some nils into the said constant, and introduced an (if ...) expression to the jetbrains-ligature-mode--make-alist function to ignore those nils and just increment the code point.
• AFAIK, the reason the Fira Code Ligature was created for Emacs compatibility, is to be compatible with the upstream haskling-mode that used those code points. But since I modified the numbers to be compatible with the JetBrains Mono original font, this is not needed.
• I should be able to do without jetbrains-ligature-mode--setup function, but I figured out it shouldn't hurt to keep it (and indeed removing it as a troubleshooting step didn't help).

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Thus the resulting code is:

(defun jetbrains-ligature-mode--make-alist (list)
   "Generate prettify-symbols alist from LIST."
   (let ((idx -1))
     (mapcar
      (lambda (s)
        (setq idx (1+ idx))
        (if s
            (let* ((code (+ #X10001 idx))
                   (width (string-width s))
                   (prefix ())
                   (suffix '(?\s (Br . Br)))
                   (n 1))
              (while (< n width)
                (setq prefix (append prefix '(?\s (Br . Bl))))
                (setq n (1+ n)))
              (cons s (append prefix suffix (list (decode-char 'ucs code)))))))
      list)))

 (defconst jetbrains-ligature-mode--ligatures
   '("-->" "//" "/**" "/*" "*/" "<!--" ":=" "->>" "<<-" "->" "<-"
     "<=>" "==" "!=" "<=" ">=" "=:=" "!==" "&&" "||" "..." ".."
     nil nil nil nil nil nil nil nil nil nil nil nil nil nil
     "|||" "///" "&&&" "===" "++" "--" "=>" "|>" "<|" "||>" "<||"
     "|||>" "<|||" ">>" "<<" nil nil "::=" "|]" "[|" "{|" "|}"
     "[<" ">]" ":?>" ":?" nil "/=" "[||]" "!!" "?:" "?." "::"
     "+++" "??" "###" "##" ":::" "####" ".?" "?=" "=!=" "<|>"
     "<:" ":<" ":>" ">:" "<>" "***" ";;" "/==" ".=" ".-" "__"
     "=/=" "<-<" "<<<" ">>>" "<=<" "<<=" "<==" "<==>" "==>" "=>>"
     ">=>" ">>=" ">>-" ">-" "<~>" "-<" "-<<" "=<<" "---" "<-|"
     "<=|" "/\\" "\\/" "|=>" "|~>" "<~~" "<~" "~~" "~~>" "~>"
     "<$>" "<$" "$>" "<+>" "<+" "+>" "<*>" "<*" "*>" "</>" "</" "/>"
     "<->" "..<" "~=" "~-" "-~" "~@" "^=" "-|" "_|_" "|-" "||-"
     "|=" "||=" "#{" "#[" "]#" "#(" "#?" "#_" "#_(" "#:" "#!" "#="
     "&="))

(defvar jetbrains-ligature-mode--old-prettify-alist)

(defun jetbrains-ligature-mode--enable ()
    "Enable JetBrains Mono ligatures in current buffer."
    (setq-local jetbrains-ligature-mode--old-prettify-alist prettify-symbols-alist)
       (setq-local prettify-symbols-alist (append (jetbrains-ligature-mode--make-alist jetbrains-ligature-mode--ligatures) jetbrains-ligature-mode--old-prettify-alist))
       (prettify-symbols-mode t))

(defun jetbrains-ligature-mode--disable ()
    "Disable JetBrains Mono ligatures in current buffer."
    (setq-local prettify-symbols-alist jetbrains-ligature-mode--old-prettify-alist)
    (prettify-symbols-mode -1))

(define-minor-mode jetbrains-ligature-mode
    "JetBrains Mono ligatures minor mode"
    :lighter " JetBrains Mono"
    (setq-local prettify-symbols-unprettify-at-point 'right-edge)
    (if jetbrains-ligature-mode
        (jetbrains-ligature-mode--enable)
      (jetbrains-ligature-mode--disable)))

(defun jetbrains-ligature-mode--setup ()
    "Setup JetBrains Mono Symbols"
    (set-fontset-font t '(#X10001 . #X1009c) "JetBrains Mono"))

(provide 'jetbrains-ligature-mode)

However the best I got so far is that the sequence is replaced with an empty space (although with the correct length).

What am I missing?

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I am using Emacs 26.3 (from Kevin Kelley's PPA) on Kubuntu 18.04

Answer 1

You want to do this with composition rules, not with prettify-symbols. Try this instead:

(defconst jetbrains-ligature-mode--ligatures
   '("-->" "//" "/**" "/*" "*/" "<!--" ":=" "->>" "<<-" "->" "<-"
     "<=>" "==" "!=" "<=" ">=" "=:=" "!==" "&&" "||" "..." ".."
     "|||" "///" "&&&" "===" "++" "--" "=>" "|>" "<|" "||>" "<||"
     "|||>" "<|||" ">>" "<<" "::=" "|]" "[|" "{|" "|}"
     "[<" ">]" ":?>" ":?" "/=" "[||]" "!!" "?:" "?." "::"
     "+++" "??" "###" "##" ":::" "####" ".?" "?=" "=!=" "<|>"
     "<:" ":<" ":>" ">:" "<>" "***" ";;" "/==" ".=" ".-" "__"
     "=/=" "<-<" "<<<" ">>>" "<=<" "<<=" "<==" "<==>" "==>" "=>>"
     ">=>" ">>=" ">>-" ">-" "<~>" "-<" "-<<" "=<<" "---" "<-|"
     "<=|" "/\\" "\\/" "|=>" "|~>" "<~~" "<~" "~~" "~~>" "~>"
     "<$>" "<$" "$>" "<+>" "<+" "+>" "<*>" "<*" "*>" "</>" "</" "/>"
     "<->" "..<" "~=" "~-" "-~" "~@" "^=" "-|" "_|_" "|-" "||-"
     "|=" "||=" "#{" "#[" "]#" "#(" "#?" "#_" "#_(" "#:" "#!" "#="
     "&="))

(sort jetbrains-ligature-mode--ligatures (lambda (x y) (> (length x) (length y))))

(dolist (pat jetbrains-ligature-mode--ligatures)
  (set-char-table-range composition-function-table
                      (aref pat 0)
                      (nconc (char-table-range composition-function-table (aref pat 0))
                             (list (vector (regexp-quote pat)
                                           0
                                    'compose-gstring-for-graphic)))))

In order to improve the performance of redisplay, you could take all the ligature patterns that start with the same character, and use regexp-opt to produce one regular expression matching all of them, ie use

(regexp-opt '("###" "## "####" "#{" "#[" "#(" "#?" "#_" "#_(" "#:" "#!" "#="))

as the pattern for #.

Answer 2

Per the upstream Fira Code instructions, you have to install the Fira Code Symbol font to use the mechanism being used here. The Fira Code Symbol font is a separate font that has the Fira Code ligatures, but puts them as regular characters in the font so that Emacs can access them. (You may note that the original Fira Code example referred to "Fira Code Symbol" rather than "Fira Code" in the set-fontset-font call.) Without that font, this mechanism won't work.

Unfortunately, I don't know of an analogous symbol font for JetBrains Mono yet, so this solution wouldn't apply. Of course, JetBrains Mono is quite new, so somebody may make one soon!

If somebody does create one, you'll need to make sure that the constant base that you use (#X10001 in your post), and the order of the elements of jetbrains-ligature-mode--ligatures, both match the (theoretical future) JetBrains Mono Symbol font.