Pretty for loops

This reddit post posed a fun challenge: display for(auto&& foo : bar){...} with mathematical notation.

In python this is trivial with prettify-symbols-mode as its syntax is simply for foo in bar:.

/img/pretty-for-loops.png

Turns out is possible... whether it is a good idea or not is still questionable.

How it works

First we write a regex for the for-auto construct. Two groups are set, one for locating the for-all symbol and another for the in symbol. The rx macro makes this rather large regex a breeze.

(setq pretty-for-rgx
      (rx (group "for(auto&&")
          (1+ space)
          (1+ word)
          (1+ space)
          (group ":")
          (1+ space)
          (1+ word)))

Now we need to compose the symbols.

Ligatures in Emacs are typically set via prettyify-symbols-mode or manually through hacking compose-region into font-lock-mode syntax highlighting.

Personal ligatures are typically done through prettify, for instance replacing lambdas with the greek lambda symbol, or simple for/in loops in Python.

Ligatures from fonts, like Fira code, are typically done through ~compose-region~ calls.

We need to go the composition route here.

We write a hook updating font-lock-keywords with the composition regex.

(defun pretty-for-add ()
  (setq font-lock-keywords nil)
  (font-lock-add-keywords
   nil
   `((,pretty-for-rgx 0 (prog1 nil
                          (compose-region
                           (match-beginning 1) (match-end 1)
                           `(?\∀ (Br . Bl) ?\())
                          (compose-region
                           (match-beginning 2) (match-end 2)
                           ,(concat "\t" (list #x2203))))))))

(add-hook 'c-mode-hook 'pretty-for-add)

Notice that in for(auto&& ... we are replacing both the for and auto but keeping the parenthesis. The (Br . Bl) is a composition rule that allows attaching the for-all symbol and the parenthesis.

We set font lock keywords to nil because some keyword(s) are interfering with the compositions. If anyone has the interest to figure out which ones, please leave a comment here.

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