Hydra for Unicode input in emacs

Sean Miller recently wrote a blog entry on emacs input methods and mentioned the prefix key C-x 8. Almost any character can be printed out with the help from the prefix key (see M-x describe-bindings). Internally Emacs uses a character encoding that is a superset of Unicode, so any conceivable character can be used. The problem is to remember the key combinations to call them out.

I have used the abbrev mode to get some characters that not in my keyboard: ccc and space prints out "°C" and 8eur gives me "€".

Sean uses a custom function to get out a certain non-printing character (ZERO WIDTH SPACE), and points to a more generic solution in Sacha Chua's emacs config. That calls out Unicode characters by name from a built-in emacs list (C-h v ucs-names) using a macro. Unfortunately this does not solve the problem of how to remember the keys to call this macro with different Unicode name strings.

For me, the solution was to use the hydra emacs package that both binds keys in very flexible ways to functions and contains a documentation string that is displayed in the emacs echo area. For some reason, the macro did not work with hydra calls but rewriting it to a function fixed it.

(defun my/insert-unicode (unicode-name)
  "Same as: C-x 8 Enter UNICODE-NAME."
  (insert-char (cdr (assoc-string unicode-name (ucs-names)))))

(global-set-key
 (kbd "C-x 9")
 (defhydra hydra-unicode (:hint nil)
   "
   Unicode  _e_ €  _s_ ZERO WIDTH SPACE
            _f_ ♀  _o_ °
            _r_ ♂  _a_ →
   "
   ("e" (my/insert-unicode "EURO SIGN"))
   ("r" (my/insert-unicode "MALE SIGN"))
   ("f" (my/insert-unicode "FEMALE SIGN"))
   ("s" (my/insert-unicode "ZERO WIDTH SPACE"))
   ("o" (my/insert-unicode "DEGREE SIGN"))
   ("a" (my/insert-unicode "RIGHTWARDS ARROW"))))

Now pressing C-x 9 displays the list of keys to press next to the Unicode characters. The list is easy to modify and expand.

Since this the default "red" hydra, it remains active as long as a key that has not been mapped is pressed. Someone should expand this to remap the entire keyboard for mathematical input or to a language with a completely different script – all within emacs.