Inserting the plus minus unicode symbol ± as a keyboard macro

Question

I insert the plus minus symbol ± frequently enough that I want to bind it to a key.

From emacs -Q, I've tried this:

(global-set-key (kbd "C-c m") "±")

However, when I enter C-c m, the minibuffer displays C-u 1-, suggesting it's waiting for me to finish entering a sequence.

I can bind C-c m to other symbols:

(global-set-key (kbd "C-c m") "⇔")

This works as expected: C-c m inserts the double-arrow ⇔.

I've tried other keys as well, e.g.

(global-set-key (kbd "C-c a") "±")

This gives the same result: emacs interprets it as C-u 1-.

Is there something special about the plus-minus glyph that makes Emacs intepret it as a prefix argument? Why can't I insert it as I would other characters?

I have solved the immediate problem by defining an abbrev to insert the plus-minus glyph, but I would like to know why the keybindings above don't work.

Update:

Inserting the character via (insert ) does work:

(global-set-key (kbd "C-c m") (lambda () (interactive) (insert "±")))

Answer 1

With respect to why you saw what you saw:

As Eli Zaretskii said in a reply to bug report #33829, the string is interpreted as a keyboard macro, and Emacs "interprets ±, which is a single byte with the 8th bit set, as a meta character".

It's interpreted as M-1, which is shown in the echo area as C-u 1-, as the beginning of a numeric prefix arg.

The mistake/gotcha is to expect that all bytes in a keyboard-macro string are interpreted as characters (which are just inserted). This is a byte that is interpreted as a key bound to a command. It just happens to be renderable also as a Unicode character, ±.

In the context of key-binding to a (keyboard-macro) string, as in the example -- especially since that is easily and commonly used to insert a Unicode character (or any char that is not so easy to type), it's easy to forget that keyboard-macro strings are command sequences.

Answer 2

You can get what you want with

(global-set-key (kbd "C-c m") [177])

or if you prefer

(global-set-key (kbd "C-c m") [?±])

Another solution uses the package key-chord which allows you to assign commands to keys pressed simultaneously. After you have installed it, you can define

(key-chord-define-global "-=" [177])

and then pressing "-" and "=" at the same time does what you want.

Using vectors to represent unicode characters seems to be more reliable than keyboard macros. Emacs calls self-insert-command on the character C when it sees the key [C] where C is the character (the decimal repersentation) of the unicode character you want.