copy-tree on global-map

Question

The following code

(define-key (copy-tree global-map) (kbd "C-r") 'forward-char)

Immediately remaps C-r to forward-char. My understanding is that copy-tree will return a deepcopy of global-map. So why does editing the resulting keymap with define-key change my current global map?

And why are you even copying the keymap? Just what is it that you are really trying to do? — Drew, Jul 13 '16 at 22:47
an alternate global map, so I can build/design a different keyboard scheme without committing to it. Bind it to a minor mode, I suppose — Dodgie, Jul 13 '16 at 22:52

score 3 · Accepted Answer · answered Jul 14 '16 at 02:46

copy-tree only makes copies of the cons cells, not the arrays. global-map is a "dense" map, which implies it has an array (well, a char-table) inside where it stores the mapping for "simple char" bindings. And (kbd "C-r") corresponds to a simple char binding, so it gets stored in the array (which was not copied by copy-tree).

This said, you should basically never copy a keymap.

Instead just make your new keymap inherit from global-map, e.g. with (defvar my-new-map (make-composed-keymap nil global-map)). This map will start out identical to global-map but you can then modify it all you want without affecting global-map itself.

Oh, that's so much better. Thank you. – Dodgie Jul 14 '16 at 02:47 — Dodgie, Jul 14 '16 at 02:47

copy-tree on global-map

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