How to properly change defcustom prefix key

Question

In a library I use a prefix key defined like this:

(defcustom ensime-mode-key-prefix [?\C-c]
  "The prefix key for ensime-mode commands."
  :group 'ensime-mode
  :type 'sexp)

In my config file I've redefined it like the following:

(setq ensime-mode-key-prefix [?\C-c ?e])

Even though this has changed the prefix, all commands refuse to work with this error:

Symbol's function definition is void: ensime-mode-key-prefix

I've also tried to set it this way:

(customize-set-value 'ensime-mode-key-prefix [?\C-c ?e])

It also doesn't work, but with a different message - C-c e is undefined, which is strange cause in description it's set:

ensime-mode-key-prefix is a variable defined in `ensime-vars.el'.
Its value is [3 101]
Original value was [3]

I want to change the prefix to C-c e, tried to do that with std kbd, but that didn't help.

Answer 1

The problem is that by changing the value of ensime-mode-key-prefix you are doing just that and nothing else. Somewhere after the definition of this option there is code which uses the value. If you haven't changed it then the default value is used, else the value you picked.

But if you change the value after the library has already been loaded, then that code won't be run again and just changing the variable value won't trigger a change in ensime-mode-map (or whatever keymap is relevant here). Such a trigger has to be implemented explicitly using defcustom's :set property. And even then this would only have an effect when changing the value through the Custom interface. The :set function won't be run if you use setq.

Answer 2

Try customize-set-variable, not customize-set-value. The former uses the custom-set property, if any, for the option; the latter does not. grep for that variable in the source code, to see where it is handled. The defcustom apparently does not have a :set form, but there might be some code somewhere in the library that does handle setting the value.