What does `(fset SYMBOL KEYMAP)` do?

Question

I was reading through the projectile source file and they do the following:

(defvar projectile-command-map
  (let ((map (make-sparse-keymap)))
    ; define-keys
    map)
  "Keymap for Projectile commands after `projectile-keymap-prefix'.")
(fset 'projectile-command-map projectile-command-map)

Why do they do this? Is it for backwards compatibility?

Answer 1

fset sets a symbol's function definition.

Here, projectile-command-map is used as a prefix command. A prefix command is a symbol whose function definition is a keymap.

The definition of a prefix key is usually the keymap to use for looking up the following event. The definition can also be a Lisp symbol whose function definition is the following keymap; the effect is the same, but it provides a command name for the prefix key that can be used as a description of what the prefix key is for.

This can also be done with define-prefix-command.

What's a bit confusing here is that the same symbol, projectile-command-map, is being used for two purposes:

• As a keymap
• As a prefix command

Another example may be clearer:

(fset 'help-command help-map)

Here, help-command is clearly a prefix command, while help-map is a keymap.