When the read-key-sequence function reads a key sequence
(see Key Sequence Input), it uses translation keymaps to
translate certain event sequences into others. The translation
keymaps are input-decode-map, local-function-key-map,
and key-translation-map (in order of priority).
Translation keymaps have the same structure as other keymaps, but are used differently: they specify translations to make while reading key sequences, rather than bindings for complete key sequences. As each key sequence is read, it is checked against each translation keymap. If one of the translation keymaps binds k to a vector v, then whenever k appears as a sub-sequence anywhere in a key sequence, that sub-sequence is replaced with the events in v.
For example, VT100 terminals send ESC O P when the
keypad key PF1 is pressed. On such terminals, Emacs must
translate that sequence of events into a single event pf1.
This is done by binding ESC O P to [pf1] in
input-decode-map. Thus, when you type C-c PF1 on
the terminal, the terminal emits the character sequence C-c
ESC O P, and read-key-sequence translates this back into
C-c PF1 and returns it as the vector [?\C-c pf1].
Translation keymaps take effect only after Emacs has decoded the
keyboard input (via the input coding system specified by
keyboard-coding-system). See Terminal I/O Encoding.
This variable holds a keymap that describes the character sequences sent by function keys on an ordinary character terminal.
The value of input-decode-map is usually set up automatically
according to the terminal’s Terminfo or Termcap entry, but sometimes
those need help from terminal-specific Lisp files. Emacs comes with
terminal-specific files for many common terminals; their main purpose is
to make entries in input-decode-map beyond those that can be
deduced from Termcap and Terminfo. See Terminal-Specific Initialization.
This variable holds a keymap similar to input-decode-map except
that it describes key sequences which should be translated to
alternative interpretations that are usually preferred. It applies
after input-decode-map and before key-translation-map.
Entries in local-function-key-map are ignored if they conflict
with bindings made in the minor mode, local, or global keymaps. I.e.,
the remapping only applies if the original key sequence would
otherwise not have any binding.
local-function-key-map inherits from function-key-map.
The latter should only be altered if you want the binding to apply in
all terminals, so using the former is almost always preferred.
This variable is another keymap used just like input-decode-map
to translate input events into other events. It differs from
input-decode-map in that it goes to work after
local-function-key-map is finished rather than before; it
receives the results of translation by local-function-key-map.
Just like input-decode-map, but unlike
local-function-key-map, this keymap is applied regardless of
whether the input key-sequence has a normal binding. Note however
that actual key bindings can have an effect on
key-translation-map, even though they are overridden by it.
Indeed, actual key bindings override local-function-key-map and
thus may alter the key sequence that key-translation-map
receives. Clearly, it is better to avoid this type of situation.
The intent of key-translation-map is for users to map one
character set to another, including ordinary characters normally bound
to self-insert-command.
You can use input-decode-map, local-function-key-map,
and key-translation-map for more than simple aliases, by using
a function, instead of a key sequence, as the translation of a
key. Then this function is called to compute the translation of that
key.
The key translation function receives one argument, which is the prompt
that was specified in read-key-sequence—or nil if the
key sequence is being read by the editor command loop. In most cases
you can ignore the prompt value.
If the function reads input itself, it can have the effect of altering the event that follows. For example, here’s how to define C-c h to turn the character that follows into a Hyper character:
(defun hyperify (prompt)
(let ((e (read-event)))
(vector (if (numberp e)
(logior (ash 1 24) e)
(if (memq 'hyper (event-modifiers e))
e
(add-event-modifier "H-" e))))))
(defun add-event-modifier (string e)
(let ((symbol (if (symbolp e) e (car e))))
(setq symbol (intern (concat string
(symbol-name symbol))))
(if (symbolp e)
symbol
(cons symbol (cdr e)))))
(keymap-set local-function-key-map "C-c h" 'hyperify)
The end of a key sequence is detected when that key sequence either is bound to a command, or when Emacs determines that no additional event can lead to a sequence that is bound to a command.
This means that, while input-decode-map and key-translation-map
apply regardless of whether the original key sequence would have a binding, the
presence of such a binding can still prevent translation from taking place.
For example, let us return to our VT100 example above and add a binding for
C-c ESC to the global map; now when the user hits C-c
PF1 Emacs will fail to decode C-c ESC O P into C-c
PF1 because it will stop reading keys right after C-c ESC,
leaving O P for later. This is in case the user really hit C-c
ESC, in which case Emacs should not sit there waiting for the next key
to decide whether the user really pressed ESC or PF1.
For that reason, it is better to avoid binding commands to key sequences where the end of the key sequence is a prefix of a key translation. The main such problematic suffixes/prefixes are ESC, M-O (which is really ESC O) and M-[ (which is really ESC [).