Rebinding a widget within a keymap

[Bart Schaefer <schaefer@brasslantern.com>]

The zsh bindkey and zle commands allow one to do most things one might
want to do with keymaps, such as copying one, setting bindings, etc.
However, there isn't a straightforward way to express transformations
such as "temporarily change all keys bound to 'self-insert' to instead
be bound to 'frob'".

This leads to workarounds such as

	zle -N self-insert frob
	zle recursive-edit
	zle -A .self-insert self-insert

This is unsatisfactory, as it's sometimes difficult to guarantee that
the "zle -A" step occurs; and if it doesn't, you're left frobbing in
a context where you should be inserting.  Furthermore, this fails if
self-insert was previously replaced by yet some other widget, so to get
that handled properly it's necessary to do

	zle -A self-insert saved-self-insert
	zle -N self-insert frob
	zle recursive-edit
	zle -A saved-self-insert self-insert
	zle -D saved-self-insert

This introduces possible name clashes on saved-self-insert; e.g., it
can't be invoked again from inside recursive-edit, and every added bit
of code to repair such a problem makes the unwinding more precarious.
Automatically saving and restoring keymaps with

	bindkey -N frobber main
	zle recursive-edit -K frobber

was meant to help with this, but without doing some sort of processing
on $(bindkey -L) it's not possible to determine which keys are already
bound to self-insert and thus need rebinding in frobber, and we're back
where we started.

Having given you all that background, I'm obviously about to propose a
solution.  Here it is.

  zmodload -i zsh/zleparameter

  for k in $keymaps
  do
    if (( $+widgets[self-insert-$k] == 0 ))
    then zle -A self-insert self-insert-$k
    fi
  done

  self-insert-by-keymap() {
    if (( $+widgets[$WIDGET-$KEYMAP] == 1 ))
    then zle $WIDGET-$KEYMAP "$@"
    else zle .$WIDGET "$@"
    fi
  }
  zle -N self-insert self-insert-by-keymap

This can of course be extended to any/all other widgets; just put a loop
"for w in ${(k)widgets}" around the whole thing and replace "self-insert"
with "$w" throughout.  (It's at that point that the lack of control over
autoremoval, which I've mentioned in other threads, becomes an issue, so
if you want to go beyond self-insert you'll have to write it out yourself.)

With this in place, you should rarely need "zle -N self-insert frob" again.
Instead you do this:

	bindkey -N frobber main
	zle -N self-insert-frobber frob

Then, whenever you wish to replace self-insert with frob, change keymaps:

	zle recursive-edit -K frobber

Here's a simple example, which improves upon the caps-lock example in the
zsh manual entry for recursive-edit:

  # Assumes self-insert-by-keymap installed as self-insert!

  self-insert-ucase() {
    LBUFFER+=${(U)KEYS[-1]}
  }
  zle -N self-insert-ucase
  caps-lock() {
    bindkey -N ucase $KEYMAP
    bindkey -M ucase "$KEYS" .accept-line
    zle recursive-edit -K ucase || zle send-break
  }
  zle -N caps-lock

To turn this on, pick a key sequence (I've chosen ctrl-x shift-L) and
bind the caps-lock widget to it:

  bindkey -M main '^XL' caps-lock

An obvious extension to this scheme is to create a variant of accept-line
that notifies the caller of recursive-edit as to whether it should treat
the end of the recursive-edit as final acceptance of the buffer, so that
it's possible e.g. to execute a command without first explicitly leaving
caps-lock "mode".

The other remaining drawback to this scheme is that it can't be employed
at the topmost level of ZLE, because the value of $KEYMAP is always "main"
(or "vicmd" as a special case) at that level.

Note that for builtin widgets the "for k in $keymaps" loop is extraneous;
so it would be possible to embed this widget-name-by-keymap logic in the
C code in ZLE, thereby making it possible to insert an "override" widget
into any specific keymap simply by creating a new widget with the keymap
name appended.  Old code that uses "zle -N builtin-widget user-function"
would continue to work, but would break new code that relies on the by-
keymap technique.

--