nadvice.el: making add-function applicable to key-binding / using a function as generalized value

nadvice.el: making add-function applicable to key-binding / using a function as generalized value

[Pontus Michael]

First problem that I would like to address with this proposal, is the
inconvenience faced by the user attempting to alter default args for
existing command.

And secondly, wouldn't it be logical to expect following form to work?

    (add-function :filter-args
          (key-binding [?\M-\(])
          (lambda (args) '(1)))

If my understanding of how this kitchen works, it would only require
implementing gv api for `key-binding' for this to work and probably
`lookup-key' along with it.

My knowledge about involved technologies is very superficial, and my
hope is that someone capable will also see the benefit in it.

Re: nadvice.el: making add-function applicable to key-binding / using a function as generalized value

[Stefan Monnier]

[ Not sure exactly what is your proposal.  Is it only adding a gv-setter
  for key-binding, or is there more to it?  ]

>     (add-function :filter-args
>           (key-binding [?\M-\(])
>           (lambda (args) '(1)))

> If my understanding of how this kitchen works, it would only require
> implementing gv api for `key-binding' for this to work and probably
> `lookup-key' along with it.

Yes.  But note that a gv-setter for `key-binding' would have the
task to "make it so that (key-binding <foo>) returns <bar>", and the
problem is that there many different ways to do that: add a binding to
the local map, to a minor-mode map, somewhere in
emulation-mode-map-alists, etc...

For `lookup-key' the task should be a bit easier, but still fraught with
risks: (define-key MAP KEY (lookup-key MAP KEY)) is not always a no-op.
Examples of problems come up if KEY is bound to a `menu-item' (in which
case lookup-key will not return the menu-item structure but only the
binding within it), or if KEY is not directly bound within MAP but only
within a parent of MAP.

Further problems come up because `add-function' is designed to handle
generalized values that contain functions, and only functions.
Whereas keymaps contain many non-function values (the most common such
value is nil, but other examples would be vectors (i.e. keyboard
macros), menu entries, sub-keymaps, ...).

So all-in-all, I think making it work half-reliably will be difficult.


        Stefan