Re: Suggestion: Simple way to make conditional key bindings.

[storm@cua.dk (Kim F. Storm)]

"Stefan Monnier" <monnier+gnu/emacs@rum.cs.yale.edu> writes:

> > > You want to have a code.  That's what functions are for.  I think `eval'
> > > should generally be avoided, and `funcall' used instead.
> > > This is especially true if we care about lexical scoping.
> > 
> > I don't quite follow, but I take your word for it :-)
> 
> When you construct a function, the Elisp interpreter/compiler knows that
> it's a piece of code and can do all kinds of things with it (byte-compile
> it, analyze it and put out warnings, build a closure to remember the
> current lexical bindings, ...).
> OTOH, when you build a piece of code to be eval'd, the Elisp
> interpreter/compiler usually has no idea that this is code rather
> than just some kind of list of elements, so it can't do anything
> clever with it.

Ok, but for the uses I can envision for the `cond' binding, there's
probably not much cleverness that can be done with it...

> 
> > > One problem with your change is "what binding do we use when we don't want
> > > to run code?".  The `menu-item' syntax provides a binding (in the example
> > > above it's "my-filter" which is not very useful indeed) for the case
> > > where code should not be evalled (for example in `where-is').
> > 
> > Who said we don't want to run code :-)
> 
> The current code says so.

But the `cond' is equivalent to (and implemented using) `menu-item',
so what's the difference?

I know we don't want to run code in current_minor_maps, but that
isn't involved here AFAICS.

> 
> > > > It will then be quite trivial to enhance `define-key' to handle
> > > > conditional bindings:
> > > 
> > > But is it desirable ?
> > 
> > Don't really know...   It seems like a simple approach to allow
> > packages to hook into "standard bindings".
> 
> How about a set of examples where the current code does not provide
> sufficient flexibility ?  I can come up with plenty of examples
> where we could use your code, but for most/all of them we could
> just as well put the bindings in minor/major-mode-maps or text-property
> keymaps.

Yes, but then the "general functionality" of `menu-item' isn't needed either?

> > The point is that you can install a package - like table.el - which is
> > then able to install its own conditional binding on C-y *without*
> > interferring with (or even knowning) the standard binding.
> 
> I don't think that's right.  I don't believe that table wants to override
> C-y sometimes: instead it wants to override `yank' sometimes.
> So what you want to do is to create `yank-careful' and then use
> either the `remap' thingy or substitute-key-definition or something
> like that.

Yes, I agree with you.  Packages should override commands, not
specific bindings.


> > Then table.el would still be able to install its own conditional
> > binding on C-y.
> 
> Now that's a more interesting example.  But maybe that calls for
> replacing `yank-careful' with a (defadvice yank ...) or something
> like that.  I admit that I don't think either of yank-careful or
> defadvice are quite satisfactory for this case.
> But maybe, using `remap', we could check for an existing `remap'
> binding before adding our own, so that table.el would not defer
> to `yank' but to `yank-careful'.

For that, using the `cond' binding would still be useful I think.
E.g. something like this:

        (define-key global-map [remap yank] 'yank-with-properties
                '(table-p (car kill-ring)))


> 
> > > The advantage is that C-h k C-y doesn't just give you one of the two
> > > bindings but a docstring that describes both.  Of course we could also
> > > improve C-h k to recognize your `cond' construct, etc... but is it
> > > really worth the trouble ?
> > 
> > I didn't think about that, but it would be a nice way to report such
> > "multiple" bindings on a key...
> > 
> > But you may also consider this as a different approach than using a
> > minor-mode-keymap, and in that case, I think C-h k doesn't report all
> > possible bindings for a key -- only the "currently active" binding, so
> > why does `cond' have to behave differently?
> 
> It depends on whether the conditional that determines if the binding
> is active changes frequently or not (or implicitly or not).

Maybe this would be a desired difference between using `cond' and
minor mode bindings.  `cond' bindings report all possible functions,
while minor mode bindings show only the currently active binding.

-- 
Kim F. Storm <storm@cua.dk> http://www.cua.dk