Re: The purpose of makunbound

[Kelly Dean <kelly@prtime.org>]

Stefan Monnier wrote:
>> To get rid of the ambiguity, I propose always using the term ‟void” for what
>> makunbound sets, and using the ‟bind/unbind” terminology exclusively for the
>> kind of binding that «let» does.
>
> Here's the problem: I don't see any difference between those two kinds
> of bindings and the implementation doesn't see them as different either.

The implementation does see them as different, and the difference is exposed to the Lisp world too. The difference is not dependent on the interpretation of whether «let» creates a new variable or temporarily changes the value of the global variable.

The difference is because makunbound works for let-bound and buffer-local variables. Regardless of the interpretation, there are two different kinds of ‟unbind” events: the one caused by exit of a «let» (which unshadows/restores the outer/previous value), and the one caused by makunbound (which sets the value to void). These are different in the implementation and different in the Lisp world (observable by doing makunbound on a let-bound or buffer-local variable, then exiting the «let» or killing the buffer-local), yet the same terminology is used for both.

I'm proposing a difference in terminology to account for those two different kinds of events.

In contrast, if makunbound didn't work for let-bound or buffer-local variables, then there wouldn't be two different kinds of events in the Lisp world (though there would still be in the implementation), because setting non-shadowing/not-temporarily-changed non-buffer-local variables to void is indistinguishable from unbinding them in the let-binding sense (because there's no shadowed/previous value to restore). And because there would be only one kind of event, there would be no need for a difference in terminology to avoid ambiguity.

>> Just to be clear, the only reason there's any ambiguity in the first place
>> is because makunbound operates on non-global variables.
>
> Dynamically-scoped vars are global.

In my message, I wrote that by ‟non-global” I meant ‟let-bound or buffer-local”, so I could use ‟non-global” and ‟global” as shorthand in the rest of the message. For your preferred interpretation of what let-binding does, just pretend I spelled out ‟let-bound-or-buffer-local” and ‟not-let-bound-and-not-buffer-local”.