Stefan Monnier <monnier@iro.umontreal.ca> writes:

>>     (macroexp-let2 nil key
>>       `(progn
>>          (map--dispatch ,map
>>            :list (setf (alist-get ,key ,map nil t) nil)
>>            :hash-table (remhash ,key ,map)
>>            :array (and (>= ,key 0)
>>                        (<= ,key (seq-length ,map))
>>                        (aset ,map ,key nil)))
>>          ,map)))
>
> Note that this will make it pretty much impossible to use
> cl-generic dispatch.

Indeed, but I already have that issue.  To use `cl-generic', map.el has
to be basically rewritten from scratch.

> A better option might be to provide a map-remove which works
> functionally (i.e. doesn't modify its argument by side-effects)

I'd love to have that, but what about performance issues with
hash-tables?

hash-tables aren't persistent collections, and having `map-remove',
`map-add', etc. work functionally, the hash-table would have
to be copied for each operation.

Cheers,
Nico