Re: Distinguishing `consp` and `functionp`

[Stefan Monnier]

>> Yup.  My impression is that this is sufficiently rare that we can afford
>> to break it.  Maybe experience will show me wrong, of course.
>
> I've definitely seen users post configs to Eglot's bug tracker where
> lambdas appear incorrectly quoted.  i.e.
>
>   (add-to-list 'eglot-server-programs '(foo-mode . (lambda (..) ...)))

Yes, that's very common and that's why I don't think we can drop support
for that in `funcall`.  What I hope is less common is for those cases
to go through `functionp`.

Of course, it also depends on happenstance of how the `functionp` test
is written.  E.g. for `eglot-server-programs` it seems that we
will/would suffer because we start by testing `functionp` instead of
first handling the non-function-like lists.

>> We could also consider an intermediate step where `functionp` returns
>> t but emits a warning.
> Indeed, though in that case I'd make the funcall warn. I think it's there
> that this proposed runtime warning  ultimately matters and is useful
> to help users correct their elisp. Runtime warnings are a bit icky though
> :-| but better than nothing.

We already have compile time warnings at those places where the compiler
easily knows that the list should be a function, but for all those other
cases (like `eglot-server-programs`), we don't have any tool currently
other than run-time warnings.

>> - it can give a wrong impression to a beginner, encouraging confusion.
>> - it can occasionally hide an error, making debugging a bit more difficult.
> Wouldn't you add "complicates type propagation, static analysis and
> optimization" to that list?

No.  All these can be blissfully unaware of what `funcall` does when
presented with a list starting with `lambda`.  In theory, it's true that
analysis&optimization could assume that *after* the `funcall` the
argument was a valid function obeying `functionp`, but I suspect our
analysis/optimization technology is pretty far from that level, and even
if we were able to make that work, the potential for gains from such
information seems vanishingly small.


        Stefan