On Sat, Aug 13, 2022, 6:51 AM Lynn Winebarger <owinebar@gmail.com> wrote:

> On Fri, Aug 12, 2022, 10:11 PM Stefan Monnier <monnier@iro.umontreal.ca>
> wrote:
>
>> > Once the VM supports proper tail recursion, it's straightforward to
>> > generate automata that never perform a function call, at least not as
>> part
>> > of the recognizer.
>>
>> It was straightforward beforehand as well (using a `while` loop instead
>> of recursion).  And if you do use recursion, then it's not very much
>> simpler with `lexical-binding` than without because you still have to
>> take into account the possibility that the function gets redefined
>> during your recursion :-(
>>
>
> I think you're mistaking self-tail recursion for tail recursion.  I mean
> proper tail recursion in Clinger's sense.  Any program written in CPS form
> will not "accumulate stack" (note the scare quotes, I know the details
> depend on implementation).  You can use a while loop with a trampoline to
> emulate it, sure, but that's not the same as having all control transfers
> take place by simple branching.   If you lift all the lambdas, there's no
> implicit memory allocation either.  I used to write code like that all the
> time - it's just "higher order assembly language".
>
> You're right about the hiccup introduced by having a "Lisp-2" without
> locally scoped function names.  That could be solved by introducing an
> explicit "function lambda" whose parameters provide lexically scoped
> function variables, or by simply using funcall to dispatch to closures on
> ordinary variables.  As long as the dispatch happens on locally scoped
> names, the compiler should be able to tell if they are constants.
>
I should have said "(eval-when-compile #'funcall)" in place of "funcall",
to guarantee the operator is a constant involving no runtime lookup of a
function symbol.



>
>> Don't get me wrong: `lexical-binding` is definitely very useful for
>> native compilation (and it does help for tail-calls in some cases,
>> e.g. in `named-let`), but I suspect that for the foreseeable future
>> it'll stay hard to be competitive with something like tree-sitter when
>> writing the code in ELisp
>
>
> This is Emacs.  Even if there was a new VM with these features, and a
> transpiler for porting existing ELC files, available today, I wouldn't be
> sure it would be integrated anytime soon.
> I just think the main barrier to introducing such improvements was,
> historically, the dynamic scoping in the massive lisp code base of Emacs.
> With that removed, I don't think Eli's skepticism is warranted.  This was
> all hashed out in the 1980s.
>
> Lynn
>
>
>
>