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From: Philipp Stephani
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Subject: bug#41988: 28.0.50;
Edebug unconditionally instruments definitions with &define specs
Date: Mon, 5 Apr 2021 16:32:16 +0200
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To: Stefan Monnier
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Am So., 4. Apr. 2021 um 22:16 Uhr schrieb Stefan Monnier
:
>
> >> [ Disclaimer: I don't understand the precise semantics of `gate`, tho
> >> I do remember using it once via trial-and-error. So maybe it wouldn't
> >> prevent it, but if doesn't prevent it, then it doesn't likely "fix"
> >> our problem ;-) ]
> > AIUI the semantics of "gate" aren't that complex, it just means "don't
> > backtrack beyond this point."
>
> [ Yes, that's the part I understand. But it's not clear where
> backtracking is possible and where it's not. At least, the code that
> I saw in edebug.el didn't match my expectations back when I looked at
> it, hence my not feeling quite sure what the semantics are (and/or
> should be).
> IIRC the issue was that the scope of that effect wasn't clear: if you
> think of Prolog's cut, its effect is local to a particular definition,
> whereas I think the scope of `gate` is not nearly as clear because
> there isn't such a notion of "definition". ]
I agree that the code isn't terribly clear and probably has some bugs.
However, the "prevent backtracking through gates" seems to work at
least somewhat (experimentally, in this case I can turn the double
definition into an error).
> >> > so I think it would be reasonable to prevent. We already
> >> > disable backtracking for literal symbols, and I think forms that require
> >> > multiple &define forms with backtracking should be exceedingly rare and can
> >> > be rewritten as you did with cl-flet.
> >> Emitting a warning would be much more helpful than just silently
> >> "cut"ting the backtracking.
> > A gate isn't silent, it would cause a hard error in this case.
>
> What I meant is that a gate would just make the old cl-flet spec fail in
> most cases, with no explanation why that spec now fails even though it
> worked in the past.
Yes, we'd need to at least announce the change in NEWS as an
incompatible Lisp change. However, I think overall that's still better
than the current situation: with your fix to cl-flet the problematic
constructs shouldn't occur any more in the Emacs codebase, and I
wouldn't expect such constructs to be very frequent "in the wild", so
the overall breakage would be very small, and if it can avoid
similarly subtle bugs then I think it's warranted.