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bug#56596: 29.0.50; void-variable cl--nm
From: |
Pierre L. Nageoire |
Subject: |
bug#56596: 29.0.50; void-variable cl--nm |
Date: |
Mon, 18 Jul 2022 05:36:11 +0200 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/29.0.50 (gnu/linux) |
Hi Stefan,
Thanks for these detailed explanations; I think I will be able to
modify my codes to make them work with this new cl-generic
implementation.
Best regards
Stefan Monnier <monnier@iro.umontreal.ca> writes:
>>> I can reproduce this -- but only when using dynamic binding. When using
>>> lexical binding, things work fine.
>
> Indeed, `cl-defmethod` (and `cl-defgeneric`) aren't guaranteed to work in
> dynbind code. They often do, admittedly.
>
>>> I've added Stefan to the CCs; perhaps he has some comments.
>>
>> I suspect that cl-generic implementation has recently changed and that
>> it should not be used nowadays exactly like it has been. Any small
>> explanations from Stefan would be greatly appreciated !
>
> The "next method" is not known when we compile `cl-defmethod` but it is
> known when we build the "effective method", which is expected to be done
> much less often than actual calls to that method.
>
> The old code for `cl-defmethod` turned
>
> (cl-defmethod I ((d raw-daughter))
> (format "the daughter of %s"
> (cl-call-next-method)))
>
> into something like
>
> (cl-..register (raw-daughter)
> (lambda (cnm d)
> (format "the daughter of %s"
> (apply cnm))))
>
> forcing the caller to build a `cnm` closure which captures the args list
> containing the value of the `d` argument. Also it made it difficult to
> implement `next-method-p` since that requires digging into this `cnm`
> closure to see if it's one of those that would signal no-next-method.
>
> The new code instead is a bit like:
>
> (cl-..register (raw-daughter)
> (lambda (nm)
> (lambda (&rest args)
> (let ((cnm (lambda (&rest cnmargs) (apply nm (or cnmargs args)))))
> (destructive-bind (d) args
> (format "the daughter of %s"
> (apply cnm)))))))
>
> This basically moves some of the code from the caller to here, which
> doesn't seem to buy us very much but:
> - it makes it much easier to implement `next-method-p` because now we
> have access to the actual "next method", rather than to a closure that
> combines the next methods with the saved arg list, so it's much easier
> to tell if the next method is the one that signals no-next-method.
> - we occasionally get to skip building the `cnm` closure because we can
> use λ-lifting instead (basically the byte-compiler gets to see both
> parts of the code together and can thus change it: with this new code
> we could actually improve the code generated by the byte-compiler
> even further).
>
> Problem is that the new code relies on the use of currying (see how
> `nm` and `args` are now passed in a curried fashion), which is only
> possible with lexical scoping.
>
>
> Stefan