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bug#56596: 29.0.50; void-variable cl--nm
From: |
Stefan Monnier |
Subject: |
bug#56596: 29.0.50; void-variable cl--nm |
Date: |
Sat, 16 Jul 2022 19:19:41 -0400 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/29.0.50 (gnu/linux) |
>> 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