bug#56596: 29.0.50; void-variable cl--nm

["Pierre L. Nageoire" <devel@pollock-nageoire.net>]

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