Subject: Re: bug#20268: 25.0.50; pcase-lambda broken

From: Philipp Stephani <[email-masked]>
Date: Wed, 08 Apr 2015 15:08:36 +0000

Stefan Monnier <[email-masked]> schrieb am Mi., 8. Apr. 2015 um
04:15 Uhr:

> > After the recent rewrite, pcase-lambda is broken. For example, eval the
> > following to get 46422 instead of the correct value 65535.
>
> >    (cl-some (pcase-lambda (`[fullsweep_after ,v]) v)
> >             '([min_bin_vheap_size 46422]
> >               [min_heap_size 233]
> >               [fullsweep_after 65535]
> >               [minor_gcs 40]))
>
> Indeed, that's the semantics I chose.
> The previous semantics was for the function to do nothing and return nil
> if the arg doesn't match.  The new semantics is the same as the one used
> by pcase-let.  It's not without its fault of course, but at least it does
> correspond to the usual idea of "destructuring" and generates more
> efficient code.
>
> I think if you prefer to return nil, then the macro should look like
>
>    (pcase-lambda ((`[fullsweep_after ,v]) v))
>
> which would then naturally let you add additional cases like
>
>    (pcase-lambda ((`[fullsweep_after ,v]) v)
>                  ((`[min_heap_size ,v]) (/ v 2)))
>
> Admittedly, for the current pcase-lambda (and pcase-let) macro, the
> pcase.el code should be refined so as to emit a warning when it ends up
> ignoring a constant like `fullsweep_after' above.
>
>
Why not raise an error instead if there is no match (or even if any quoted
or self-quoting expressions are used at all)? The current behavior is quite
confusing, and warnings tend to be ignored.

Given that pcase-let, pcase-lambda and pcase-dolist don't do any
case-by-case analysis, maybe they should even be renamed and moved out of
pcase.el.