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bug#20268: 25.0.50; pcase-lambda broken
From: |
Stefan Monnier |
Subject: |
bug#20268: 25.0.50; pcase-lambda broken |
Date: |
Tue, 07 Apr 2015 22:14:29 -0400 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/25.0.50 (gnu/linux) |
> 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.
Stefan