GNU bug report logs -
#20268
25.0.50; pcase-lambda broken
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Reported by: Leo Liu <sdl.web <at> gmail.com>
Date: Tue, 7 Apr 2015 07:43:02 UTC
Severity: wishlist
Found in version 25.0.50
Fixed in version 29.1
Done: Lars Ingebrigtsen <larsi <at> gnus.org>
Bug is archived. No further changes may be made.
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Forgive me to rely here. None of our messages shown up in bug#19670.
> After recent rewrite of pcase-lambda I am seeing:
> (cl-some (pcase-lambda (`[fullsweep_after ,v]) v)
> '([min_bin_vheap_size 46422]
> [min_heap_size 233]
> [fullsweep_after 65535]
>
> [minor_gcs 10]))
>
> 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 don't mind new semantics but I want to make sense of it so as to use
it with confidence. What I am seeing is:
(funcall (pcase-lambda (`[fullsweep_after ,v]) v) [min_bin_vheap_size 46422])
=> 46422
That doesn't make sense to me. Could you explain why this is the right
thing?
> 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)))
I didn't choose the multiple-clause version because it is a cheap
shortcut for (lambda (...) (pcase ... ...)).
The single-clause pcase-lambda makes a lot of higher-order functions fun
to use and that is where I am reaping the benefits.
> 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.
Yes, I am having a lot of trouble understanding and using pcase-let*
except for trivial cases. I don't want the same thing to happen to
pcase-lambda.
> Stefan
Thanks,
Leo
This bug report was last modified 3 years and 156 days ago.
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