Re: pcase-dolist

[Thierry Volpiatto <thierry.volpiatto@gmail.com>]

Tassilo Horn <tsdh@gnu.org> writes:

> Stefan Monnier <monnier@iro.umontreal.ca> writes:
>
>>>   (pcase-let ((`(,w ,x ,y ,z) '(1 2 3)))
>>>     (message "%s %s %s %s" w x y z))
>>
>>> prints "1 2 3 nil" although the pattern when used in `pcase' wouldn't
>>> match that list but only 4-element lists.  So a pcase pattern has
>>> different semantics depending on whether it is used in `pcase' or
>>> `pcase-let' or `pcase-dolist' which doesn't feel right.
>>
>>> I'd prefer if `pcase-let' would signal an error if some pattern doesn't
>>> match.
>>
>> So you'd want the above to signal an error in the case of:
>>
>>    (pcase-let ((`(,w ,x) '(1 2 3)))
>>      (message "%s %s" w x))
>>
>> ?
>
> Yes, if you want to pick the first two elements of a list with 2 or more
> elements, you should use `(,w ,x . ,_).  I see that the above is
> slightly more convenient and concise but I'd value consistency more.

This would be a regression IMHO, what if you want to do e.g:

(pcase-let ((`(,a ,b) (file-attributes "foo")))
  (list a b))

It would be inconvenient to have to bind the 10 unused remaining elements.

Of course one can use `cl-multiple-value-bind' instead in such situation.

-- 
Thierry
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