Re: Pattern matching on match-string groups #elisp #question

[Mattias Engdegård]

25 feb. 2021 kl. 16.32 skrev Stefan Monnier <monnier@iro.umontreal.ca>:

>> The same won't work with pcase-let, so it's either unsupported or a bug.
> 
> I'd say it's a bug.  The patch below would fix it.  Mattias, WDYT?

Thank you, it looks like multiple bugs. The lack of (pred stringp) was of 
course an oversight but unrelated to pcase-let, right?

>   (let* ((rx--pcase-vars nil)
>          (regexp (rx--to-expr (rx--pcase-transform (cons 'seq regexps)))))
> -    `(and (pred (string-match ,regexp))
> +    `(and (pred stringp)
> +          (app (lambda (s) (string-match ,regexp s)) (pred identity))

It does seem to work, but why exactly do we need this monstrosity instead of 
(pred (string-match ,regexp))? Is it because pcase-let throws away all `pred` 
clauses somewhere? It makes sense to do so but I haven't found exactly where 
this takes place in pcase.el yet...

Perhaps the assumption that non-binding clauses like `pred` (and what else, 
`guard`?) are all side-effect free and can be thrown away in pcase-let[*] 
should be documented? Not that I would have read it, of course...

I'll push a fix as soon as I understand the machinery a bit better, but right 
now I'm wary of getting my fingers snapped off by the gears and knives.