bug#27659: 26.0.50; Add string-matched-text: string-match + match-string

[Philipp Stephani <p.stephani2@gmail.com>]

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Michael Heerdegen <michael_heerdegen@web.de> schrieb am Sa., 22. Juli 2017
um 03:46 Uhr:

> Hi Philipp,
>
> nice idea!  I have some questions:
>
>
> +(pcase-defmacro rx (&rest regexps)
> +  "Build a `pcase' pattern matching `rx' regexps.
> +The REGEXPS are interpreted as by `rx'.
>
> Should we tell what the semantics of multiple REGEXPS is?  I guess they
> are implicitly wrapped inside rx-`and' (but FWIW, the doc of `rx' also
> fails to tell that).
>


yes, probably something should be added to the docstring of `rx'.


>
>
> +Within the case code, the match data is bound as usual, but you
>
> This makes it sound like match data is bound pcase-branch-locally.
> This isn't the case, right?
>

Yes, I've reworded it to sound less like a variable binding.


>
>
> +In addition to the usual `rx' constructs, REGEXPS can contain the
> +following constructs:
> +
> +  (let VAR FORM...)  creates a new explicitly numbered submatch
> +                     that matches FORM and binds the match to
> +                     VAR.
>
> This made me wonder what FORM should be.  I think it means any rx
> symbolic expression, so the name FORM seems misleading.
>

I think the words `form' and `sexp' are used mostly interchangeably
nowadays, and the docstring of `rx' speaks of "forms" several times.


>
>
> +(ert-deftest pcase-tests-rx ()
> +  (should (equal (pcase "a 1 2 3 1 b"
> +                   ((rx (let u (+ digit)) space
> +                        (let v (+ digit)) space
> +                        (let v (+ digit)) space
> +                        (backref-var u))
> +                    (list u v)))
> +                 '("1" "3"))))
> +
>
> I don't understand the example (or test).  Is v first bound to 2, and
> after that rebound to 3?  This seems at least surprising, since let
> behaves differently in pcase, e.g.
>
> (pcase 'foo
>   ((and (let x 1) (let x 2)) x))
> ==> nil
>
> Hmm, in general I see the risk of confusing this `let' with `pcase' let.
> It seems to be something very different.  Maybe you could just pick a
> different name, `bind' maybe?
>
>
>
I'd rather stick with the short and common `let'. That there's a name clash
is unfortunate, but there are lots of similar cases (e.g. pcase `let' vs.
Lisp `let', rx `and' and `or').

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