GNU bug report logs - #50136
28.0.50; A problem with rx-let expansion

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From: Michael Heerdegen <michael_heerdegen <at> web.de>
To: 50136 <at> debbugs.gnu.org
Cc: Mattias Engdegård <mattiase <at> acm.org>, Stefan Monnier <monnier <at> iro.umontreal.ca>
Subject: bug#50136: 28.0.50; A problem with rx-let expansion
Date: Fri, 20 Aug 2021 15:59:44 +0200

Hello,

I'm referring to this paragraph in rx.el:

;; FIXME: Consider adding extensions in Lisp macro style, where
;; arguments are passed unevaluated to code that returns the rx form
;; to use.
;; [...]
;; While this would permit more powerful extensions, it's unclear just
;; how often they would be used in practice.  Let's wait until there is
;; demand for it.

Ok - here is!  I would find that approach much more natural than the
current one.  Look:

Already the first `rx-let' pseudo macro I tried hit a problem (bug) with
the current approach:

#+begin_src emacs-lisp
(rx-let ((scatter (string)
                  (regex (mapconcat #'string (string-to-list string) ".*"))))
  (rx (scatter "abc"))) => useless error message
#+end_src

while the following version (the only difference is the argument name)
works as expected:

#+begin_src emacs-lisp
(rx-let ((scatter (s) (regex (mapconcat #'string (string-to-list s) ".*"))))
  (rx (scatter "abc"))) ==> "a.*b.*c"
#+end_src

Seems the function form #'string gets replaced and ends as #'"abc" in
the first version because the function name accidentally collides with
the argument name.

Personally I would be more happy with a thing called `rx-macrolet' than
with an extended `rx-let' to support this additional macro-like kind of
syntax.

TIA,

Michael.

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