GNU bug report logs - #26073
How should cl-symbol-macrolet interact with rebindings?

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Message #29 received at 26073 <at> debbugs.gnu.org (full text, mbox):

From: Stefan Monnier <monnier <at> IRO.UMontreal.CA>
To: Paul Pogonyshev <pogonyshev <at> gmail.com>
Cc: 26073 <at> debbugs.gnu.org
Subject: Re: bug#26073: workaround
Date: Tue, 26 Sep 2017 10:38:36 -0400

retitle 26073 How should cl-symbol-macrolet interact with rebindings?
thanks

The problem here is indeed different from the one in bug#26068.
bug#26068 was clearly triggering a (known) bug in cl-symbol-macrolet.

Here it's triggering a (known) misfeature.  The source code has the
following comments about it:

             ;; CL's symbol-macrolet treats re-bindings as candidates for
             ;; expansion (turning the let into a letf if needed), contrary to
             ;; Common-Lisp where such re-bindings hide the symbol-macro.
and
            ;; FIXME: The behavior of CL made sense in a dynamically scoped
            ;; language, but for lexical scoping, Common-Lisp's behavior might
            ;; make more sense (and indeed, CL behaves like Common-Lisp w.r.t
            ;; lexical-let), so maybe we should adjust the behavior based on
            ;; the use of lexical-binding.

more concretely cl-symbol-macrolet implements the following semantics:

      (cl-symbol-macrolet ((x <e>))
        ... (let ((x <foo>)) ..x..))
    =>
        ... (cl-letf ((<e> <foo>)) ..<e>..)

whereas Common-Lisp's symbol-macrolet wants the following semantics instead:

    => ... (let ((x <foo>)) ..x..)

As mentioned in the comment, it probably makes sense to change
cl-symbol-macrolet in lexical-binding code to follow Common-Lisp's
semantics (tho we'd want to give access to the old semantics if the user
explicitly uses cl-letf).

Not sure what might break if we do that: the main user of
cl-symbol-macrolet outside of generator.el AFAIK is the with-slots of
eieio, so the question is whether some users of with-slots expect
a subsequent `let` binding to temporarily change the slot's value.
I just checked and it seems that no code in Emacs itself relies on this
behavior, so maybe it's "safe" to change it.


        Stefan

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