|Date:||Thu, 28 Sep 2017 19:18:05 +0200|
retitle 26073 How should cl-symbol-macrolet interact with rebindings?
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.
;; 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.
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