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bug#31641: 26.1; iter-do variable not left unused warning
From: |
Stefan Monnier |
Subject: |
bug#31641: 26.1; iter-do variable not left unused warning |
Date: |
Thu, 04 Feb 2021 11:36:27 -0500 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/28.0.50 (gnu/linux) |
>>> When byte-compiling an iter-do form with a variable intended to be
>>> left unused, the compiler emits a false warning:
>>>
>>> ;;; -*- lexical-binding: t; -*-
>>> (require 'generator)
>>> (iter-do (_ i))
>>> ;; -> "Warning: variable ‘_’ not left unused"
>>
>> Looking at the expansion, I guess the setf should be dropped if the
>> variable name starts with _.
>>
>> (let (_
>> #3=#:iter-do-result11
>> (#1=#:iter-do-iterator-done8 nil)
>> (#2=#:iter-do-iterator10 i))
>> (while (not #1#)
>> (condition-case #4=#:iter-do-condition9
>> (setf _ (iter-next #2#))
>> (iter-end-of-sequence
>> (setf #3# (cdr #4#))
>> (setf #1# t)))
>> (unless #1#))
>> #3#)
FWIW, I find the above expansion to provide somewhat "dirty" semantics
in the sense that
(let ((funs '()))
(iter-do (n i) (push (lambda () n) funs))
funs)
will return a list of functions which all return the same value (the
last `n`).
You can clean up this semantics and the warning at the same time by
using an expansion like:
(let (#3=#:iter-do-result11
(#1=#:iter-do-iterator-done8 nil)
(#2=#:iter-do-iterator10 i))
(while (not #1#)
(let ((_ (condition-case #4=#:iter-do-condition9
(iter-next #2#)
(iter-end-of-sequence
(setf #3# (cdr #4#))
(setf #1# t)))
(unless #1# [BODY])
#3#)
BTW, I think we can remove the duplicate #1 test by moving the body of
the `while` into its test, e.g.:
(let (#3=#:iter-do-result11
(#1=#:iter-do-iterator-done8 nil)
(#2=#:iter-do-iterator10 i))
(while
(let ((_ (condition-case #4=#:iter-do-condition9
(iter-next #2#)
(iter-end-of-sequence
(setf #3# (cdr #4#))
(setf #1# t)))))
(unless #1#
[BODY]
t)))
#3#)
It's too bad that [BODY] can throw `iter-end-of-sequence`, since
otherwise we could move the `condition-case` outside of the loop and get
something more efficient.
Stefan
> The following patch does this, but I'm not sure whether this is correct
> or not -- in other cases, the _ convention just removes the warning, but
> doesn't change the semantics.
>
> I wondered whether we could just suppress this warning like this:
>
> ,(if (string-match-p "\\`_" (symbol-name var))
> `(with-suppressed-warnings ((not-unused ,var))
> (setf ,var (iter-next ,it-symbol)))
> `(setf ,var (iter-next ,it-symbol)))
>
> But no, cconv--analyze-use is called too early, and would have to be
> taught about `with-suppressed-warnings'... which, looking at the code,
> isn't immediately obvious how to do.
>
> So does anybody have any ideas here?
>
> diff --git a/lisp/emacs-lisp/generator.el b/lisp/emacs-lisp/generator.el
> index 9eb6d95964..0b644cc72c 100644
> --- a/lisp/emacs-lisp/generator.el
> +++ b/lisp/emacs-lisp/generator.el
> @@ -731,7 +731,10 @@ iter-do
> (,it-symbol ,iterator))
> (while (not ,done-symbol)
> (condition-case ,condition-symbol
> - (setf ,var (iter-next ,it-symbol))
> + ;; Variables that start with an underscore shouldn't be set.
> + ,(if (string-match-p "\\`_" (symbol-name var))
> + nil
> + `(setf ,var (iter-next ,it-symbol)))
> (iter-end-of-sequence
> (setf ,result-symbol (cdr ,condition-symbol))
> (setf ,done-symbol t)))