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bug#8711: 24.0.50; binding _ to unused values with lexical-binding
From: |
Helmut Eller |
Subject: |
bug#8711: 24.0.50; binding _ to unused values with lexical-binding |
Date: |
Mon, 23 May 2011 20:23:52 +0200 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/24.0.50 (gnu/linux) |
* Stefan Monnier [2011-05-23 14:24] writes:
> The two warnings come from different analyzes:
> - The warning for `bar' comes from cconv.el which is intended to check
> whether the variable is syntactically used, rather than semantically.
> - The warning for `foo' checks whether pure functions are not called for
> their side-effects and it's applied after optimizations so in the
> above code, the code generated by destructuring-bind ends up optimized
> to something that calls `car' without binding the result to _ because
> it figured that _ is not used and just got rid of it without warning.
>
> So most likely the answers I give here aren't satisfactory to the OP,
> since his real problem is probably different than (destructuring-bind
> (_) x) and the solution for that problem is probably going to be
> yet different.
My problem is basically that I have a macro "destructure-case" that
expands to destructuring-bind, e.g.:
(destructure-case location
((:error _) nil)) ; do nothing
expands to
(ecase (car location)
(:error (destructuring-bind (_) (cdr location)
(ignore _)
nil)))
The macro inserts the (ignore _) to suppress the "value returned from
(car --cl-rest--) is unused" warning. But that trick only works for the
non-lexical-binding case.
I think that rewriting (let ((_ foo))) to (progn foo nil) is not quite
right because that loses the information that the value that flows to
the variable _ is intentionally ignored, but I guess that is somewhat
hard to fix this. (Maybe byte-optimize-letX could call
byte-optimize-form-code-walker directly but instead of specifying that
the form is evaluated "for-effect" somehow say that the value is
"ignorable".)
It's perhaps easier to teach destructuring-bind the _ convention so that
it can produce more direct code.
There is also a somewhat related problem with loop:
;; -*- lexical-binding: t -*-
(defun foo (alist) (loop for (_key . value) in alist collect value))
produces a "variable `_key' not left unused" warning.
Helmut
- bug#8711: 24.0.50; binding _ to unused values with lexical-binding, Helmut Eller, 2011/05/21
- bug#8711: 24.0.50; binding _ to unused values with lexical-binding, Lawrence Mitchell, 2011/05/23
- bug#8711: 24.0.50; binding _ to unused values with lexical-binding, Stefan Monnier, 2011/05/23
- bug#8711: 24.0.50; binding _ to unused values with lexical-binding,
Helmut Eller <=
- bug#8711: 24.0.50; binding _ to unused values with lexical-binding, Stefan Monnier, 2011/05/23
- bug#8711: 24.0.50; binding _ to unused values with lexical-binding, Helmut Eller, 2011/05/23
- bug#8711: 24.0.50; binding _ to unused values with lexical-binding, Stefan Monnier, 2011/05/23
- bug#8711: 24.0.50; binding _ to unused values with lexical-binding, Helmut Eller, 2011/05/24
- bug#8711: 24.0.50; binding _ to unused values with lexical-binding, Stefan Monnier, 2011/05/24
- bug#8711: 24.0.50; binding _ to unused values with lexical-binding, Helmut Eller, 2011/05/23
- bug#8711: 24.0.50; binding _ to unused values with lexical-binding, Stefan Monnier, 2011/05/23