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bug#8711: 24.0.50; binding _ to unused values with lexical-binding
From: |
Stefan Monnier |
Subject: |
bug#8711: 24.0.50; binding _ to unused values with lexical-binding |
Date: |
Mon, 23 May 2011 11:24:31 -0300 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/24.0.50 (gnu/linux) |
>> There seems be no way to avoid a warning in code like this:
>> ;; -*- lexical-binding: t -*-
>> (eval-when-compile (require 'cl))
>> (defun foo (x) (destructuring-bind (_) x))
Hmm... the byte-compiler complains because the above gets turned into
(let* ((--cl-rest-- x)
(_
(if (= (length --cl-rest--) 1)
(car --cl-rest--)
(signal 'wrong-number-of-arguments (list nil (length
--cl-rest--))))))
nil)
which gets changes by byte-opt.el into
(let* ((--cl-rest-- x))
(if (= (length --cl-rest--) 1)
(car --cl-rest--)
(signal 'wrong-number-of-arguments (list nil (length --cl-rest--))))
nil)
And note that this is only done for a `nil' body (tho it can also be
something like (progn nil) because that would be optimized to nil as
well), and not for a body like `5' (although the optimization is just
as valid for `5' as for nil).
>> (defun bar (x) (destructuring-bind (_) x (ignore _)))
(defun bar (x) (destructuring-bind (_) x (ignore nil)))
works (because (ignore nil) happens not to be optimized to nil).
> This latter is because cconv-analyse-use is not smart enough when
> checking for use of variables starting with an underscore.
> ;; -*- lexical-binding: t -*-
> (let ((_ nil))
> (ignore _))
> =>
> Warning: variable `_' not left unused.
> This despite ignore not touching it.
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.
Stefan
- 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 <=
- 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, 2011/05/23
- 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