bug#21185: 25.0.50; byte-compiler warn on quote lexical var

bug#21185: 25.0.50; byte-compiler warn on quote lexical var

[stephen_leake]

This file gives an example where a new byte-compiler warning would be
useful:

;; quote-lexical-var.el -*- lexical-binding: t -*-

(defun display-var (var)
  "Display variable value of VAR (a symbol), if any."
  (cond
   ((boundp var)
    (message "%s: %s" (symbol-name var) (symbol-value var)))

   (t
    (message "%s not bound" (symbol-name var)))))

(defun quote-lexical-var ()
  (let ((my-var "foo"))
    (message "non-quoted my-var: %s" my-var)
    (display-var 'my-var)))

(provide 'quote-lexical-var)
;; end of file

To see the problem, execute (in emacs master, -Q):

    (add-to-list 'load-path "/home/stephe/Elisp_Work/")
    ;; path containing above file

    (byte-compile-file  "/home/stephe/Elisp_Work/quote-lexical-var.el")
    (require 'quote-lexical-var)

    (display-var 'default-directory)

This gives "default-directory: /home/stephe" as expected.

    (quote-lexical-var)

This gives:

non-quoted my-var: foo
my-var: not bound

Quoting lexical variables does not work; there is no symbol available at
run-time.

I've been bitten by this problem when converting code to use lexical
binding; it would be helpful if the byte-compiler warned about it.


In GNU Emacs 25.0.50.4 (x86_64-w64-mingw32)
 of 2015-08-02 on TAKVER2
Repository revision: d2c4309d46529bac656f332677d2c5e199577845
Windowing system distributor `Microsoft Corp.', version 6.3.9600
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bug#21185: 25.0.50; byte-compiler warn on quote lexical var

[Stefan Monnier]

> I've been bitten by this problem when converting code to use lexical
> binding; it would be helpful if the byte-compiler warned about it.

Agreed, but I haven't figured out how to make this work.
There are two problems:
1- detect not just things like « 'var » but also things like « '(var1
   var2 var3) » since such occur in a few places as well.
2- don't swamp the user with false positives.

Problem 2 is by far the biggest.  It's very common to have a local
variable `face' and at the same time to use the symbol `face' (quoted),
for example.

I do have such a warning in my local hacks (it outputs a warning for any
'<var> where <var> is a locally bound lexical variable) and find it
unbearable, even though I'm usually considered to tolerate warnings much
better than most.


        Stefan

bug#21185: 25.0.50; byte-compiler warn on quote lexical var

[Stephen Leake]

Stefan Monnier <monnier@iro.umontreal.ca> writes:

>> I've been bitten by this problem when converting code to use lexical
>> binding; it would be helpful if the byte-compiler warned about it.
>
> Agreed, but I haven't figured out how to make this work.
> There are two problems:
> 1- detect not just things like « 'var » but also things like « '(var1
>    var2 var3) » since such occur in a few places as well.
> 2- don't swamp the user with false positives.
>
> Problem 2 is by far the biggest.  It's very common to have a local
> variable `face' and at the same time to use the symbol `face' (quoted),
> for example.
>
> I do have such a warning in my local hacks (it outputs a warning for any
> '<var> where <var> is a locally bound lexical variable) and find it
> unbearable, even though I'm usually considered to tolerate warnings much
> better than most.

Ok. Can you post your local hack? That would at least be useful during a
transition to lexical-binding: t; after that, testing should suffice.

-- 
-- Stephe

bug#21185: 25.0.50; byte-compiler warn on quote lexical var

[Lars Ingebrigtsen]

Stefan Monnier <monnier@iro.umontreal.ca> writes:

>> I've been bitten by this problem when converting code to use lexical
>> binding; it would be helpful if the byte-compiler warned about it.
>
> Agreed, but I haven't figured out how to make this work.
> There are two problems:
> 1- detect not just things like « 'var » but also things like « '(var1
>    var2 var3) » since such occur in a few places as well.
> 2- don't swamp the user with false positives.
>
> Problem 2 is by far the biggest.  It's very common to have a local
> variable `face' and at the same time to use the symbol `face' (quoted),
> for example.

So the suggestion is to have code like

(let ((foo "bar"))
  (when (zot 'foo)
     ...))

give a warning (in lexical code)?  That does sound useful, especially if
the `zot' here is `bound-and-true-p', which somebody was using in a
different bug report the other week.

But I think that Stefan is totally correct in 2) -- there will be so
many false positives that it won't be useful.  So I'm closing this bug
report.

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no