Re: Lexical binding

[David Kastrup <dak@gnu.org>]

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

>>> Check the `dolist' macro, it has a FIXME for that.
>> OK, fair enough. Sorry for the noise.
>> Is this one also documented (or has a logical explanation that I'm
>> again missing)?
>
> No, it's not documented tho it's the same underlying problem:
>
>> ;; test.el  -*- lexical-binding: t -*-
>> (condition-case test
>>     (ignore)
>>   (quit test)
>>   (error test))
>> ;; end
>
>> No error. Now, if you remove `test' in either handler:
>
> Right.  Internally, the condition-case above is turned into
>
>   (condition-case :fun-body
>       (lambda () (ignore))
>     (quit (lambda (test) test))
>     (error (lambda (test) test)))
>
> I.e. the underlying problem (shared with dolist and some pcase
> situations as well) is that one binding occurrence of a variable (above,
> `test') is turned into 2 or more, so you can get warnings about an
> unused variable because some of the its binders aren't used, but since
> it really corresponds to a single binder in the source code the
> programmer can't really fix it, hence the warning is an annoyance.
>
> I'm not sure yet how best to solve the issue.

Go through a stage

(condition-case :fun-body (lambda () ignore)
  (multi-lambda (x) (quit (x test) test) (error (x test) test)))

or something similar where the byte compiler does the "unused variable"
check (or a setup for it) during the multi-lambda expansion stage.  What
kind of form is exactly needed here in order to cater for all such
situations will require careful thinking.

-- 
David Kastrup