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Re: Do we need C's extern in Emacs Lisp?
From: |
Alan Mackenzie |
Subject: |
Re: Do we need C's extern in Emacs Lisp? |
Date: |
Thu, 31 Mar 2022 20:59:36 +0000 |
Hello, Stefan.
On Tue, Mar 29, 2022 at 17:09:10 -0400, Stefan Monnier wrote:
> > For example, if
> > (defvar foo)
> > is in a Lisp file, then subsequent use of foo will generate an unknown
> > variable warning.
> That's not my experience. Do you have a recipe?
This happened in an attempt to fix bug #21466, a bug you opened in 2015
proposing to replace cc-bytecomp-defun and cc-bytecomp-defvar by
(declare-function foo nil) and (defvar foo).
I was wrong about the "subsequent use ... generate an unknown variable
warning.". Sorry. The problem happened during a more unusual
compilation situation best explained by the following comment from
cc-fonts.el:
;; Need to declare these local symbols during compilation since
;; they're referenced from lambdas in `byte-compile' calls that are
;; executed at compile time. They don't need to have the proper
;; definitions, though, since the generated functions aren't called
;; during compilation.
The "need to declare" means "need to give the value/function cell of the
symbol a binding", because the byte compiler seems to use boundp and
fboundp to check whether to emit "not known" warnings.
> > For example(2), if in the Lisp file there is
> > (declare-function foo nil)
> > , this generates a warning when the function foo is later defined.
> I haven't verified that it's indeed the case, but it sounds likely,
> indeed. We could consider silencing this warning, but I wonder why
> you'd put such a `declare-function` in the same file where you define
> `foo`, since it's redundant anyway.
In the above cc-fonts.el scenario, foo needs a function binding at the
time the `byte-compile' form processes it, which happens to be before it
gets defined in the file.
> Do you have a good use-case for it?
See above. ;-) It may well be that such facilities would enable me to
dismantle large parts of cc-bytecomp.el sooner rather than later.
"This variable/function is defined somewhere" is a natural thing to want
to say. The (defvar foo) and (declare-function foo nil) don't say this,
exactly.
> Stefan
--
Alan Mackenzie (Nuremberg, Germany).