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Re: AC_DEFUN_ONCE semantics

From: Eric Blake
Subject: Re: AC_DEFUN_ONCE semantics
Date: Wed, 04 Feb 2009 06:04:52 -0700
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According to Ralf Wildenhues on 2/3/2009 11:55 AM:
> At the same time, you lose out on a probably slightly unusual case which
> previously was pretty harmless but has now turned into a mine: a case
> where the user used these macros in several branches of a shell
> conditional construct:
>   case $foo in
>     bar) MACRO ;;
>     baz) ;;
>     *) MACRO ;;
>   esac
> These things (used to) work fine more often than not, and now you break
> them silently.  And what's more important: there really are cases where
> it is so much easier to invoke macros shell-conditionally.

In the case of macros that were already AC_DEFUN_ONCE, there is no
semantic change.  But for macros which were previously AC_DEFUN, but which
we convert to AC_DEFUN_ONCE, yes, this is a potential mine, which is why
we should minimize which macros undergo the conversion unless we have good
reason to believe that the macro will not be used conditionally.

> Why not instead let autoconf warn about instances other than the first
> one?  Whether you then decide to expand them nonetheless, or not is then
> less of a correctness concern.

Warning would defeat the purpose of using AC_DEFUN_ONCE to avoid the
warning of multiple expansions.  We could, however, rewrite the
problematic macros as AC_DEFUN with a self-redefinition that warns on
subsequent expansions, and have that be the state of affairs for a release
cycle prior to switching over to AC_DEFUN_ONCE.

> Anyway, I do think that turning macros to be defun-once'd is something
> akin of an API change of these macros, and as such, the list of affected
> macros should be put in NEWS at least (and probably also annotated in
> the manual).

Highly agreed; I'll add a NEWS entry with all of the affected macros.

- --
Don't work too hard, make some time for fun as well!

Eric Blake             address@hidden
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