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Re: AS_IF breakage

From: Eric Blake
Subject: Re: AS_IF breakage
Date: Tue, 14 Apr 2009 15:52:06 +0000 (UTC)
User-agent: Loom/3.14 (

Eric Blake <ebb9 <at>> writes:

> which puts the burden back on the user, where calling:
> AS_IF([false], [], [AC_REQUIRE(...)])
> causes a syntax error.
> If I could get AC_REQUIRE to play nicely with m4_expand, then that would be 
> solution - pre-expand the user's text, at which point we then know for 
> whether the expansion is empty.  But right now, I don't know how to pull that 
> trick off.

Here's a thought - right now, AC_REQUIRE is identical to m4_require, and 
expands to an empty string.  What if we instead did:

m4_define([AC_REQUIRE], [m4_require($@):;])

which makes every use of AC_REQUIRE output a valid shell command?  The ; is 
necessary, since "AC_REQUIRE(...)dnl" is a common idiom.

That only helps one particular case of a macro that used to expand to an empty 
string, but it is probably the most common case.

Eric Blake

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