Re: AC_TRY_COMPILE() annoyances with 2.63b

[Mike Frysinger]

On Monday 06 April 2009 08:59:32 Eric Blake wrote:
> According to Mike Frysinger on 4/5/2009 7:19 PM:
> >> i'm not an expert by any means with internal autoconf/m4.  if there's a
> >> m4 helper function to test whether an argument contains something other
> >> than whitespace, then the change to m4sh.m4:_AS_IF is trivial ... use
> >> that rather than m4_ifvaln([$1],....
> >
> > to answer my own question and to post a patch, m4_normalize can be used:
>
> Indeed.  However, m4_normalize is an expensive choice for doing this check
> (it involves regular expressions).  I'm thinking of introducing a new
> m4sugar macro (untested in this email) that can be used for this purpose
> with less overhead - any suggestions for a better name?

"blank" is actually what i thought of at first when i went looking for a 
preexisting function ;)

> # m4_ifblank(COND, [IF-BLANK], [IF-TEXT])
> # ---------------------------------------
> # If COND is empty, or consists only of blanks (space, tab, newline),
> # expand IF-BLANK, else expand IF-TEXT.
> m4_define([m4_ifblank],
> [m4_if(m4_translit([[$1]],  [ ][      ][
> ]), [], [$2], [$3])])
>
> Hmm, this means that in addition to differing on a COND consisting of just
> whitespace, m4_ifval and m4_ifblank have opposite if-then sense:
> m4_ifval([$1], [$1], [])
> m4_ifblank([$1], [], [$1])

call it m4_ifnblank() ?  or have both so people can swap as needed ?  i 
imagine people would favor the form for their specific case which only 
requires one argument.  i know i would.
m4_ifblank(COND, [IF-BLANK], [IF-TEXT])
m4_ifnblank(COND, [IF-TEXT], [IF-BLANK])

> > this should "fix" every macro that uses AS_IF with an "else" argument. 
> > it certainly fixes my test case.
>
> I think you are right that fixing AS_IF to detect blank instead of empty
> arguments when performing optimizations will probably help a number of
> poorly-written configure.ac, so I will probably be checking in a patch
> along these lines soon.  Note, however, that this still doesn't help
> situations like:
>
> AS_IF([foo], [AC_REQUIRE([bar])])
>
> since the presence of AC_REQUIRE will trigger the IF-TEXT expansion,
> although the subsequent expansion pass will result in a blank body.

true, although i dont really know squat about m4 to propose any solution along 
those lines.  perhaps have the else case always emit a leading null cmd ":" ?  
seems like adding 2 bytes is a small price to pay for the resulting syntax 
safety ...

--- a/lib/m4sugar/m4sh.m4
+++ b/lib/m4sugar/m4sh.m4
@@ -607,7 +607,7 @@ m4_define([_AS_IF],
 ])
 m4_define([_AS_IF_ELSE],
 [m4_ifvaln([$1],
-[else
+[else :
   $1])])
 
 m4_defun([AS_IF],
-mike

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