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Re: AS_CASE vs case/esac


From: Ralf Wildenhues
Subject: Re: AS_CASE vs case/esac
Date: Thu, 13 Mar 2008 22:25:46 +0100
User-agent: Mutt/1.5.17+20080114 (2008-01-14)

* NightStrike wrote on Thu, Mar 13, 2008 at 10:07:31PM CET:
> On 3/13/08, Ralf Wildenhues <address@hidden> wrote:
> >
> > AC_INIT
> > AC_DEFUN([FOO], [echo foo])
> > AC_DEFUN([BAR], [AC_REQUIRE([FOO])
> >                 echo bar])
> > x=zork
> > AS_CASE([$x], [y*], [BAR])
> >
> > will print 'foo' because FOO will be expanded outside of the AS_CASE.

> I'm trying to follow this logic, but I'm not putting it together in my
> head.  Why isn't the result nothing?

Because it expands to this:

x=zork
echo foo
case $x in
  y*)
    echo bar
    ;;
esac

That is, required macros of macro expansions that happen inside
defun'ed macros get pushed right before the outermost defun'ed macro.
<http://www.gnu.org/software/autoconf/manual/html_node/Dependencies-Between-Macros.html>

Here's a slightly more useful example:

# FIND(PATTERN, FILE)
# -------------------
AC_DEFUN([FIND],
[AC_REQUIRE([AC_PROG_GREP])
$GREP $1 $2
])

AS_CASE([$whatever],
        [blabla],    [FIND([pattern], [file])])
FIND([pattern2], [file2])


The AC_PROG_GREP will be expanded only once (that's what AC_REQUIRE
does).  If it were expanded inside the case..esac, then if $whatever
did not match, the second FIND would provoke a syntax error because
$GREP has not been initialized at that point.

One should not that this also frequently leads to confusion, because
users don't realize AC_REQUIRE'd macros end up quite a bit earlier than
they expect.

Cheers,
Ralf




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