autoconf: should I always use AS_IF over if and m4_if?

Question

The autoconf manual suggest using AS_IF instead of if, but it doesn't mention m4_if. Should AS_IF also be preferred over m4_if?

The "Limitations of Builtins" section of the manual says to use AS_IF for it's portability, but the AC_REQUIRE description says to use it to expand required macros.

Should I always use AS_IF? The answer seems to be yes, but then again I see if and m4_if being used in other macros. When is it ok not to use AS_IF?

tprk77 · Accepted Answer · 2013-02-26T19:39:12.580

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I was confused about this because I didn't realize the fundamental difference between if/AS_IF and m4_if. The normal if and AS_IF have their conditionals evaluated when the user runs the script, i.e. configure. Conversely, m4_if has its conditional evaluated during macro expansion, i.e. autoreconf.

if/AS_IF affects the flow of the script. m4_if affects the contents of the script.

This seems obvious now, but I didn't get it at first. Hopefully this helps someone else.

edited Feb 26 '13 at 19:39

answered Jan 14 '13 at 01:54

tprk77

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That doesn't say when to use `if` and when `AS_IF`. – Pavel Šimerda Dec 30 '14 at 08:51
I think the consensus is to always use `AS_IF` over `if`, because you get better quoting, etc. – tprk77 Dec 30 '14 at 19:21

score 2 · Answer 2 · answered Jan 08 '13 at 00:16

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The other benefit of AS_IF is that when it expands, any macros that are required by macros in the branches are expanded before the test.

See the manual.

answered Jan 08 '13 at 00:16

Jack Kelly

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autoconf: should I always use AS_IF over if and m4_if?

2 Answers2