On 05/15/2014 02:13 PM, Daniel Goldman wrote: > Hi, > > I would like to suggest the small change to add an m4_ifndef builtin > macro to some future version of m4.
Why? You can already write your own m4_ifndef macro that wraps the builtin. We intentionally want a minimal set of builtins, and leave the bulk of the power in wrappers around the builtins. m4_define(`m4_ifndef', `m4_ifdef(`$1', `$3', `$2')') > Reason #1 I suggest adding m4_ifndef is for better communication, to > make the language easier to express the logic. Perhaps a lot of m4 usage > is obscure, but I don't consider that a virtue. I would argue that: > > m4_ifndef(`MACRO_1', `MACRO_1' NOT defined) > > is clearer than: > > m4_ifdef(`MACRO_1', , `MACRO_1' NOT defined) Yes, and that's why the autoconf language has an m4_ifndef wrapper alongside the m4_ifdef wrapper. But that didn't require a new builtin. > Reason #1 in favor of NOT adding is because m4_ifdef is adequate to do > the logical operations. To my understanding the following two macros are > 100% equivalent: > > m4_ifdef(`MACRO_1', `MACRO_1' defined, `MACRO_1' NOT defined) > m4_ifndef(`MACRO_1', `MACRO_1' NOT defined, `MACRO_1' defined) > Yes, if you write your m4_ifndef wrapper as above. > > $ cat ifndef-02.m4 > m4_define(`m4_ifndef', `m4_ifdef(`$1', `$3', `$2')') Yep, you already wrote the correct composite macro. -- Eric Blake eblake redhat com +1-919-301-3266 Libvirt virtualization library http://libvirt.org
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