Re: AC_TRY_COMPILE() annoyances with 2.63b

[Eric Blake]

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According to Mike Frysinger on 4/6/2009 3:43 PM:
>>>  m4_define([_AS_IF_ELSE],
>>>  [m4_ifvaln([$1],
>>> -[else
>>> +[else :
>>>    $1])])
>>>
>>>  m4_defun([AS_IF],
>> I like this one, for simplicity and obvious correctness alone, but will
>> defer to Eric for the final decision.
> 
> personally, i prefer both :).  dont emit extraneous brackets when they arent 
> needed, and be safe when we think we must.

I'm also leaning towards omitting the else if $1 is provably blank,
otherwise providing the : in case non-blank $1 ultimately expands to a
blank (unless someone finds a shell where 'if false ; then :; fi; echo $?'
fails to output 0).  AS_CASE and AS_FOR are also candidates for this.  And
it will be easier with m4_ifblank.  Stay tuned; I'll have a patch by the
end of the week.

Now for a question - right now, m4_default([$1], [$2]) is a nice shorthand
for m4_ifval([$1], [$1], [$2]); is there any reason to create a shorthand
for m4_ifnblank([$1], [$1], [$2]) that likewise only needs two arguments?
 And if so, what to call it?

>> BTW, m4_normalize would have been wrong as it can change expanded text
>> that you wouldn't want changed, too.  (I think.)
> 
> but that wouldnt matter would it ?  we're only expanding to check it 
> ifvaln(normalize($1)).  if we like the result, then we emit $1 as part of the 
> else.  but again, m4 subtleties are beyond me.

Actually, you nailed the subtleties on that one:

m4_ifval(m4_normalize([$1]), [else :
  $1
])

uses two copies of $1; the first can undergo arbitrary change to check for
blanks, while the second is used as-is.  I'd use exactly that if it
weren't for the fact that the future m4_ifblank is more efficient than
m4_ifval/m4_normalize.

- --
Don't work too hard, make some time for fun as well!

Eric Blake             address@hidden
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