How do I check the return value of wantarray?

Alien has asked for the wisdom of the Perl Monks concerning the following question:

Hi Monks !
I was writing some code today and I stumbled across something I find curious :

sub testc1 {
if(defined wantarray()) {
        print "scalar\n";
}
elsif(wantarray()) {
        print "array\n";
}
else {
        print "void context\n";
  }
}
testc1();
$a=testc1();
@a=testc1();
[download]

and

sub testc2 {
if(wantarray()) {
        print "array\n";
}
elsif(defined wantarray()) {
        print "scalar\n";
}
else {
        print "void context\n";
}
}
$x=testc2();
@x=testc2();
testc2();
[download]

My question is : how come testc2 prints the right calling contexts but testc1 doesn't ?

Comment on How do I check the return value of wantarray? Select or Download Code

Replies are listed 'Best First'.
Re: How do I check the return value of wantarray? by imp (Priest) on Apr 04, 2007 at 15:09 UTC
Because in the first case it isn't possible to reach the branch that tests if wantarray is true, since it is an elsif after testing if it is defined. `if(defined wantarray()) { } elsif(wantarray()) { }` [download]	[reply] [d/l]
Re: How do I check the return value of wantarray? by mreece (Friar) on Apr 04, 2007 at 15:35 UTC
as this demonstates, wantarray is defined in both scalar and list context (which is why your testc1 fails), but true only in list context. `sub testc { if (defined wantarray() and not wantarray()) { print "scalar\n"; } elsif (defined wantarray() and wantarray()) { print "array\n"; } elsif (not defined wantarray()) { print "void\n"; } } my $x = testc(); my @x = testc(); testc();` [download]	[reply] [d/l]
Re: How do I check the return value of wantarray? by kyle (Abbot) on Apr 04, 2007 at 15:14 UTC
Well, one's written correctly and the other isn't. It seems as though you think that wantarray being defined is separate from whether it's true or false when actually your `testc1` will say 'scalar' for the instance where it is both defined and true.	[reply] [d/l]
Re: How do I check the return value of wantarray? by derby (Abbot) on Apr 04, 2007 at 15:32 UTC
wantarray's kinda wonky - returning 1 if list context, "" if scalar and undefined if void -- you can normalize that to 1 for list, 0 for scalar and -1 for void. The only trick is to force the "" into an integer (IV) by doing an integer operation (+0). `my $w = defined wantarray() ? wantarray()+0 : -1; my %v = ( -1 => 'void', 0 => 'scalar', 1 => 'array' ); print $v{$w}, " context\n";` [download] -derby	[reply] [d/l]