in reply to Perl disproves commutativity of Addition/Multiplication

The subs pragma is a red herring. Remove it, and the problem still holds.

The explanation is in the constant pragma, and some book I cannot remember explains that you should use () for constant-style sub calls because non_commutative * 10 parses as non_commutative(*10).

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Re^2: Perl disproves commutativity of Addition/Multiplication
by syphilis (Archbishop) on Jan 23, 2012 at 23:41 UTC
    The subs pragma is a red herring. Remove it, and the problem still holds.

    Quite so.
    If I'm not mistaken, moving the definition of the non_commutative sub to the end of the script *does* create a purpose for loading the 'subs' pragma iff we want to avoid the use of parens - and it would have been better if the scripts I had presented did precisely that: 
    use warnings;
use subs qw(non_commutative); #necessary

$x = 10 * non_commutative;
$y = non_commutative * 10;

print "$x\n$y\n\n";

$x = 10 + non_commutative;
$y = non_commutative + 10;

print "$x\n$y\n";

sub non_commutative {return 17}

__END__
Outputs:
170
17

27
17

    [download]
    and 
    use warnings;
use subs qw(non_commutative); #unnecessary

$x = 10 * non_commutative();
$y = non_commutative() * 10;

print "$x\n$y\n\n";

$x = 10 + non_commutative();
$y = non_commutative() + 10;

print "$x\n$y\n";

sub non_commutative {return 17}

__END__
Outputs:
170
170

27
27

    [download]
    Cheers,
    Rob
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