Fun with Prototypes

[haukex]

Can you tell me:

1. How does this get parsed? (e.g. try adding parentheses to show precedence)
2. Given that the body of each of the four subs is { print Dumper(\@_) }, what is the output?

$_ = "#x";
my @a = qw/b c/;

foo /#(\w+)/, @a;
bar /#(\w+)/, @a;
quz /#(\w+)/, @a;
baz /#(\w+)/, @a;

Hint:

<Reveal this spoiler or all spoilers in this node or all in this thread>

Solution:

1. Parsing according to perl -MO=Deparse,-p (with some edits for clarity):

foo( /#(\w+)/, @a );
(  bar() /  # division; "#(\w+)/, @a;" is a comment
   quz( /#(\w+)/ )
), @a;  # void context
&baz( scalar(/#(\w+)/), \@a );

2. Output (again slightly edited, and without warnings):

$VAR1 = ['x','b','c'];   # sub foo
$VAR1 = [];              # sub bar ()
$VAR1 = [1];             # sub quz ($)
$VAR1 = [1, ['b','c']];  # sub baz ($\@)

See Far More than Everything You've Ever Wanted to Know about Prototypes in Perl -- by Tom Christiansen. The code was inspired by merlyn's great node On Parsing Perl.

[LanX]

I think you shouldn't hide the "hint", or at least rename it to "missing info".

Hint implies that it's already solvable.

[haukex]

Thanks for the comments, and I did think about it - but I think I want to keep the "trick question" aspect of the node for now ;-)

Also thank you for your other replies, that's certainly getting pretty tricky with the parsing!

[LanX]

More fun for you: ;)

Some people want debug functions to disappear without footprint in production.

Like with Smart::Comments but without source filter.

Try to find a way where changing the prototype of a function &dmp will turn the call to a no op.

Your bar /#(\w+)/, @a; is already quite close, but it's still involving a division. :)

[tobyink]

Isn't this sufficiently clear?

use constant DEBUG => 0;
use Data::Dumper;

# ...

warn Dumper($myvar) if DEBUG;

Or PerX::Assert.

[LanX]

> Isn't this sufficiently clear?

> ...

> warn Dumper($myvar) if DEBUG;

clear but unnecessarily verbose.

[LanX]

an example avoiding a sub-call, unfortunately the division in void context is not optimized away.
<Reveal this spoiler or all in this thread>

and the warning will create runtime overhead, too :/

update

But I think a little overhead is acceptable when debugging.

another edge case are empty arrays causing a division by zero...

update:

fixed minor bug

[Laurent_R]

I don't quite understand what you're after, but you can check this module: B::Deparse

Update: Sorry haukex, the spoilers did not work when I first tried to click on them. Now that I can see them, I understand your point.