Usage patterns strike back

[kappa]

Not that evil as most of the posts here. Ok, not evil at all. But my colleagues are regularly falling into this trap so I thought it could be of some interest.

while(<DATA>){last};while(-<DATA>){print}
__DATA__
Just another Perl hacker
This gotcha bites too many...

[chip]

Interestingly, unary minus could default to string or numeric behavior and the trap would still bite.

[halley]

The difference is the magical parsing of while (<HANDLE>). Try with B::Deparse:

while (defined($_ = <DATA>)) {
    last;
}

-but-

while (-<DATA>) {
    print $_;
}

The second form doesn't actually assign the line into any variable.

(Of course, a negative "Just" is still not negative.)

[theguvnor]

Explanation please? I'd like to avoid the trap but I'm not savvy enough to quite take your meaning.

[kappa]

You can run it. Then remove that '-' and run again. Compare results. Hm, work it out for yourself, you'll get much more satisfaction, really :)

[muba]

Ok, I do understand what's happening, but I don't understand why somebody would want to read data from a HANDLE and then get a negative value of it? My first thought was that -<HANDLE> would maybe read from the last line of the file towards the first line, but that does not seem to be the case. So, what's the trap? Who wants to do this? And what is their target?

[kappa]

Oh, that's just a short and 'obfu' way of turning <HANDLE> into an expression involving <HANDLE> for the sake of this very example.