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Re: Unusual Closure Behaviour

by artist (Parson)
on Jul 12, 2001 at 20:12 UTC ( #96088=note: print w/replies, xml ) Need Help??


in reply to Unusual Closure Behaviour

HI, Try this, Here it says that $x is not defined and therefore it creates a new local variable $x with 'my'.
Otherwise it uses the last value of x which is the default behaviour.
sub foo { my $x if !defined $x; return ++$x; } for (1..7){ print foo(); };

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(tye)Re2: Unusual Closure Behaviour
by tye (Sage) on Jul 13, 2001 at 03:20 UTC

    Actually, that code isn't doing what you think. One way to see is to just add "use strict" to it. You'll get: Global symbol "$x" requires explicit package name

    A more fun way is:

    sub foo { my $x= $_[0] if ! defined $x; print "(@_):", ++$x, " "; undef $x if @_ && shift; } foo($_) for( 0,0,5,4,3,0,5,0,4,0,3 ); print $/, $x= 10, $/; foo($_) for( 0,0,5,4,3,0,5,0,4,0,3 ); print $/;
    which produces:
    (0):1 (0):1 (5):6 (4):5 (3):4 (0):1 (5):6 (0):1 (4):5 (0):1 (3):4 10 (0):1 (0):2 (5):3 (4):1 (3):1 (0):1 (5):2 (0):1 (4):2 (0):1 (3):2
    The $x in "if !defined $x" is the global $main::x because my variables can't be used until after the end of the statement in which they were declared.

    So your code is always initializing the lexical $x in your subroutine because you have never defined the global $main::x that you are checking against.

            - tye (but my friends call me "Tye")

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[Corion]: You'll have to look somewhere esoteric for that. Maybe some tied variable or special dualvar can also trigger that. But it's certainly not a common occurrence
[Corion]: And on 5.20, the following also outputs no find:perl -wle 'for my $x ("\x{2000}".."\ x{1fffff}") { if( $x && ! length $x ) { warn qq(<$x>); warn length $x; die } }'
[Corion]: (this time on Unix)
[hippo]: Understood. I'll have to go through the code and see if it's doing anything fancy with ties, dual-vars or non-scalars. In the end, it's probably a bug though.
[Corion]: Aaah - you should be able to do this with overload, but I would hit somebody really hard if they constructed objects that are true but the empty string, and you not knowing about the domain knowledge where this makes sense
[Eily]: you could tie a variable into not having the same value each time, if you like to make people who try to debug your code facepalm
[Corion]: perl -wle 'package o; use overload q("") => sub {warn "str"; ""}, bool => sub{warn "bool"; 1}; package main; my $o={}; bless $o => o; print "Yay" if ($o && !length($o))'
[Corion]: But people writing such code should document the objects they construct and why it makes sense for an object to be invisible as string while being true in a boolean context

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