http://www.perlmonks.org?node_id=96339


in reply to Re: Re: Unusual Closure Behaviour
in thread Unusual Closure Behaviour

Well, technically what is happening is not violating Camel III. See, it *does* get its own scratchpad, which can be quickly checked by: 
    #!/opt/perl/bin/perl -w
    use strict;
    sub foo;
    sub foo {
        return unless $_ [0];
        my $x if 0;
        print ++ $x, " ";
        foo $_ [0] - 1;
    }
    
    foo 1; print "\n";
    foo 2; print "\n";
    foo 3; print "\n";
    foo 4; print "\n";
    __END__
    1 
    2 1 
    3 2 1 
    4 3 2 1

[download]
Scary, isn't? When the subroutine recurses, it notices there is still a reference to $x and hence it will create a new scratchpad. But when there is no reference, it will reuse an old scratchpad....

I do agree that using my in this way doesn't tend to lead to well understood code. I wouldn't go as far as to say it's never useful, but such cases will be very rare and it's not a technique I would teach in my classes.

-- Abigail

Replies are listed 'Best First'.
Re^2: Unusual Closure Behaviour
by BrowserUk (Patriarch) on Sep 12, 2015 at 23:55 UTC
[reply]
      This place needs you.

      So true.

      What happens here? Does this mean that each scratchpad is persistent at its recursion level?

      use strict;
sub foo;
sub foo {
    return unless $_ [0];
    my $x if undef;
    print ++ $x, " ";
    foo $_ [0] - 1;
}
foo 1; print "\n";
foo 2; print "\n";
foo 3; print "\n";
foo 7; print "\n";
foo 11; print "\n";
foo 9; print "\n";
foo 15; print "\n";
__END__
1 
2 1 
3 2 1 
4 3 2 1 1 1 1 
5 4 3 2 2 2 2 1 1 1 1 
6 5 4 3 3 3 3 2 2 
7 6 5 4 4 4 4 3 3 2 2 1 1 1 1

      [download]
      perl -le'print map{pack c,($-++?1:13)+ord}split//,ESEL'
[reply]
[d/l]
Re^2: Unusual Closure Behaviour
by Anonymous Monk on Sep 12, 2015 at 23:24 UTC
    Have you tried this? 
    #!/usr/bin/perl
use strict;
    sub foo;
    sub foo {
        return unless $_ [0];
        my $x if undef;
        print ++ $x, " ";
        foo $_ [0] - 1;
    }

    foo 29; print "\n";

    [download]
    It prints:

    1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

    Hum...

[reply]
[d/l]
       Deprecated use of my() in false conditional at - line 6.
[reply]
[d/l]

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