Re: Unusual Closure Behaviour

[Abigail]

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

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

[BrowserUk]

Come back Abigail(II). This place needs you.

[shmem]

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

[Anonymous Monk]

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";

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...

[Anonymous Monk]

 Deprecated use of my() in false conditional at - line 6.