All,
There was a recent post on the Perl6 Language mailing list asking if there would be a way to identify code as "do it once only". Here is an example. 
while ( <FH> ) {
    next if $_ eq 'foo'; # Where foo can only appear once in the file
    print;
}

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Once the match has been made, why must every subsequent line in the file be tested? I have also desired this ability in long tight running loops when performance was a concern and tried to figure out a way to make it work. I came up with the following inelegant code: 
#!/usr/bin/perl
use strict;
use warnings;

do_something(5);

sub do_something {
    my $target = shift;
    my $block;
    my $block2 = sub {
        print "No need to check $_ against $target anymore\n";
    };
    my $block1 = sub {
        if ( $_ == $target ) {
            print "Skipping $target\n";
            $block = $block2;
        }
        else {
            print "$_ is not $target\n";
        }
    };
    $block = $block1;
    for ( 1 .. 9 ) {
        $block->();
    }
}

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Once the condition has been met, the code dynamically changes to no longer check for the condition. Keep in mind, to get this luxury I am now paying for the overhead of dereferencing and invoking a sub each time to get this "performance" benefit.

I was wondering what your thoughts on this are? If you think of running code as a work flow and each piece of functionality a widget then what we are talking about is a special kind of widget that can replace another widget while the work flow is running. Would building this functionality in the Perl 6 core be of great benefit? Obviously providing the functionality would open the door for other applications then just the "run once" feature. If this ability existed with negligible costs - what other things could you think of to do with it?

Cheers - L~R

Update: I removed some irrelevant pieces of the code which were leftover from my trial and error attempts

In reply to Doing "it" only once by Limbic~Region

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