The keyword 'once' is very misleading. We're talking about optimizing away checking of a conditional - but it's not true that this conditional is checked only once. It continues to be checked (and needs to be checked) until it happens to be true.

Perl6 will have a FIRST block, meaning the block will be executed only once, and to be specific, the first time the surrounding block was entered. If you write this: 

while (...) {
    if (/foo/) {
        FIRST {...; next}
    }
    ...
}

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and perl can determine there are no side-effects in the conditional, perl could be made to optimize away the conditional once it was true once.

Having said that, the original problem isn't clearly defined. Suppose there would be this 'once-only if', what should the following print: 

my @list = qw /foo bar foo/;
for my $x (1, 2) {
    for (@list) {
        if (/foo/) {  # Assume a 'once-only if'
            print "foo";
        }
     }
}

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Should it print 'foo' once, or twice? That is, is the "once-only" a property of the enclosing loop, to be reset when that loop is reentered, or a property of the condition itself, meaning that after it became true once, it will never be true in the life-time of the program?

In reply to Re^2: Doing "it" only once by Anonymous Monk
in thread Doing "it" only once by Limbic~Region

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