Well, that took longer than I expected. ;-)

I just uploaded a new release of Var::Pairs (version 0.003005)
that solves the problem of resetting an iterator on premature termination
of the surrounding loop. It does so by using Scope::Upper::reap()
to install a "destructor" for each iterator when execution leaves
the surrounding scope.

Take another look; I think it's likely this technique would also work for you.

Damian

I think it's likely this technique would also work for you.

In fact, more than just likely...

sub from (&$) {
    use Devel::Callsite qw< callsite context >;
    use Scope::Upper    qw< reap UP >;

    my ($iterator_source) = @_;

    # Uniquely identify this call, according to its lexical context...
    my $ID = callsite() . context();

    # Install the iterator...
    state %iterator_for;
    $iterator_for{$ID} //= $iterator_source->();

    # Install destructor for iterator at end of caller's block...
    reap { delete $iterator_for{$ID} } UP UP;

    # Iterate...
    return $iterator_for{$ID}->($_[1]);
}


for my $limit (reverse 1..5) {
    while ( from{countdown($limit)} my $a ) {
        print "$a: ";
    }
    print "\n";
}

sub countdown{
    my $val = shift;

    my $iter = sub {
        if ($val--) {
            $_[0]=$val;
            return 1;
        }
        return;                         # stop iteration
    };

    return $iter;

}
[download]

What the heck...I just uploaded another new release of Var::Pairs
(version 0.004000), which lets you write this:

use Var::Pairs;

sub countdown {
    my $n = shift;

    return sub {
        return if $n < 0;  # Indicates iterator exhausted
        return $n--;       # Next value iterated
    }
}

for my $limit (reverse 1..5) {
    while (my ($a) = each_value countdown($limit)) {
        print $a, ':';
    }
    say q{};
}
[download]

...which is very close to what you wanted, I believe.

(And, yes, the each_value() subroutine is doing a lot of very hard paddling
under the surface to correctly ignore all those repeated calls to countdown($limit)
in the while loop. ;-)

And, obviously, it would be far more efficient to write the iteration:

for my $limit (reverse 1..5) {
    my $iter = countdown($limit);
    while (my ($a) = each_value $iter) {
        print $a, ':';
    }
    say q{};
}
[download]

Damian