Junctions

Junctions are one of the simplest examples of where Perl 6 can work in parallel. In the version of Rakudo, which is available at the time of writing this book, this feature is not fully implemented.

A junction is a value that keeps many values at the same time. Examine the following code:

my $j = 1 | 3 | 5;
say 'OK' if $j == 3;
say 'Not OK' if $j != 2;

The variable $j is a junction that keeps three odd numbers, 1, 3, and 5. You may compare $j with an integer and get the Boolean True if the value is one of the values hosted by the junction. In the case of comparing with 3, the result is True , while the second comparison with 2 fails.

Autothreading

Now try passing a junction to a function that takes a scalar:

sub f($x) {
    say $x;
    return $x;
}

say 'OK' if f(1 | 3 | 5) == 3;
say 'Not OK' if f(1 | 3 | 5) != 2;

The behavior is intuitively understandable—the function is executed for each of the values of the junction separately. The values that the function returns are then used as...