Strangely Consistent

Musings about programming, Perl 6, and programming Perl 6

June 18 2011: Junctions

We've seen junctions a bit already. They showed up as a sidekick to the slightly longer logical operators:

if $answer == 1 || $answer == 3 { ... }

if $answer == 1 | 3 { ... }               # same thing

There's a third way to write it too, and that's by using a function form:

if $answer == any(1, 3) { ... }

The two latter forms mean exactly the same thing; both create a junction, a scalar value masquerading as several different scalar values.

The main thing we'd want to use a junction for is to highlight what it is we want to calculate. It's often the case that we could write things out with a for loop:

my $matched = False;
for @candidates {
    if $answer == $_ {
        $matched = True;
        last;
    }
}
if $matched { ... }

But it's much easier to just "pack" the whole looping into a junction:

if $answer == any(@candidates) { ... }   # same thing!

Under the hood, what happens with a junction is that several different alternatives are tried, and then the final result of the junction is extracted from that. With any, we want at least one alternative to evaluate to a true value; with all; we want all of them to yield something true.

You're free to think of any and all as running all of its alternatives in parallel and coming back with a result. That might actually come true sometime — things going off in sepearate threads and running concurrently — but right now it's just a possible future optimization.

Just to have this said: it's common for people to look at junctions, think they're really cool, and then expect them to do the following:

if any(@contestants).won {
    # do something with the winner
}

But we can't. Junctions answer questions like "does any of these..." or "do all of these..."; they don't pick out the things that match or fail to match and hand them back to you. Their purpose is to treat several things as a unit. If you find you wanted to tease out values of that unit, what you wanted was probably something more like this:

my $winner = @contestants.first({ .won });

if $winner {
    # do something with $winner
}

And with that, we're all set. See you tomorrow with more blogging.