There Is More Than One Way At The Same Time

The Perl 6 Rosattacode section for parallel calculations is terribly outdated and missing all the goodies that where added or fixed in the last few weeks. With this post I would like to propose an updated version for Rosettacode. If you believe that I missed something feel free to comment below. Please keep in mind that Rosettacode is for showing off, not for being comprehensive.

use v6.d.PREVIEW;

 

Perl 6 provides parallel execution of code via threads. There are low level custructs that start a thread or safely pause execution.

my $t1 = Thread.start({ say [+] 1..10_000_000 });
my $t2 = Thread.start({ say [*] 1..10_000 });
$t1.finish;
$t2.finish;

my $l = Lock.new;
$l.lock;
$t1 = Thread.start: { $l.lock; say 'got the lock'; $l.unlock };
sleep 2; $l.unlock;

$t1.finish;

When processing lists, one can use a highlevel Iterator created by the methods hyper and race. The latter may return values out of order. Those Iterators will distribute the elements of the list to worker threads that are in turn assigned to OS level threads by Rakudos ThreadPoolScheduler. The whole construct will block until the last element is processed.

my @values = 1..100;

sub postfix:<!> (Int $n) { [*] 1..$n }

say [+] @values.hyper.map( -> $i { print '.' if $i %% 100; $i!.chars });

For for-lovers there are the race for and hyper for keyword for distributing work over threads in the same way as their respective methods forms.

race for 1..100 {
    say .Str; # There be out of order dragons!
}

my @a = do hyper for 1..100 {
   .Int! # Here be thread dragons!
}

say [+] @a;

Perl 6 sports constructs that follow the reactive programming model. One can spin up many worker threads and use threadsafe Channels or Supplys to move values from one thread to another. A react-block can combine those streams of values, process them and react to conditions like cleaning up after a worker thread is done producing values or dealing with errors. The latter is done by bottling up Exception-objects into Failure-objects that keep track of where the error first occured and where it was used instead of a proper value.

my \pipe = Supplier::Preserving.new;

start {
    for $*HOME {
        pipe.emit: .IO if .f & .ends-with('.txt');

        say „Looking in ⟨{.Str}⟩ for files that end in ".txt"“ if .IO.d;
        .IO.dir()».&?BLOCK when .IO.d;

        CATCH {
            default {
                note .^name, ': ', .Str;
                pipe.emit: Failure.new(.item);
            }
        }
    }
    pipe.done;
}

react {
    whenever pipe.Supply {
        say „Checking ⟨{.Str}⟩ for "Rosetta".“;
        say „I found Rosetta in ⟨{.Str}⟩“ if try .open.slurp.contains('Rosetta');
        LAST {
            say ‚Done looking for files.‘;
            done;
        }
        CATCH {
            default {
                note .^name, ': ', .Str;
            }
        }
    }
    whenever Promise.in(60*10) {
        say „I gave up to find Rosetta after 10 minutes.“;
        pipe.done;
        done;
    }
}

Many build-in objects will return a Supply or a Promise. The latter will return a single value or just convey an event like a timeout. In the example above we used a Promise in that fashion. Below we shell out to find and process its output line by line. This could be used in a react block if there are many different types of events to process. Here we just tap into a stream of values and process them one by one. Since we don’t got a react block to provide a blocking event loop, we wait for find to finish with await and process it’s exitcode. Anything inside the block given to .tap will run in its own thread.

my $find = Proc::Async.new('find', $*HOME, '-iname', '*.txt');
$find.stdout.lines.tap: {
    say „Looking for "Rosetta" in ⟨$_⟩“;
    say „Found "Rosetta" in ⟨$_⟩“ if try .open.slurp.contains('Rosetta');
};

await $find.start.then: {
    say „find finished with exitcode: “, .result.exitcode;
};

Having operators process values in parallel via threads or vector units is yet to be done. Both hyper operators and Junctions are candidates for autothreading. If you use them today please keep in mind side effects may provide foot guns in the future.