in reply to [OT]: threading recursive subroutines.

Good question! I've been working on it to see what the real issue is. I don't have an answer for you; I'm just documenting what I've found for everyone's benefit.

First, I presume that "inherently recursive" means "cannot be rewritten to be tail-recursive". Fibonacci is an example of an inherently recursive algorithm.

sub fibonacci { my ($n) = @_; return 0 if $n == 0; return 1 if $n == 1; return sum fibonacci($n-2), fibonacci($n-1); }

At face value, it's quite easy to make it threaded:

sub fibonacci { my ($n) = @_; return 0 if $n == 0; return 1 if $n == 1; return sum map $_->join(), async { fibonacci($n-2) }, async { fibonacci($n-1) }; }

But what if you wanted to limit the number of threads (perhaps to limit overhead)? One solution would be to have a pool of threads and to use them if available.

sub fibonacci { my ($n) = @_; return 0 if $n == 0; return 1 if $n == 1; return sum map $_->get(), async_maybe { fibonacci($n-2) }, async_maybe { fibonacci($n-1) }; }

(The above assumes closures can be shared, but it can be rewritten to not use closures.)

When implementing async_maybe (and the get of the object it returns), one must be extra careful to avoid the situation where a thread is waiting to have it's result collected.

But what if you want a worker model (perhaps to distribute the work to other machines)? Now, that's hard. One would need some kind of a callback system.

sub fibonacci { my ($n) = @_; my $result; process_and_wait(sub { fibonacci_task(sub { $result = $_[0]; }, $n ); }); return $result; } sub fibonacci_task { my ($on_complete, $n) = @_; return $on_complete->(0) if $n == 0; return $on_complete->(1) if $n == 1; my ($x,$y); process(sub { fibonacci_task(sub { $x = $_[0] }, $n-2) }); process(sub { fibonacci_task(sub { $y = $_[0] }, $n-1) }); #TODO: The last of the two tasks to complete # must call $on_complete->($x+$y). }

The TODO item is hard to do cleanly. And of course, I'm still using closures even though I think that's not an option for threads threads. Rewriting to avoid closures will likely make the code even longer.