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 tailrecursive". Fibonacci is an example of an inherently recursive algorithm.
sub fibonacci {
my ($n) = @_;
return 0 if $n == 0;
return 1 if $n == 1;
return sum fibonacci($n2), fibonacci($n1);
}
At face value, it's quite easy to make it threaded:
sub fibonacci {
my ($n) = @_;
return 0 if $n == 0;
return 1 if $n == 1;
return sum map $_>join(),
async { fibonacci($n2) },
async { fibonacci($n1) };
}
But what if you wanted to limit the number of threads (perhaps to limit overhead)? One solution would be to have a pool of threads and to use them if available.
sub fibonacci {
my ($n) = @_;
return 0 if $n == 0;
return 1 if $n == 1;
return sum map $_>get(),
async_maybe { fibonacci($n2) },
async_maybe { fibonacci($n1) };
}
(The above assumes closures can be shared, but it can be rewritten to not use closures.)
When implementing async_maybe (and the get of the object it returns), one must be extra careful to avoid the situation where a thread is waiting to have it's result collected.
But what if you want a worker model (perhaps to distribute the work to other machines)? Now, that's hard. One would need some kind of a callback system.
sub fibonacci {
my ($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] }, $n2) });
process(sub { fibonacci_task(sub { $y = $_[0] }, $n1) });
#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.
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