http://www.perlmonks.org?node_id=885839

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;
}

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).

}