Software Transactional Memory comes to Pugs.

As of r10528, the internal STM engine in the Haskell runtime is finally exposed to the userland:

my ($x, $y);
async {
    atomically {
        $x = $y + 1;
        $y = $x + 1;
    }   
} for 1..5;
say "($x, $y)" # Guaranteed to be (9, 10) 

I'd like to thank nothingmuch++ for demanding -- and sponsoring -- this wonderful language feature. See the slides and this paper and another paper for more about STM. More exciting combinators such as orElse, retry and assert coming soon!