Concurrent Haskell

I am trying to create a program that prints interleaved sequences of 'a's and 'z's, using Haskell Direct (under GHC). I've tryied the following: import Concurrent main :: IO() main = let loop ch = putStr(ch) >> loop ch in forkIO (loop "a") >> loop "z" But this program prints only 'z's, and no 'a's. I was surprised by this, since GHC uses preemptive multitasking, not cooperative multitasking. Does anyone knows what's going wrong? Thanks, Andre W b Furtado