Segfaults somewhere between 500 and 1000 iterations.

fork.hs

import Control.Monad (when)
import System.Exit (exitSuccess)
import System.Posix.Process (forkProcess)

fork_ :: Integer -> IO ()
fork_ n | n > 0 = do pid <- forkProcess (fork_ $ n - 1)
                     when (pid /= 0) exitSuccess
        | otherwise = putStrLn "I'm done!"

main :: IO ()
main = fork_ 1000