Am Samstag 05 September 2009 00:06:50 schrieb Alberto G. Corona:
Hi
I need to execute a procedure not in the IO monad, but in an any monad:
I defined:
forkM :: Monad m=> m a -> IO ThreadId forkM proc=forkIO $ proc `seq` return()
I assumed that seq will force the evaluation of proc and after, it will discard his type (m a) and return () in the IO monad.as forkIO expect.
however proc is not executed
Prelude> Control.Concurrent.forkIO $ print "hola" ThreadId 331 "hola" Prelude> Prelude> let forkM p=Control.Concurrent.forkIO $ p `seq` return () Prelude> forkM $ print "hola" ThreadId 493 Prelude>
Any idea?. Thanks in advance
seq forces proc to weak head normal form, that could be e.g. a lambda. You would have to force the value returned by proc, like class (Monad m) => RunnableMonad m where runM :: m a -> a forkM :: (RunnableMonad m) => m a -> IO () forkM proc = let a = runM proc in forkIO $ a `seq` return () (untested, might also not work)