Is there a way in Haskell to query thread state using ThreadID after a forkIO?
This is not possible with the standard base libraries as far as I know, but you can use the GHC specific API to get a thread's status:
import GHC.Concalive :: ThreadID -> IO Boolalive = fmap (== ThreadRunning) . threadStatus
Different definition extending on dflemstr's answer to also account for when the thread is blocked. I think it also counts as alive, as pretty soon it's executing code again, once the reason for why it's blocked is taken care of (e.g. an MVar is written to, an STM transaction on retry completes, etc.):
import GHC.Concimport Control.MonadisThreadStatusBlocked :: ThreadStatus -> BoolisThreadStatusBlocked (ThreadBlocked _) = TrueisThreadStatusBlocked _ = FalseisAlive :: ThreadId -> IO BoolisAlive = fmap (liftM2 (||) (ThreadRunning ==) isThreadStatusBlocked) . threadStatus