Thank you for your reply, I'd come up with the following:

timed :: Int → IO a → b → IO (Either b a)
timed max act def = do

  r ← new
 
  t ← forkIO $ do
    a ← act
    r ≔ Right a

  s ← forkIO $ do
    wait max
    e ← em r
    case e of
      True  → do
        kill t
        r ≔ Left def

      False → return ()

  takeMVar r

---------------------

*Network.Port.Scan> timed 500 (wait 50000 >> return 0) 'x'
Left 'x'
*Network.Port.Scan> timed 500000 (wait 50000 >> return 0) 'x'
Right 0

---------------------

before reading your reply:

timed timeout act fallback = do

   res <- newEmptyMVar

   tid <- forkIO $ act >>= writeMVar res

   threadDelay timeout

   stillRunning <- isEmptyMVar res

   if stillRunning then killThread tid >> return fallback else takeMVar res

---------------------

*Network.Port.Scan> timed2 500 (wait 50000 >> return 0) 'x'

<interactive>:1:33:
    No instance for (Num Char)
      arising from the literal `0' at <interactive>:1:33
    Possible fix: add an instance declaration for (Num Char)
    In the first argument of `return', namely `0'
    In the second argument of `(>>)', namely `return 0'
    In the second argument of `timed2', namely
        `(wait 50000 >> return 0)'