Given the following code: {-# LANGUAGE GADTs #-} data Any a where AInt :: Int -> Any Int -- demo 1 does not compile {- demo1 a = do case a of (AInt i) -> print i Couldn't match expected type ‘t’ with actual type ‘IO ()’ ‘t’ is untouchable inside the constraints (t1 ~ Int) bound by a pattern with constructor AInt :: Int -> Any Int, in a case alternative at /Users/doaitse/TryHaskell/TestGADT.hs:6:17-22 ‘t’ is a rigid type variable bound by the inferred type of demo1 :: Any t1 -> t at /Users/doaitse/TryHaskell/TestGADT.hs:5:1 Relevant bindings include demo1 :: Any t1 -> t (bound at /Users/doaitse/TryHaskell/TestGADT.hs:5:1) In the expression: print i In a case alternative: (AInt i) -> print i Failed, modules loaded: none. -} -- all the following go through without complaints: a = AInt 3 demo2 = do case a of (AInt i) -> print i demo3 :: IO () demo3 = do case a of (AInt i) -> print i demo4 = do case AInt 3 of (AInt i) -> print i demo5 :: Any Int -> IO () demo5 a = do case a of (AInt i) -> print i I do not see why the error message in demo1 arises. It claims it can't match some t with the type IO (), but when I tell that the result is IO () it can? I think at least the error message is confusing, and not very helpful. I would have in no way been able to get the clue that add a type signature as in the case of demo5 would solve the problem. What am I overlooking? Doaitse