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
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