I just hit a similar error the other day. I think the gist of it is that there are two perfectly good types, and neither is more general than the other. A slightly different example shows why more clearly: foo (AInt i) = (3 :: Int) Now, what type should this have? foo :: Any a -> a foo :: Any a -> Int both seem pretty good, and neither is clearly better. It's not *as* obvious what the two candidates might be for your example, but maybe you could imagine something like these two types: demo1 :: AInt a -> IO () type family Foo a type instance Foo Int = IO () demo1 :: AInt a -> Foo a Again, not too clear that one is better, I think. ~d On 2014-05-13 14:20, S. Doaitse Swierstra wrote:
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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