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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