Can't infer type (type family as "element" type)
Suppose I have a class C,
class C a where type E a c :: E a -> a -> a
a datatype T,
data T a = T a
and an instance of C for T
instance C (T a) where type E (T a) = a c x (T _) = T x
I would like to write a function such as f
f t@(T x) = c x t
without a type signature. Unfortunately, I can't because GHC tells me
Couldn't match expected type `t' against inferred type `T (E t)' In the second argument of `c', namely `t' In the expression: c x t In the definition of `f': f (t@(T x)) = c x t
There are at least three possible ways to write the above code such that it works.
(1) Give a type signature for f
f :: T a -> T a
(2) Define the class C using an equality constraint
class C t where type E t c :: (E t ~ e) => e -> t -> t
(3) Define the class C using functional dependencies
class C t e | t -> e where c :: e -> t -> t
But the real question is why don't I get a type for f?
This has been tested in GHC 6.10.1 and 6.12.1.
|Component||Compiler (Type checker)|