Can't infer type (type family + "element" type)

18 Jan 2010

      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.

Thanks,
Sean

Sean Leather

Simon Peyton-Jones

Sean Leather

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