[Haskell-cafe] Funct dependency conflict when using types

[Brian Hulley]

Frank wrote:
> I have observed the error message (ghci 6.4)
>
> Functional dependencies conflict between instance declarations:
>      LocTest.hs:16:0: instance B S Bool
>      LocTest.hs:22:0: instance (F f, B a b) => B (f a) (f b)
>
> when I changed in an instance from a simple type to a type with a
> parameter (i.e. from Int to (Int, Bool)).
>
> The following minimal program demonstrates the problem:
>
> an instance with type S (= T Float) conflicts with B (f a) (f b)
>
> whereas the comparable instance with data R (= R Float) does not.
>
> Is this intentional? How can it be avoided?
> Help is greatly appreciated!
>
> Andrew
>
> ----------------------------------------
> class  F f where
>    m2 :: (a -> b -> c) -> f a -> f b -> f c        --zipWith
>
> class B a b | a -> b where
>    dist2 :: a -> a -> b
>
> instance B S Bool where -- line 16
>    dist2 a d = True
>
> instance B R Float where
>    dist2 a d = 1
>
> instance (F f, B a b) => B (f a) (f b) where -- line 22
>    dist2 a b = m2 dist2 a b
>
> data T f = T f
> type S = T Float
> data R = R Float

Suppose you later declared T as an instance of F so that one particular F f 
would be F T.

Then if S = T Float then one instantiation of (f a) could be (T a) where f = 
T, so the two instantiated instance decls would look like:

instance B (T Float) Bool where -- line 16
instance B (T a) (T b) where -- line 22

and the functional dependency says that the 'b' (in the class decl) would be 
Bool by line 16 and T b for some b by line 22 if 'a' was instantiated to 
Float. However there is no 'b' such that Bool = T b.

Regards, Brian.