[Haskell-cafe] Type families - how to resolve ambiguities?

25 Aug 2010

      Hi,

The code below defines a type synonym family:

{-# LANGUAGE MultiParamTypeClasses, TypeFamilies #-}
{-# LANGUAGE FlexibleInstances, TypeSynonymInstances #-}

data Vect k b = V [(b,k)] deriving (Eq,Show)

data TensorBasis a b = T a b deriving (Eq, Ord, Show)

type family Tensor k u v :: *

type instance Tensor k (Vect k a) (Vect k b) = Vect k (TensorBasis a b) 

class Algebra k v where -- "v is a k-algebra"
    unit :: k -> v
    mult :: Tensor k v v -> v

instance Algebra Integer (Vect Integer [Int]) where
    unit 0 = V []
    unit x = V [([],x)]
    mult (V ts) = V [(g++h,x) | (T g h, x) <- ts]

Naively I think of the type instance declaration as saying that the two types
are synonyms, so I should be able to use one in a context where the other
is expected. However, when I try to use the code, I don't seem to be able
to get the type inferencer to recognise an object as being of both "types" at
the same time. For example:

*> mult $ (V [(T [1] [2],3)] :: Vect Integer (TensorBasis [Int] [Int]))

<interactive>:1:8:
    Couldn't match expected type `Tensor k v v'
           against inferred type `Vect Integer (TensorBasis [Int] [Int])'
      NB: `Tensor' is a type function, and may not be injective
    In the second argument of `($)', namely
        `(V [(T [1] [2], 3)] :: Vect Integer (TensorBasis [Int] [Int]))'
    In the expression:
          mult
        $ (V [(T [1] [2], 3)] :: Vect Integer (TensorBasis [Int] [Int]))
    In the definition of `it':
        it = mult
           $ (V [(T [1] [2], 3)] :: Vect Integer (TensorBasis [Int] [Int]))

*> mult $ (V [(T [1] [2],3)] :: Tensor Integer (Vect Integer [Int])
(Vect Integer [Int]))

<interactive>:1:8:
    Couldn't match expected type `Tensor k v v'
           against inferred type `Vect Integer (TensorBasis [Int] [Int])'
      NB: `Tensor' is a type function, and may not be injective
    In the second argument of `($)', namely
        `(V [(T [1] [2], 3)] ::
            Tensor Integer (Vect Integer [Int]) (Vect Integer [Int]))'
    In the expression:
          mult
        $ (V [(T [1] [2], 3)] ::
             Tensor Integer (Vect Integer [Int]) (Vect Integer [Int]))
    In the definition of `it':
        it = mult
           $ (V [(T [1] [2], 3)] ::
                Tensor Integer (Vect Integer [Int]) (Vect Integer [Int]))

Are type families the right mechanism to express what I'm trying to express?
If so, what am I doing wrong, and how do I fix it?

[Haskell-cafe] Type families - how to resolve ambiguities?

DavidA