In the future you might get more responses if you also post the error
message. I pasted your code, commented out the external stuff, and
got:
• Illegal instance declaration for
‘Operation (Tree t) (Tree t) (Vect k (Tree t))’
The coverage condition fails in class ‘Operation’
for functional dependency: ‘a b -> c’
Reason: lhs types ‘Tree t’, ‘Tree t’
do not jointly determine rhs type ‘Vect k (Tree t)’
Un-determined variable: k
• In the instance declaration for
‘Operation (Tree t) (Tree t) (Vect k (Tree t))’
So it's saying that, while you state with `a b -> c` that types `a`
and `b` together determine `c`, this is not the case for the `(Tree t)
(Tree t)` instance, since they don't determine what type variable `k`
should be. I don't know the package or domain you're working with, but
that might help you get further.
Erik
On 28 February 2016 at 22:00, Kristoffer Føllesdal
I am trying to use Functional dependencies to overload a operation on Vector space and its basis (Use the Vector spaces module Math.Algebras.VectorSpace). I have tried to mimic the example for matrices and vectors from https://wiki.haskell.org/Functional_dependencies. I have tried different ways of defining classes and instances, but I do not get it to work.
What I want is to have the «same» function for these cases:
operation :: a -> a -> Vect k a operation :: a -> Vect k a -> Vect k a operation :: Vect k a -> a -> Vect k a operation :: Vect k a -> Vect k a -> Vect k a
Her are som sample code to illustrate what I want. Do anybody have an idea to how to solve it?
{-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE FunctionalDependencies #-}
import Math.Algebras.VectorSpace
linearExtension :: (Eq k, Num k, Ord a) => (a -> a -> Vect k a) -> Vect k a -> Vect k a -> Vect k a linearExtension f xs ys = linear (\x -> linear (f x) ys) xs
data Tree t = Root t [Tree t] deriving(Eq, Show, Ord)
op :: (Eq k, Num k, Ord t) => Tree t -> Tree t -> Vect k (Tree t) op x y = return x <+> return y
opA :: (Eq k, Num k, Ord t) => Vect k (Tree t) -> Vect k (Tree t) -> Vect k (Tree t) opA = linearExtension op
class Operation a b c | a b -> c where (<.>) :: a -> b -> c
instance (Ord t) => Operation (Tree t) (Tree t) (Vect k (Tree t)) where (<.>)= op
instance (Ord t) => Operation (Vect k (Tree t)) (Vect k (Tree t)) (Vect k (Tree t)) where (<.>) = opA
instance (Ord t) => Operation (Tree t) (Vect k (Tree t)) (Vect k (Tree t)) where (<.>) x = opA (return x)
instance (Ord t) => Operation (Vect k (Tree t)) (Tree t) (Vect k (Tree t)) where (<.>) x y = opA x (return y)
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