I want a function fun :: q -> Either a b where q is of type a or b. fun shall work in the following way fun x gives Left x if x :: a fun x gives Right x if x :: b Following is a more precise description of what I want. I have function fun1 and fun2 newtype Vec k b = V [(b,k)] deriving (Eq,Ord,Show) fun1 :: (Num k) => a -> Either a (Vec k a) fun1 = Left fun2 :: (Num k) => Vec k a -> Either a (Vec k a) fun2 = Right that work in the following way {-
fun1 6 Left 6
fun2 (V[(6,1)]) Right (V [(6,1)]) -}
I want a overloaded function ‘fun' such that {-
fun 6 Left 6
fun (V[(6,1)]) Right (V[(6,1)]) -}
I have tried to use a type class to do this (see code below). But when I try
fun 6
I get the following error Could not deduce (Num a0) from the context (Num a, Fun a b v) bound by the inferred type for ‘it’: (Num a, Fun a b v) => Either b (v b) at <interactive>:70:1-5 The type variable ‘a0’ is ambiguous When checking that ‘it’ has the inferred type it :: forall a b (v :: * -> *). (Num a, Fun a b v) => Either b (v b) Probable cause: the inferred type is ambiguous Is the someone that have know how I can solve this? Kristoffer ——— CODE ——— {-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE FlexibleInstances #-} newtype Vec k b = V [(b,k)] deriving (Eq,Ord,Show) fun1 :: (Num k) => a -> Either a (Vec k a) fun1 = Left fun2 :: (Num k) => Vec k a -> Either a (Vec k a) fun2 = Right class Fun a b v where fun :: a -> Either b (v b) instance (Num k) => Fun t t (Vec k) where fun = fun1 instance (Num k) => Fun (Vec k t) t (Vec k) where fun = fun2