{-# LANGUAGE FlexibleContexts #-}

module SkiFree where

import Test.QuickCheck
import Test.QuickCheck.Checkers

data S n a = S (n a) a deriving (Eq, Show)

instance ( Functor n
         , Arbitrary (n a)
         , Arbitrary a)
  => Arbitrary (S n a) where
      arbitrary = S <$> arbitrary <*> arbitrary

--instance ( Applicative n
--         , Testable (n Property)
--         , EqProp a)
--  => EqProp (S n a) where
--    (S x y) =-= (S p q) =
--        (property $ (=-=) <$> x <*> p) .&. (y =-= q)
instance (Eq (n a), Eq a) => EqProp (S n a) where
    (=-=) = eq

instance Functor n => Functor (S n) where
    fmap f (S x y) = S (fmap f x) (f y)

instance Foldable n => Foldable (S n) where
    foldMap f (S n a) = mappend (foldMap f n) (f a)

instance Traversable n => Traversable (S n) where
    traverse f (S x y) = S <$> (traverse f x) <*> (f y)