{-# LANGUAGE FlexibleInstances #-}
module Combine where

import Data.Semigroup
import Test.QuickCheck

newtype Combine a b =
    Combine { unCombine :: (a -> b) }

instance Semigroup b => Semigroup (Combine a b) where
    (Combine f1) <> (Combine f2) = Combine f
        where f x = (f1 x) <> (f2 x)

instance (Monoid b) => Monoid (Combine a b) where
    mempty = Combine $ \_ -> mempty
    mappend (Combine f1) (Combine f2) = Combine f
        where f x = mappend (f1 x) (f2 x)

instance (Num a, CoArbitrary a, Arbitrary b) => Arbitrary (Combine a b) where
    arbitrary = do
        f <- arbitrary
        return $ Combine (\n -> f n)

instance (Show a, Show b) => Show (Combine a b) where
    show _ = "a -> b"