Semigroup-556.hs

module Example where

import Data.Semigroup
import Test.QuickCheck

data Trivial = Trivial deriving (Eq, Show)

instance Semigroup Trivial where
  _ <> _ = Trivial

instance Arbitrary Trivial where
  arbitrary = return Trivial

newtype Identity a = Identity a deriving (Eq, Show)


instance Semigroup (Identity a) where
  x <> y = x

instance (Num a) => Arbitrary (Identity a) where
  arbitrary = undefined

newtype BoolConj = BoolConj Bool deriving (Eq, Show)

instance Semigroup BoolConj where
  BoolConj x <> BoolConj y = BoolConj $ x || y

instance Arbitrary BoolConj where
  arbitrary = elements [BoolConj True, BoolConj False]


semigroupAssoc :: (Eq m, Semigroup m) => m -> m -> m -> Bool
semigroupAssoc a b c = (a <> (b <> c)) == ((a <> b) <> c)

type TrivialAssoc = Trivial -> Trivial -> Trivial -> Bool

type BoolConjAssoc = BoolConj -> BoolConj -> BoolConj -> Bool


main :: IO ()
main = do
  quickCheck (semigroupAssoc :: TrivialAssoc)
  -- @todo:
  -- quickCheck (semigroupAssoc :: Identity Int -> Identity Int -> Identity Int -> Bool)
  quickCheck (semigroupAssoc :: BoolConjAssoc)