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module Sum where
import Test.QuickCheck
import Test.QuickCheck.Checkers
data Sum a b =
First a
| Second b
deriving (Eq, Show)
instance Functor (Sum a) where
fmap f (First x) = First x
fmap f (Second x) = Second $ f x
instance Applicative (Sum a) where
pure x = Second x
First f <*> _ = First f
Second f <*> x = fmap f x
instance Monad (Sum a) where
return = pure
(First x) >>= f = First x
(Second x) >>= f = f x
instance (Arbitrary a, Arbitrary b) => Arbitrary (Sum a b) where
arbitrary = frequency [ (1, fmap First arbitrary)
, (1, fmap Second arbitrary)
]
instance (Eq a, Eq b) => EqProp (Sum a b) where
(=-=) = eq
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