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{-# LANGUAGE FlexibleInstances #-} | |
{-# LANGUAGE GeneralizedNewtypeDeriving #-} | |
{-# LANGUAGE ScopedTypeVariables #-} | |
import Control.Applicative | |
import Data.Monoid | |
import Test.QuickCheck | |
data JoinList m a = Empty | |
| Single m a | |
| Append m (JoinList m a) (JoinList m a) | |
deriving (Eq, Show) | |
-- where `m` is a Monoid | |
newtype Size = Size Int | |
deriving (Eq, Ord, Show, Num) | |
getSize :: Size -> Int | |
getSize (Size i) = i | |
class Sized a where | |
size :: a -> Size | |
instance Sized Size where | |
size = id | |
-- This instance means that things like | |
-- (Foo, Size) | |
-- (Foo, (Bar, Size)) | |
-- ... | |
-- are all instances of Sized. | |
instance Sized b => Sized (a,b) where | |
size = size . snd | |
instance Monoid Size where | |
mempty = Size 0 | |
mappend = (+) | |
instance (Arbitrary m, Arbitrary a) => Arbitrary (JoinList m a) where | |
arbitrary = oneof [ pure Empty | |
, Single <$> arbitrary <*> arbitrary | |
, Append <$> arbitrary <*> arbitrary <*> arbitrary | |
] | |
main :: IO () | |
main = quickCheck \(jl :: JoinList [Int] Int) (error "your property here") |
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