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{-# LANGUAGE GeneralizedNewtypeDeriving, FlexibleInstances #-}
module Sized where
import Data.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 = (+)
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