108 lines
3 KiB
Haskell
108 lines
3 KiB
Haskell
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module FoldableScratch where
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import Data.Function ((&))
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--------------------------------------------------------------------------------
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sum :: (Foldable t, Num a) => t a -> a
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sum xs =
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foldr (+) 0 xs
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product :: (Foldable t, Num a) => t a -> a
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product xs =
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foldr (*) 1 xs
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elem :: (Foldable t, Eq a) => a -> t a -> Bool
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elem y xs =
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foldr (\x acc -> if acc then acc else y == x) False xs
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minimum :: (Foldable t, Ord a) => t a -> Maybe a
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minimum xs =
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foldr (\x acc ->
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case acc of
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Nothing -> Just x
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Just curr -> Just (min curr x)) Nothing xs
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maximum :: (Foldable t, Ord a) => t a -> Maybe a
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maximum xs =
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foldr (\x acc ->
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case acc of
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Nothing -> Nothing
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Just curr -> Just (max curr x)) Nothing xs
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-- TODO: How could I use QuickCheck to see if Prelude.null and this null return
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-- the same results for the same inputs?
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null :: (Foldable t) => t a -> Bool
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null xs =
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foldr (\_ _ -> False) True xs
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length :: (Foldable t) => t a -> Int
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length xs =
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foldr (\_ acc -> acc + 1) 0 xs
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toList :: (Foldable t) => t a -> [a]
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toList xs =
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reverse $ foldr (\x acc -> x : acc) [] xs
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fold :: (Foldable t, Monoid m) => t m -> m
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fold xs =
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foldr mappend mempty xs
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foldMap :: (Foldable t, Monoid m) => (a -> m) -> t a -> m
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foldMap f xs =
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foldr (\x acc -> mappend (f x) acc) mempty xs
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--------------------------------------------------------------------------------
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data List a = Nil | Cons a (List a) deriving (Eq, Show)
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instance Foldable List where
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foldr f acc (Cons x rest) = foldr f (f x acc) rest
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foldr f acc Nil = acc
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fromList :: [a] -> List a
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fromList [] = Nil
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fromList (x:rest) = Cons x (fromList rest)
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--------------------------------------------------------------------------------
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data Constant a b = Constant b deriving (Eq, Show)
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-- TODO: Is this correct?
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instance Foldable (Constant a) where
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foldr f acc (Constant x) = f x acc
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--------------------------------------------------------------------------------
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data Two a b = Two a b deriving (Eq, Show)
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instance Foldable (Two a) where
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foldr f acc (Two x y) = f y acc
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--------------------------------------------------------------------------------
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data Three a b c = Three a b c deriving (Eq, Show)
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instance Foldable (Three a b) where
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foldr f acc (Three x y z) = f z acc
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--------------------------------------------------------------------------------
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data Three' a b = Three' a b b deriving (Eq, Show)
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instance Foldable (Three' a) where
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foldr f acc (Three' x y z) = acc & f z & f y
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--------------------------------------------------------------------------------
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data Four' a b = Four' a b b b deriving (Eq, Show)
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instance Foldable (Four' a) where
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foldr f acc (Four' w x y z) = acc & f z & f y & f x
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--------------------------------------------------------------------------------
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filterF :: (Applicative f, Foldable t, Monoid (f a)) => (a -> Bool) -> t a -> f a
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filterF pred xs =
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foldr (\x acc -> if pred x then pure x `mappend` acc else acc) mempty xs
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