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module Exercises where
-- 1
data Constant a b = Constant b deriving (Show)
instance Foldable (Constant a) where
foldr f acc (Constant x) = f x acc
-- 2
data Two a b = Two a b deriving (Show)
instance Foldable (Two a) where
foldr f acc (Two _ x) = f x acc
-- 3
data Three a b c = Three a b c deriving (Show)
instance Foldable (Three a b) where
foldr f acc (Three _ _ x) = f x acc
-- 4
data Three' a b = Three' a b b deriving (Show)
instance Foldable (Three' a) where
foldr f acc (Three' _ x y) = f y $ f x acc
-- 5
data Four' a b = Four' a b b b deriving (Show)
instance Foldable (Four' a) where
foldr f acc (Four' _ x y z) = f z $ f y $ f x acc
filterF :: ( Applicative f
, Foldable t
, Monoid (f a))
=> (a -> Bool) -> t a -> f a
filterF f x = foldMap y x
where y k = if f k then pure k else mempty
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