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