31 lines
763 B
Haskell
31 lines
763 B
Haskell
module FoldlHelp
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( minimumOn
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, premapMaybe
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, module Control.Foldl
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)
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where
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import Control.Foldl
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data Maybe' a = Just' !a | Nothing'
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data Pair a b = !a :!: !b
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minimumOn :: Ord b => (a -> b) -> Fold a (Maybe a)
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minimumOn f = Fold step Nothing' extract
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where
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extract Nothing' = Nothing
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extract (Just' (a :!: _)) = Just a
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step Nothing' b = Just' (b :!: f b)
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step (Just' (a :!: x)) b
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| y < x = Just' (b :!: y)
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| otherwise = Just' (a :!: x)
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where
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y = f b
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premapMaybe :: (a -> Maybe b) -> Fold b r -> Fold a r
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{-# INLINABLE premapMaybe #-}
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premapMaybe f (Fold step initial extract) = Fold step' initial extract
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where
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step' x a = case f a of
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Nothing -> x
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Just s -> step x s
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