29 lines
916 B
Haskell
29 lines
916 B
Haskell
module FoldableHelp
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( safeMinimumOn
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, module Data.Foldable
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)
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where
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import Data.Foldable
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-- TODO check up whether and how it is necessary to specialise this
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-- | A version of 'minimum' where the comparison is done on some extracted value.
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-- Returns Nothing if the list is empty. Only calls the function once per element.
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--
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-- > safeMinimumOn id [] == Nothing
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-- > safeMinimumOn length ["test","extra","a"] == Just "a"
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safeMinimumOn :: (Foldable t,Ord b) => (a -> b) -> t a -> Maybe a
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safeMinimumOn f = foldl' g Nothing
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where
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g (Just x) y
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| f x < f y = Just x
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| otherwise = Just y
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g Nothing y = Just y
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--{- | Partial. -}
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--minimumOn :: Ord b => (a -> b) -> [a] -> a
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--minimumOn f [] = error "tried to take minimumOn of empty list"
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--minimumOn f (x:xs) = go x xs
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-- where
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-- go x (y:xs) | f x < f y = go x xs
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-- | otherwise = go y xs
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-- go x [] = x
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