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