clean2-test.hs

{-# LANGUAGE ExistentialQuantification, RankNTypes #-}

module Scanl where

import           Control.Foldl (Fold(..))
import           Data.Foldable (Foldable)
import qualified Data.Foldable as F
import           Data.Traversable

scan :: (Foldable f) => Fold a b -> f a -> [b]
scan (Fold step begin done) as = F.foldr step' done' as begin'
  where
    step' x k z = k $! (step (head z) x:z)
    done' = map done . reverse
    begin' = [begin]
{-# INLINABLE scan #-}

scan' :: (Traversable t) => Fold a b -> t a -> t b
scan' (Fold step begin done) as = snd (mapAccumL step' begin as)
  where
    step' x a = (step x a, done (step x a))
{-# INLINABLE scan' #-}