Jackknife

jackknife.hs

import Data.Monoid

-- | Type class for monoidal accumulators
class Monoid m => Accumulator m a where
  -- | Convert value to 1-element accumulator
  unit :: a -> m
  unit a = cons a mempty
  -- | Prepend value to accumulator
  cons :: a -> m -> m
  cons a m = unit a <> m
  -- | Append value to accumulator
  snoc :: m -> a -> m
  snoc m a = m <> unit a

jackknifeMean :: Accumulator m a => [a] -> [m]
jackknifeMean xs =
    zipWith mappend
    (init (scanl snoc mempty xs))
    (tail (scanr cons mempty xs))