everyN.hs

module Every where

import Control.Lens
import Control.Monad.Fix

-- |
--
-- >>> everyN 2 []
-- []
--
-- >>> everyN 2 [1]
-- []
--
-- >>> everyN 2 [1,2,3,4,5]
-- [2,4]
--
-- >>> everyN (-1) []
-- []
--
-- >>> everyN (-1) [1,2,3]
-- [1,2,3]
--
-- >>> everyN (-4) [1,2,3]
-- [1,2,3]
--
-- >>> everyN 4 [1..20]
-- [4,8,12,16,20]
--
-- prop> everyN 1 x == (x :: [Int])
--
-- prop> everyN n [] == []
everyN :: Int -> [a] -> [a]
everyN n =
  over _tail (everyN n) . snd . splitAt (pred n)

everyN' :: Int -> [a] -> [a]
everyN' n =
  fix (\w -> over _tail w . snd . splitAt (pred n))