99 Elm 问题/问题 16/解决方案

解决方案 1：使用 List.foldr 和 List.indexedMap

dropEvery n list =
  let
    indexed = List.indexedMap (,) list
    maybeAdd (i, x) xs =
      if (i + 1) % n == 0 then xs else x :: xs        
  in
    List.foldr maybeAdd [] indexed