Elixir

Other Elixir solutions.

defmodule ListOps do
  # Please don't use any external modules (especially List or Enum) in your
  # implementation. The point of this exercise is to create these basic
  # functions yourself. You may use basic Kernel functions (like `Kernel.+/2`
  # for adding numbers), but please do not use Kernel functions for Lists like
  # `++`, `--`, `hd`, `tl`, `in`, and `length`.

  @spec count(list) :: non_neg_integer
  def count([]), do: 0
  def count([_ | tail]), do: 1 + count(tail)

  @spec reverse(list) :: list
  def reverse([], acc), do: acc
  def reverse([head | tail], acc), do: reverse(tail, [head | acc])
  def reverse(l), do: reverse(l, [])

  @spec map(list, (any -> any)) :: list
  def map([], _), do: []
  def map([head | tail], f), do: [f.(head) | map(tail, f)]

  @spec filter(list, (any -> as_boolean(term))) :: list
  def filter(l, f), do: for(x <- l, f.(x), do: x)

  @type acc :: any
  @spec reduce(list, acc, (any, acc -> acc)) :: acc
  def reduce([], acc, _), do: acc
  def reduce([head | tail], acc, f), do: f.(head, reduce(tail, acc, f))

  @spec append(list, list) :: list
  def append([], b), do: b
  def append([head | tail], b), do: [head | append(tail, b)]

  @spec concat([[any]]) :: [any]
  def concat([head | tail]), do: append(head, concat(tail))
  def concat([]), do: []
end