defmodule Strain do
  @doc """
  Given a `list` of items and a function `fun`, return the list of items where
  `fun` returns true.

  Do not use `Enum.filter`.
  """
  @spec keep(list :: list(any), fun :: (any -> boolean)) :: list(any)
  def keep([], _fun), do: []
  def keep([head | tail], fun) do
    if fun.(head), do: [head | keep(tail, fun)], else: keep(tail, fun)
  end

  @doc """
  Given a `list` of items and a function `fun`, return the list of items where
  `fun` returns false.

  Do not use `Enum.reject`.
  """
  @spec discard(list :: list(any), fun :: (any -> boolean)) :: list(any)
  def discard(list, fun), do: keep(list, &!fun.(&1))
end