strain

elixir/strain/lib/strain.ex

@@ -0,0 +1,22 @@
+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