Implement Enum.member?

lib/elixir/lib/enum.ex

@@ -759,6 +759,28 @@ defmodule Enum do
     end
   end
 
+  @doc """
+  Checks if the given `term` is included in the `collection`.
+
+  ## Examples
+
+      iex> Enum.member?(1 .. 3, 1)
+      true
+      iex> Enum.member?([1, 2, 3, 4], 10)
+      false
+
+  """
+  @spec member?(t, term) :: boolean
+  def member?(collection, term) do
+    case I.iterator(collection) do
+      { iterator, pointer } ->
+        do_member?(pointer, iterator, term)
+
+      list when is_list(list) ->
+        List.member?(list, term)
+    end
+  end
+
   @doc """
   Partitions `collection` into two where the first one contains elements
   for which `fun` returns a truthy value, and the second one -- for which `fun`
@@ -1549,6 +1571,20 @@ defmodule Enum do
     { :lists.reverse(list_acc), acc }
   end
 
+  ## member?
+
+  def do_member?(:stop, _, _) do
+    false
+  end
+
+  def do_member?({ h, _ }, _, term) when h == term do
+    true
+  end
+
+  def do_member?({ h, next }, iterator, term) do
+    do_member?(iterator.(next), iterator, term)
+  end
+
   ## partition
 
   defp do_partition([h|t], fun, acc1, acc2) do

lib/elixir/test/elixir/enum_test.exs

@@ -182,6 +182,12 @@ defmodule EnumTest.List do
     assert Enum.map_reduce([1,2,3], 1, fn(x, acc) -> { x * 2, x + acc } end) == { [2,4,6], 7 }
   end
 
+  test :member? do
+    refute Enum.member?([], 0)
+    assert Enum.member?([1, 2, 3], 2)
+    assert Enum.member?(1 .. 3, 1)
+  end
+
   test :partition do
     assert Enum.partition([1,2,3], fn(x) -> rem(x, 2) == 0 end) == { [2], [1,3] }
     assert Enum.partition([2,4,6], fn(x) -> rem(x, 2) == 0 end) == { [2,4,6], [] }