Add Stream.each/2

Author: José Valim

lib/elixir/lib/stream.ex

@@ -182,6 +182,31 @@ defmodule Stream do
     lazy enum, true, fn(f1) -> R.drop_while(fun, f1) end
   end
 
+  @doc """
+  Execute the given function for each item.
+
+  Useful for adding side effects (like printing) to a stream.
+
+  ## Examples
+
+      iex> stream = Stream.each([1, 2, 3], fn(x) -> IO.puts x end)
+      iex> Enum.to_list(stream)
+      1
+      2
+      3
+      [1,2,3]
+
+  """
+  @spec each(Enumerable.t, (element -> term)) :: Enumerable.t
+  def each(enum, fun) do
+    lazy enum, fn(f1) ->
+      fn(x, acc) ->
+        fun.(x)
+        f1.(x, acc)
+      end
+    end
+  end
+
   @doc """
   Creates a stream that will filter elements according to
   the given function on enumeration.

lib/elixir/test/elixir/stream_test.exs

@@ -105,6 +105,18 @@ defmodule StreamTest do
     assert Stream.drop_while(nats, &(&1 <= 5)) |> Enum.take(5) == [6,7,8,9,10]
   end
 
+  test "each" do
+    Process.put(:stream_each, [])
+
+    stream = Stream.each([1,2,3], fn x ->
+      Process.put(:stream_each, [x|Process.get(:stream_each)])
+    end)
+
+    assert is_lazy(stream)
+    assert Enum.to_list(stream) == [1,2,3]
+    assert Process.get(:stream_each) == [3,2,1]
+  end
+
   test "filter" do
     stream = Stream.filter([1,2,3], fn(x) -> rem(x, 2) == 0 end)
     assert is_lazy(stream)