Add Stream.repeatedly

Author: pragdave

lib/elixir/lib/stream.ex

@@ -284,6 +284,26 @@ defmodule Stream do
   end
 
 
+  @doc """
+  Returns a stream generated by calling `generator_fun` repeatedly.
+
+  ## Examples
+
+      iex> Stream.repeatedly(function(:random.uniform/0)) |> Enum.take(3)
+      [0.4435846174457203, 0.7230402056221108, 0.94581636451987]
+
+  """
+  @spec repeatedly(( -> element)) :: Lazy.t
+  def repeatedly(generator_fun) 
+  when is_function(generator_fun, 0) do
+    do_repeatedly(generator_fun, &1, &2)
+  end
+
+  defp do_repeatedly(generator_fun, acc, fun) do
+    do_repeatedly(generator_fun, fun.(generator_fun.(), acc), fun)
+  end
+
+
   @doc """
   Lazily takes the next `n` items from the enumerable and stops
   enumeration.

lib/elixir/test/elixir/stream_test.exs

@@ -74,6 +74,14 @@ defmodule StreamTest do
     assert Enum.to_list(stream) == [1,3]
   end
 
+  test :repeatedly do 
+    stream = Stream.repeatedly(fn -> 1 end)
+    assert Enum.take(stream, 5) == [1,1,1,1,1]
+    stream = Stream.repeatedly(function(:random.uniform/0))
+    [r1,r2] = Enum.take(stream, 2)
+    assert r1 != r2
+  end
+
   test :take do
     stream = Stream.take(1..1000, 5)
     assert is_lazy(stream)