Merge pull request #1390 from pragdave/iterate

Iterate

Author: José Valim

lib/elixir/lib/stream.ex

@@ -89,7 +89,7 @@ defmodule Stream do
   to `Stream.map/2`, we have automatically created a stream that will
   multiply the items in the list by 2 on enumeration.
 
-  This module also provides other functions for creating streams, like
+  This module also provides other functions for creating streams, such as
   `Stream.cycle/1`.
   """
 
@@ -220,6 +220,27 @@ defmodule Stream do
          end]
   end
 
+  @doc """
+  Emit a sequence of values, starting with `start_value`. Successive
+  values are generated by calling `next_fun` on the previous value.
+
+
+  ## Examples
+
+      iex> Stream.iterate(0, &1+1)  |> Enum.take(5)
+      [0,1,2,3,4]
+
+  """
+
+  @spec iterate(element, (element -> element)) :: Lazy.t
+  def iterate(start_value, next_fun) do
+     do_iterate(start_value, next_fun, &1, &2)
+  end
+
+  defp do_iterate(start_value, next_fun, acc, fun) do
+    do_iterate(next_fun.(start_value), next_fun, fun.(start_value, acc), fun)
+  end
+
   @doc """
   Creates a stream that will apply the given function on
   enumeration.
@@ -262,6 +283,27 @@ defmodule Stream do
          end]
   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

@@ -57,6 +57,13 @@ defmodule StreamTest do
     assert Enum.to_list(stream) == [2]
   end
 
+  test :iterate do
+    stream = Stream.iterate(0, &1+2)
+    assert Enum.take(stream, 5) == [0,2,4,6,8]
+    stream = Stream.iterate(5, &1+2)
+    assert Enum.take(stream, 5) == [5,7,9,11,13]
+  end
+
   test :map do
     stream = Stream.map([1,2,3], &1 * 2)
     assert is_lazy(stream)
@@ -69,6 +76,14 @@ 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)