Just iterate the next value if we are going to use it

Author: José Valim

lib/elixir/lib/stream.ex

@@ -224,21 +224,23 @@ defmodule Stream do
   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)
+      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)
+    fn acc, fun ->
+      do_iterate(start_value, next_fun, fun.(start_value, acc), fun)
+    end
   end
 
-  defp do_iterate(start_value, next_fun, acc, fun) do
-    do_iterate(next_fun.(start_value), next_fun, fun.(start_value, acc), fun)
+  defp do_iterate(value, next_fun, acc, fun) do
+    next = next_fun.(value)
+    do_iterate(next, next_fun, fun.(next, acc), fun)
   end
 
   @doc """

lib/elixir/test/elixir/stream_test.exs

@@ -62,6 +62,10 @@ defmodule StreamTest do
     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]
+
+    # Only calculate values if needed
+    stream = Stream.iterate("HELLO", raise(&1))
+    assert Enum.take(stream, 1) == ["HELLO"]
   end
 
   test :map do