Add Stream.seq() for generating potentially infinite streams of values

Author: pragdave

lib/elixir/lib/stream.ex

@@ -93,6 +93,11 @@ defmodule Stream do
   `Stream.cycle/1`.
   """
 
+  @compile {:inline, increment_func: 0}
+
+  @doc false
+  def increment_func(), do: &1+1
+
   defrecord Lazy, [:enumerable, :fun, :acc]
 
   defimpl Enumerable, for: Lazy do
@@ -262,6 +267,28 @@ defmodule Stream do
          end]
   end
 
+  @doc """
+  Emit a sequence of values, starting with `start_value`. Successive
+  values are generated by calling `incr_func` on the previous value.
+  The default parameters generate a sequence of integers starting at 0.
+
+  ## Examples
+
+      iex> Stream.seq  |> Enum.take(5)
+      [0,1,2,3,4]
+
+  """
+
+  @spec seq((element -> element), element) :: Lazy.t
+  def seq(incr_fun // &1+1, start_value // 0) do
+     do_seq(incr_fun, start_value, &1, &2)
+  end
+
+  defp do_seq(incr_fun, start_value, acc, fun) do
+    do_seq(incr_fun, incr_fun.(start_value), fun.(start_value, acc), fun)
+  end
+
+
   @doc """
   Lazily takes the next `n` items from the enumerable and stops
   enumeration.

lib/elixir/test/elixir/stream_test.exs

@@ -67,6 +67,23 @@ defmodule StreamTest do
     assert Enum.to_list(stream) == [1,3]
   end
 
+  test :seq_default do
+    stream = Stream.seq
+    assert Enum.take(stream, 5) == [0,1,2,3,4]
+  end
+
+  test :seq_with_explicit_but_default do
+    stream = Stream.seq(&1+1, 0)
+    assert Enum.take(stream, 5) == [0,1,2,3,4]
+  end
+
+  test :seq_for_every_second_number do
+    stream = Stream.seq(&1+2)
+    assert Enum.take(stream, 5) == [0,2,4,6,8]
+    stream = Stream.seq(&1+2, 5)
+    assert Enum.take(stream, 5) == [5,7,9,11,13]
+  end
+
   test :take do
     stream = Stream.take(1..1000, 5)
     assert is_lazy(stream)