Add read from a streams koans

lib/koans/19_streams.ex

@@ -0,0 +1,27 @@
+defmodule Streams do
+  use Koans
+
+  @intro "Streams"
+
+  koan "Streams are lazier than Enum, they only return a struct" do
+    assert Stream.map([1, 2, 3], &(&1 + 1)).__struct__ == ___
+  end
+
+  koan "You have to ask it explicitly to do the operation" do
+    updated_list =
+      [1, 2, 3]
+      |> Stream.map(&(&1 + 1))
+      |> Enum.to_list
+
+    assert updated_list == ___
+  end
+
+  koan "Which will let you decide which element you want to compute in the stream" do
+    computed_element =
+      1..42
+      |> Stream.map(&("I computed: #{&1}"))
+      |> Enum.take(1)
+
+    assert computed_element == ___
+  end
+end

lib/runner.ex

@@ -18,6 +18,7 @@ defmodule Runner do
     Tasks,
     Agents,
     Protocols,
+    Streams,
   ]
 
   def koan?(koan), do: Enum.member?(@modules, koan)

test/koans/streams_koans_test.exs

@@ -0,0 +1,14 @@
+defmodule StreamsTests do
+  use ExUnit.Case
+  import TestHarness
+
+  test "Streams" do
+    answers = [
+      Stream,
+      [2, 3, 4],
+      ["I computed: 1"],
+    ]
+
+    test_all(Streams, answers)
+  end
+end