Adds a koan to demonstrate the use of deriving a default behaviour

lib/koans/16_protocols.ex

@@ -5,14 +5,25 @@ defmodule Protocols do
 
   defprotocol School, do: def enrol(person)
 
-  defmodule Student, do: defstruct name: ""
-  defmodule Baker, do: defstruct name: ""
-  defmodule Dancer do
-    defstruct name: "", dance_style: ""
+  defimpl School, for: Any do
+    def enrol(person) do
+      "#{person.name} enrolled at school"
+    end
   end
 
-  defimpl School, for: Student do
-    def enrol(student), do: "#{student.name} enrolled at secondary school"
+  defmodule Student do
+    @derive School
+    defstruct name: ""
+  end
+
+  defmodule Musician, do: defstruct name: "", instrument: ""
+  defmodule Dancer,  do: defstruct name: "", dance_style: ""
+  defmodule Baker, do: defstruct name: ""
+
+  defimpl School, for: Musician do
+    def enrol(musician) do
+      "#{musician.name} signed up for #{musician.instrument}"
+    end
   end
 
   defimpl School, for: Dancer do
@@ -20,13 +31,18 @@ defmodule Protocols do
   end
 
   koan "Sharing an interface is the secret at school" do
-    student = %Student{name: "Emily"}
+    musician = %Musician{name: "Andre", instrument: "violin"}
     dancer = %Dancer{name: "Darcy", dance_style: "ballet"}
 
-    assert School.enrol(student) == ___
+    assert School.enrol(musician) == ___
     assert School.enrol(dancer) == ___
   end
 
+  koan "Sometimes we all use the same" do
+    student = %Student{name: "Emily"}
+    assert School.enrol(student) == ___
+  end
+
   koan "If you don't comply you can't get in" do
     assert_raise ___, fn ->
       School.enrol(%Baker{name: "Delia"})

test/koans/protocols_koans_test.exs

@@ -4,7 +4,8 @@ defmodule ProtocolsTests do
 
   test "Protocols" do
     answers = [
-      {:multiple, ["Emily enrolled at secondary school", "Darcy enrolled for ballet"]},
+      {:multiple, ["Andre signed up for violin", "Darcy enrolled for ballet"]},
+      "Emily enrolled at school",
       Protocol.UndefinedError
     ]