To enable adding the intro, the tracker knows visited modules.

lib/tracker.ex

@@ -4,27 +4,34 @@ defmodule Tracker do
             |> Enum.flat_map(&(&1.all_koans))
             |> Enum.count
 
-    Agent.start_link(fn -> {total, MapSet.new()} end, name: __MODULE__)
+    Agent.start_link(fn -> %{total: total,
+                             koans: MapSet.new(),
+                             visited_modules: MapSet.new()} end, name: __MODULE__)
     modules
   end
 
-  def get do
-    Agent.get(__MODULE__, &(&1))
-  end
+  defp get(),  do: Agent.get(__MODULE__, &(&1))
 
-  def completed(koan) do
-    Agent.update(__MODULE__, fn({total, completed}) ->
-      {total, MapSet.put(completed, koan)}
+  def completed(module, koan) do
+    Agent.update(__MODULE__, fn(%{koans: completed, visited_modules: modules} = all) ->
+      %{ all | koans: MapSet.put(completed, koan),
+               visited_modules: MapSet.put(modules, module)}
     end)
   end
 
   def complete? do
-    {total, completed} = get
-    total == Enum.count(completed)
+    %{total: total, current: completed} = summarize
+    total == completed
   end
 
   def summarize, do: get |> summarize
-  defp summarize({total, completed}) do
-    %{total: total, current: MapSet.size(completed)}
+  defp summarize(%{total: total,
+                  koans: completed,
+                  visited_modules: modules}) do
+    %{
+      total: total,
+      current: MapSet.size(completed),
+      visited_modules: MapSet.to_list(modules)
+    }
   end
 end

test/tracker_test.exs

@@ -5,19 +5,24 @@ defmodule TrackerTest do
 
   test "can start" do
     Tracker.start(@sample_modules)
-    assert Tracker.summarize == %{total: 2, current: 0}
+    assert Tracker.summarize == %{total: 2, current: 0, visited_modules: []}
   end
 
   test "can be notified of completed koans" do
     Tracker.start(@sample_modules)
-    Tracker.completed(:"Hi there")
-    assert Tracker.summarize == %{total: 2, current: 1}
+    Tracker.completed(SampleKoan, :"Hi there")
+    assert Tracker.summarize == %{total: 2, current: 1, visited_modules: [SampleKoan]}
   end
 
   test "multiple comletions of the same koan count only once" do
     Tracker.start(@sample_modules)
-    Tracker.completed(:"Hi there")
-    Tracker.completed(:"Hi there")
-    assert Tracker.summarize == %{total: 2, current: 1}
+    Tracker.completed(SampleKoan, :"Hi there")
+    Tracker.completed(SampleKoan, :"Hi there")
+    assert Tracker.summarize == %{total: 2, current: 1, visited_modules: [SampleKoan]}
+  end
+
+  test "knows when koans are not complete" do
+    Tracker.start(@sample_modules)
+    refute Tracker.complete?
   end
 end