Reposition MapSets koans. Reposition some of the koans to make it clear why MapSets are interesting.

lib/koans/17_agents.ex

@@ -0,0 +1,43 @@
+defmodule Agents do
+  use Koans
+
+  @intro "Agents"
+
+  koan "Agents maintain state, so you can ask them about it" do
+    {:ok, pid} = Agent.start_link(fn -> "Hi there" end)
+    assert Agent.get(pid, &(&1)) == ___
+  end
+
+  koan "Agents may also be named so that you don't have to keep the pid around" do
+    Agent.start_link(fn -> "Why hello" end, name: AgentSmith)
+    assert Agent.get(AgentSmith, &(&1)) == ___
+  end
+
+  koan "Update to update the state" do
+    Agent.start_link(fn() -> "Hi there" end, name: __MODULE__)
+
+    Agent.update(__MODULE__, fn(old) ->
+      String.upcase(old)
+    end)
+    assert Agent.get(__MODULE__, &(&1)) == ___
+  end
+
+  koan "Use get_and_update when you need to read and change a value in one go" do
+    Agent.start_link(fn() -> ["Milk"] end, name: __MODULE__)
+
+    old_list = Agent.get_and_update(__MODULE__, fn(old) ->
+      {old, ["Bread" | old]}
+    end)
+
+    assert old_list == ___
+    assert Agent.get(__MODULE__, &(&1)) == ___
+  end
+
+  koan "Somebody has to switch off the light at the end of the day" do
+    {:ok, pid} = Agent.start_link(fn() -> ["Milk"] end, name: __MODULE__)
+
+    Agent.stop(__MODULE__)
+
+    assert Process.alive?(pid) == ___
+  end
+end