Add argument to koan to allow it to be tested

lib/ast_manger.ex

@@ -0,0 +1,24 @@
+defmodule ASTMangler do
+  def expand([do: thing], replacement) do
+    [do: expand(thing, replacement)]
+	end
+
+  def expand({fun, context, args}, replacement) do
+    new_args = replace(args, replacement)
+    {fun, context, new_args}
+  end
+
+  def replace(args, b) do
+    args
+    |> Enum.find_index(fn(x) -> x == :__ end)
+    |> replace(args, b)
+  end
+
+	def replace(nil, [], _b), do: []
+	def replace(nil, [ast], b) do
+    [expand(ast,b)]
+	end
+  def replace(index, list, b) do
+    List.update_at(list, index, fn(_)-> b end)
+  end
+end

lib/koans.ex

@@ -1,11 +1,17 @@
 defmodule Koans do
   defmacro koan(name, body) do
     compiled_name = String.to_atom(name)
+    x = quote do: answer
+    mangled_body = ASTMangler.expand(body, x)
     quote do
       @koans unquote(compiled_name)
-      def unquote(compiled_name)() do
+      def unquote(compiled_name)(answer \\ :nothing) do
         try do
-          unquote(body)
+            if answer == :nothing do
+              unquote(body)
+            else
+              unquote(mangled_body)
+            end
           :ok
         rescue
           e in _ -> e

lib/runner.ex

@@ -40,6 +40,13 @@ defmodule Runner do
     end
   end
 
+  def test_single_koan(module, name, answer) do
+    case apply(module, name, []) do
+      :ok -> :passed
+      error -> IO.inspect error
+    end
+  end
+
   def run_koan(module, name) do
     case apply(module, name, []) do
       :ok   -> :passed