Move error line detection into runner itself due to processes.

lib/runner.ex

@@ -14,25 +14,23 @@ defmodule Runner do
     Tasks,
   ]
 
-  def koan?(module) do
-    Enum.member?(@modules, module)
-  end
+  def koan?(koan), do: Enum.member?(@modules, koan)
 
   def run do
     Options.initial_koan
     |>run
   end
 
-  def run(start_module) when start_module in @modules do
+  def run(start_module) when start_module in @modules, do: run(start_module, @modules)
+  def run(start_module), do: Display.invalid_koan(start_module, @modules)
+
+  def run(start_module, modules) do
     Display.clear_screen()
 
-    start_idx = Enum.find_index(@modules, &(&1 == start_module))
-
-    @modules
-    |> Enum.drop(start_idx)
+    modules
+    |> Enum.drop_while( &(&1 != start_module))
     |> Enum.take_while( &(run_module(&1) == :passed))
   end
-  def run(koan), do: Display.invalid_koan(koan, @modules)
 
   def run_module(module) do
     Display.considering(module)
@@ -64,7 +62,22 @@ defmodule Runner do
 
   def exec(module, name, args, parent) do
     result = apply(module, name, args)
-    send parent, result
+    send parent, expand(result)
     Process.exit(self(), :kill)
   end
+
+  def expand(:ok), do: :ok
+  def expand(error) do
+    {file, line} = System.stacktrace
+      |> Enum.drop_while(&in_ex_unit?/1)
+      |> List.first
+      |> extract_file_and_line
+
+      %{error: error, file: file, line: line}
+  end
+
+  defp in_ex_unit?({ExUnit.Assertions, _, _, _}), do: true
+  defp in_ex_unit?(_), do: false
+
+  defp extract_file_and_line({_, _, _, [file: file, line: line]}), do: {file, line}
 end