Rely on stacktraces to get the file and line of last failure

lib/display.ex

@@ -7,7 +7,7 @@ defmodule Display do
 
     IO.puts("Now meditate upon #{display_module(module)}")
     IO.puts("---------------------------------------")
-    IO.puts(format_cyan(display_failed_assertion(module, expr)))
+    IO.puts(format_cyan(last_failure_location))
     IO.puts(display_koan(name))
     IO.puts(format_red(Macro.to_string(expr)))
   end
@@ -23,9 +23,20 @@ defmodule Display do
     end
   end
 
-  def display_failed_assertion(module, expr) do
-    file = source_file(module)
-    "Assertion failed in #{file}:#{line_number(expr, in: file)}"
+  def last_failure_location do
+    {file, line} = System.stacktrace
+      |> Enum.drop_while(&in_ex_unit?/1)
+      |> List.first
+      |> extract_file_and_line
+
+    "Assertion failed in #{file}:#{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
 
   def format_compile_error(error) do
@@ -33,45 +44,6 @@ defmodule Display do
     IO.puts(format_red(Exception.format(:error, error, trace)))
   end
 
-  defp line_number({_, [line: line], _}, in: _), do: line
-  defp line_number(expr, in: file) do
-    expr
-    |> to_s
-    |> find_in_file(file)
-    |> interpret
-  end
-
-  defp interpret({:ok, n}), do: n
-
-  defp find_in_file(line, file) do
-    File.open(file, fn(handle) ->
-      IO.read(handle, :all)
-      |> String.split("\n")
-      |> Enum.find_index(fn(candidate) -> String.contains?(candidate, line) end)
-      |> one_based
-    end)
-  end
-
-  defp one_based(nil), do: "???"
-  defp one_based(line), do: line + 1
-
-  defp to_s(atom) when is_atom(atom), do: ":#{to_string(atom)}"
-  defp to_s(tuple) when is_tuple(tuple) do
-    elements = tuple
-                 |> Tuple.to_list
-                 |> Enum.map(fn(x) -> to_s(x) end)
-
-   "{#{Enum.join(elements, ", ")}}"
-  end
-  defp to_s(x), do: "\"#{x}\""
-
-
-  defp source_file(module) do
-    module.__info__(:compile)
-    |> Dict.get(:source)
-    |> Path.relative_to(@current_dir)
-  end
-
   defp format_red(str) do
     Enum.join([ANSI.red, str, ANSI.reset], "")
   end