Warn about reserved names

As mentioned in #10, you cannot use a TwiML verb name as a parameter in
a function definition, like this:

```elixir
Enum.each [1, 2] fn(number) ->
  say "Press #{number}"
end
```

This commit adds a nice warning message if a user attempts to do this,
telling them that the "number" verb is reserved.

lib/ex_twiml.ex

@@ -67,6 +67,8 @@ defmodule ExTwiml do
 
   import ExTwiml.Utilities
 
+  alias ExTwiml.ReservedNameError
+
   @verbs [ 
     # Nested
     :gather, :dial, :message,    
@@ -99,15 +101,17 @@ defmodule ExTwiml do
       # The buffer's state is a list of XML fragments. New fragments are 
       # inserted by other macros. Finally, all the fragments are joined
       # together in a string.
-      {:ok, var!(buffer, Twiml)}  = start_buffer([header])
+      {:ok, var!(buffer, Twiml)} = start_buffer([header])
 
       # Wrap the whole block in a <Response> tag
       tag :response do
         # Postwalk the AST, expanding all of the TwiML verbs into proper
         # `tag` and `text` macros. This gives the impression that there
         # is a macro for each verb, when in fact it all expands to only
         # two macros.
-        unquote(Macro.postwalk(block, &postwalk/1))
+        unquote(block
+                |> Macro.prewalk(&prewalk(&1, __CALLER__.file))
+                |> Macro.postwalk(&postwalk/1))
       end
 
       xml  = render(var!(buffer, Twiml))      # Convert buffer to string
@@ -174,6 +178,14 @@ defmodule ExTwiml do
   # Private API
   ##
 
+  # Check function definitions for reserved variable names
+  defp prewalk({:fn, _, [{:-> , _, [[vars], _]}]} = ast, file_name) do
+    assert_no_verbs!(vars, file_name)
+    ast
+  end
+
+  defp prewalk(ast, _file_name), do: ast
+
   # {:text, [], ["Hello World"]}
   defp postwalk({:text, _meta, [string]}) do
     # Just add the text to the buffer. Nothing else needed.
@@ -239,4 +251,17 @@ defmodule ExTwiml do
       put_buffer var!(buffer, Twiml), create_tag(:self_closed, unquote(verb), unquote(options))
     end
   end
+
+  defp assert_no_verbs!({name, _, _} = var, file_name)
+  when is_atom(name) and name in @verbs do
+    raise ReservedNameError, [var, file_name]
+  end
+
+  defp assert_no_verbs!(vars, file_name) when is_tuple(elem(vars, 0)) do
+    vars
+    |> Tuple.to_list
+    |> Enum.each(&assert_no_verbs!(&1, file_name))
+  end
+
+  defp assert_no_verbs!(vars, _file_name), do: vars
 end

lib/ex_twiml/reserved_name_error.ex

@@ -0,0 +1,33 @@
+defmodule ExTwiml.ReservedNameError do
+  @moduledoc """
+  This error is thrown if you try to use TwiML verb name as a variable name
+  in your `twiml` block.
+
+  ## Example
+
+  This code will raise the error, because `number` is a reserved name.
+
+      twiml do
+        Enum.each [1, 2], fn(number) ->
+          # ...
+        end
+      end
+  """
+
+  defexception [:message]
+
+  @doc false
+  def exception([{name, context, _}, file_name]) do
+    file_name = Path.relative_to_cwd(file_name)
+    name = to_string(name)
+
+    message = ~s"""
+    "#{name}" is a reserved name in #{file_name}:#{context[:line]}, because it
+    is used to generate the <#{String.capitalize(name)} /> TwiML verb.
+
+    Please use a different variable name.
+    """
+
+    %__MODULE__{message: message}
+  end
+end

test/ex_twiml/reserved_name_error_test.exs

@@ -0,0 +1,17 @@
+defmodule ExTwiml.ReservedNameErrorTest do
+  use ExUnit.Case
+
+  alias ExTwiml.ReservedNameError
+
+  test ".exception returns a nice exception" do
+    %{message: message} = ReservedNameError.exception([{:number, [line: 1], []},
+                                                      "test/test.ex"])
+
+    assert message == ~s"""
+    "number" is a reserved name in test/test.ex:1, because it
+    is used to generate the <Number /> TwiML verb.
+
+    Please use a different variable name.
+    """
+  end
+end

test/ex_twiml_test.exs

@@ -1,7 +1,10 @@
 defmodule ExTwimlTest do
   use ExUnit.Case, async: false
+
   import ExTwiml
 
+  alias ExTwiml.ReservedNameError
+
   doctest ExTwiml
 
   test "can render the <Gather> verb" do
@@ -226,6 +229,21 @@ defmodule ExTwimlTest do
     assert_twiml markup, "<Say>123</Say>"
   end
 
+  test ".twiml warns of reserved variable names" do
+    ast = quote do
+      twiml do
+        Enum.each [1, 2], fn(number) ->
+          say "#{number}"
+        end
+      end
+    end
+
+    assert_raise ReservedNameError, fn ->
+      # Simulate compiling the macro
+      Macro.expand(ast, __ENV__)
+    end
+  end
+
   defp assert_twiml(lhs, rhs) do
     assert lhs == "<?xml version=\"1.0\" encoding=\"UTF-8\"?><Response>#{rhs}</Response>"
   end