Implement Exception.message/1 for exceptions (#242)

We generate Elixir exception structs via defexception for Thrift
exception types. That macro always provides an exception/1
implementation, but message/1 is only generated if the exception is
defined with a :message field. When it doesn't, the generated code
produces a warning because the full Exception behaviour hasn't been
implemented.

This change generates a message/1 implementation for all of our Thrift
exception structs which don't have a natural :message field. It returns
the Kernel.inspect/2 string representation of the exception, which is
both simple and unambiguous.

lib/thrift/generator/struct_generator.ex

@@ -24,12 +24,17 @@ defmodule Thrift.Generator.StructGenerator do
     |> Utils.sort_defs
 
     define_block = case label do
-      :struct ->
-        quote do: defstruct unquote(struct_parts)
-      :union ->
-        quote do: defstruct unquote(struct_parts)
       :exception ->
         quote do: defexception unquote(struct_parts)
+      _ ->
+        quote do: defstruct unquote(struct_parts)
+    end
+
+    extra_defs = if label == :exception and not Keyword.has_key?(struct_parts, :message) do
+      quote do
+        @spec message(Exception.t) :: String.t
+        def message(exception), do: inspect(exception)
+      end
     end
 
     quote do
@@ -43,6 +48,7 @@ defmodule Thrift.Generator.StructGenerator do
         unquote(define_block)
         @type t :: %__MODULE__{}
         def new, do: %__MODULE__{}
+        unquote_splicing(List.wrap(extra_defs))
         defmodule BinaryProtocol do
           unquote_splicing(binary_protocol_defs)
         end

test/thrift/generator/binary_protocol_test.exs

@@ -306,6 +306,9 @@ defmodule Thrift.Generator.BinaryProtocolTest do
      1: optional string message,
     99: optional byte num;
   }
+  exception Ex2 {
+    1: optional i16 error_code;
+  }
   struct Exception {
     1: optional Ex val;
     2: optional map<Ex, Ex> val_map;
@@ -327,6 +330,12 @@ defmodule Thrift.Generator.BinaryProtocolTest do
     assert_serializes %Exception{val_list: [%Ex{num: 91}]},             <<15, 0, 4, 12, 0, 0, 0, 1, 3, 0, 99, 91, 0, 0>>
   end
 
+  thrift_test "exceptions always provide message/1" do
+    assert Ex.message(%Ex{message: "text", num: 1}) == "text"
+    assert Ex2.message(%Ex2{error_code: 1}) ==
+      ~s(%Thrift.Generator.BinaryProtocolTest.Ex2{error_code: 1})
+  end
+
   @thrift_file name: "composite.thrift", contents: """
   struct Composite {
     1: optional map<map<byte, byte>, map<byte, byte>> map_of_maps;