From 753477518f87860dcc4981762e3799d6ffc44f30 Mon Sep 17 00:00:00 2001
From: Michael Snoyman <michael@snoyman.com>
Date: Thu, 3 Feb 2011 06:57:37 +0200
Subject: [PATCH] Exception instance for ErrorHandler

---
 Yesod/Handler.hs  | 7 ++++---
 Yesod/Internal.hs | 6 +++++-
 2 files changed, 9 insertions(+), 4 deletions(-)

diff --git a/Yesod/Handler.hs b/Yesod/Handler.hs
index 0490c92b..ac3330ae 100644
--- a/Yesod/Handler.hs
+++ b/Yesod/Handler.hs
@@ -348,9 +348,10 @@ runHandler :: HasReps c
            -> YesodApp
 runHandler handler mrender sroute tomr ma sa =
   YesodApp $ \eh rr cts initSession -> do
-    let toErrorHandler =
-            InternalError
-          . (show :: Control.Exception.SomeException -> String)
+    let toErrorHandler e =
+            case fromException e of
+                Just x -> x
+                Nothing -> InternalError $ show e
     let hd = HandlerData
             { handlerRequest = rr
             , handlerSub = sa
diff --git a/Yesod/Internal.hs b/Yesod/Internal.hs
index f3e6ee30..6d9eb8fc 100644
--- a/Yesod/Internal.hs
+++ b/Yesod/Internal.hs
@@ -2,6 +2,7 @@
 {-# LANGUAGE GeneralizedNewtypeDeriving #-}
 {-# LANGUAGE CPP #-}
 {-# LANGUAGE OverloadedStrings #-}
+{-# LANGUAGE DeriveDataTypeable #-}
 -- | Normal users should never need access to these.
 module Yesod.Internal
     ( -- * Error responses
@@ -46,6 +47,8 @@ import qualified Data.Text.Lazy as LT
 import qualified Data.Text.Lazy.Encoding as LT
 
 import qualified Network.Wai as W
+import Data.Typeable (Typeable)
+import Control.Exception (Exception)
 
 #if GHC7
 #define HAMLET hamlet
@@ -61,7 +64,8 @@ data ErrorResponse =
     | InvalidArgs [String]
     | PermissionDenied String
     | BadMethod String
-    deriving (Show, Eq)
+    deriving (Show, Eq, Typeable)
+instance Exception ErrorResponse
 
 ----- header stuff
 -- | Headers to be added to a 'Result'.