diff --git a/Network/Wai.hs b/Network/Wai.hs
index 9807c44..4466b2c 100644
--- a/Network/Wai.hs
+++ b/Network/Wai.hs
@@ -1,4 +1,4 @@
-{-# LANGUAGE ExistentialQuantification #-}
+{-# LANGUAGE Rank2Types #-}
 module Network.Wai
     ( -- * Data types
       -- ** Request method
@@ -23,6 +23,8 @@ module Network.Wai
     , Status (..)
     , statusCode
     , statusMessage
+      -- * Enumerator
+    , Enumerator
       -- * WAI interface
     , Request (..)
     , Response (..)
@@ -233,6 +235,8 @@ statusMessage Status405 = B8.pack "Method Not Allowed"
 statusMessage Status500 = B8.pack "Internal Server Error"
 statusMessage (Status _ m) = m
 
+type Enumerator a = (a -> B.ByteString -> IO (Either a a)) -> a -> IO a
+
 data Request = Request
   {  requestMethod  :: Method
   ,  httpVersion    :: HttpVersion
@@ -242,7 +246,7 @@ data Request = Request
   ,  serverPort     :: Int
   ,  httpHeaders    :: [(RequestHeader, B.ByteString)]
   ,  urlScheme      :: UrlScheme
-  ,  requestBody    :: IO (Maybe B.ByteString)
+  ,  requestBody    :: forall a. Enumerator a
   ,  errorHandler   :: String -> IO ()
   ,  remoteHost     :: String
   }
@@ -250,7 +254,7 @@ data Request = Request
 data Response = Response
   { status        :: Status
   , headers       :: [(ResponseHeader, B.ByteString)]
-  , body          :: Either FilePath ((B.ByteString -> IO ()) -> IO ())
+  , body          :: forall a. Either FilePath (Enumerator a)
   }
 
 type Application = Request -> IO Response
diff --git a/Network/Wai/Handler/SimpleServer.hs b/Network/Wai/Handler/SimpleServer.hs
index 72f7280..6e4d3ba 100644
--- a/Network/Wai/Handler/SimpleServer.hs
+++ b/Network/Wai/Handler/SimpleServer.hs
@@ -21,7 +21,6 @@ import Network.Wai
 import qualified System.IO
 
 import qualified Data.ByteString as BS
-import qualified Data.ByteString.Lazy as BL
 import qualified Data.ByteString.Char8 as B8
 import Network
     ( listenOn, accept, sClose, PortID(PortNumber), Socket
@@ -135,14 +134,16 @@ parseRequest port lines' handle remoteHost' = do
                 , remoteHost = remoteHost'
                 }
 
-requestBodyHandle :: Handle -> MVar Int -> IO (Maybe BS.ByteString)
-requestBodyHandle h mlen = modifyMVar mlen helper where
-    helper :: Int -> IO (Int, Maybe BS.ByteString)
-    helper 0 = return (0, Nothing)
-    helper len = do
+requestBodyHandle :: Handle -> MVar Int -> Enumerator a
+requestBodyHandle h mlen iter accum = modifyMVar mlen (helper accum) where
+    helper a 0 = return (0, a)
+    helper a len = do
         bs <- BS.hGet h len
         let newLen = len - BS.length bs
-        return (newLen, Just bs)
+        ea' <- iter a bs
+        case ea' of
+            Left a' -> return (newLen, a')
+            Right a' -> helper a' newLen
 
 parseFirst :: (StringLike s, MonadFailure InvalidRequest m) =>
               s
@@ -167,8 +168,9 @@ sendResponse h res = do
     BS.hPut h $ SL.pack "\r\n"
     case body res of
         Left fp -> unsafeSendFile h fp
-        Right enum -> enum $ BS.hPut h
+        Right enum -> enum myPut h >> return ()
     where
+        myPut _ bs = BS.hPut h bs >> return (Right h)
         putHeader (x, y) = do
             BS.hPut h $ responseHeaderToBS x
             BS.hPut h $ SL.pack ": "
diff --git a/test.hs b/test.hs
index 1e8b5c1..a92d1e5 100644
--- a/test.hs
+++ b/test.hs
@@ -1,7 +1,7 @@
+{-# LANGUAGE Rank2Types #-}
 import Network.Wai
 import Network.Wai.Handler.SimpleServer
 import qualified Data.ByteString.Char8 as B8
-import qualified Data.ByteString as B
 
 main :: IO ()
 main = putStrLn "http://localhost:3000/" >> run 3000 app
@@ -18,19 +18,12 @@ indexResponse = return Response
     , body = index
     }
 
-postResponse :: IO (Maybe B.ByteString) -> IO Response
+postResponse :: (forall a. Enumerator a) -> IO Response
 postResponse rb = return Response
     { status = Status200
     , headers = [(ContentType, B8.pack "text/plain")]
-    , body = Right $ postBody rb
+    , body = Right rb
     }
 
 index :: Either FilePath a
 index = Left "index.html"
-
-postBody :: IO (Maybe B.ByteString) -> (B.ByteString -> IO ()) -> IO ()
-postBody req res = do
-    mbs <- req
-    case mbs of
-        Nothing -> return ()
-        Just bs -> res bs >> postBody req res