diff --git a/haskell/src/Data/MessagePack/Base.hsc b/haskell/src/Data/MessagePack/Base.hsc
index ad717120..72c421ce 100644
--- a/haskell/src/Data/MessagePack/Base.hsc
+++ b/haskell/src/Data/MessagePack/Base.hsc
@@ -297,7 +297,7 @@ foreign import ccall "msgpack_pack_raw_body_wrap" msgpack_pack_raw_body ::
 -- | Pack a single byte stream. It calls 'packRAW' and 'packRAWBody'.
 packRAW' :: Packer -> ByteString -> IO Int
 packRAW' pc bs = do
-  packRAW pc (BS.length bs)
+  _ <- packRAW pc (BS.length bs)
   packRAWBody pc bs
 
 type Unpacker = ForeignPtr ()
@@ -475,7 +475,7 @@ peekObject ptr = do
     (#const MSGPACK_OBJECT_MAP) ->
       peekObjectMap ptr
     _ ->
-      fail "peekObject: unknown object type"
+      fail $ "peekObject: unknown object type (" ++ show typ ++ ")"
 
 peekObjectBool :: Ptr a -> IO Object
 peekObjectBool ptr = do
@@ -541,11 +541,11 @@ packObject pc (ObjectDouble d) = packDouble pc d >> return ()
 packObject pc (ObjectRAW bs) = packRAW' pc bs >> return ()
 
 packObject pc (ObjectArray ls) = do
-  packArray pc (length ls)
+  _ <- packArray pc (length ls)
   mapM_ (packObject pc) ls
 
 packObject pc (ObjectMap ls) = do
-  packMap pc (length ls)
+  _ <- packMap pc (length ls)
   mapM_ (\(a, b) -> packObject pc a >> packObject pc b) ls
 
 data UnpackReturn =
diff --git a/haskell/src/Data/MessagePack/Class.hs b/haskell/src/Data/MessagePack/Class.hs
index 34a7772f..365acc5f 100644
--- a/haskell/src/Data/MessagePack/Class.hs
+++ b/haskell/src/Data/MessagePack/Class.hs
@@ -27,7 +27,6 @@ module Data.MessagePack.Class(
 import Control.Monad.Error
 import Data.ByteString.Char8 (ByteString)
 import qualified Data.ByteString.Char8 as C8
-import Data.Either
 
 import Data.MessagePack.Base
 
diff --git a/haskell/src/Data/MessagePack/Feed.hs b/haskell/src/Data/MessagePack/Feed.hs
index 93bdd9b5..4b486396 100644
--- a/haskell/src/Data/MessagePack/Feed.hs
+++ b/haskell/src/Data/MessagePack/Feed.hs
@@ -21,7 +21,6 @@ module Data.MessagePack.Feed(
   feederFromString,
   ) where
 
-import Control.Monad
 import Data.ByteString (ByteString)
 import qualified Data.ByteString as BS
 import Data.IORef
@@ -37,9 +36,9 @@ feederFromHandle h = return $ do
   if BS.length bs > 0
     then do return $ Just bs
     else do
-    bs <- BS.hGet h 1
-    if BS.length bs > 0
-      then do return $ Just bs
+    c <- BS.hGet h 1
+    if BS.length c > 0
+      then do return $ Just c
       else do
       hClose h
       return Nothing
diff --git a/haskell/src/Data/MessagePack/Monad.hs b/haskell/src/Data/MessagePack/Monad.hs
index cf3a0fd9..c718b8ad 100644
--- a/haskell/src/Data/MessagePack/Monad.hs
+++ b/haskell/src/Data/MessagePack/Monad.hs
@@ -79,7 +79,7 @@ packToString :: MonadIO m => PackerT m r -> m ByteString
 packToString m = do
   sb <- liftIO $ newSimpleBuffer
   pc <- liftIO $ newPacker sb
-  runPackerT m pc
+  _ <- runPackerT m pc
   liftIO $ simpleBufferData sb
 
 -- | Execcute given serializer and write byte sequence to Handle.
diff --git a/haskell/src/Data/MessagePack/Stream.hs b/haskell/src/Data/MessagePack/Stream.hs
index bd17f467..c56fe8d4 100644
--- a/haskell/src/Data/MessagePack/Stream.hs
+++ b/haskell/src/Data/MessagePack/Stream.hs
@@ -19,9 +19,7 @@ module Data.MessagePack.Stream(
   unpackObjectsFromString,
   ) where
 
-import Control.Monad
 import Data.ByteString (ByteString)
-import qualified Data.ByteString as BS
 import System.IO
 import System.IO.Unsafe