diff --git a/CHANGELOG.md b/CHANGELOG.md
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,10 @@
 See also https://pvp.haskell.org/faq
 
+#### 1.3.1.0 *(minor)*
+
+- Evaluate message before taking lock in simple handler ([#49](https://github.com/haskell-hvr/hslogger/pull/49))
+- Define `Typeable`, `Data`, `Generic` and `NFData` instances for `System.Log.Priority` ([#43](https://github.com/haskell-hvr/hslogger/pull/43))
+
 ## 1.3.0.0 *(major)*
 
 - **[semantic change]** Messages are encoded as UTF-8 (previously the encoding was locale dependent) for the Syslog and Growl backends
diff --git a/hslogger.cabal b/hslogger.cabal
--- a/hslogger.cabal
+++ b/hslogger.cabal
@@ -1,7 +1,7 @@
 cabal-version: 1.12
 build-type: Simple
 name: hslogger
-version: 1.3.0.0
+version: 1.3.1.0
 
 maintainer: hvr@gnu.org
 author: John Goerzen
@@ -61,24 +61,27 @@
         UTF8
 
     default-language: Haskell2010
-    other-extensions: CPP ExistentialQuantification
+    other-extensions: CPP ExistentialQuantification DeriveDataTypeable
 
     build-depends:
-        base       >= 4.3 && < 4.13
+        base       >= 4.3 && < 4.14
       , bytestring >= 0.9 && < 0.11
       , containers >= 0.4 && < 0.7
+      , deepseq    >= 1.1 && < 1.5
       , time       >= 1.2 && < 1.10
       , old-locale >= 1.0 && < 1.1
 
     if flag(network--GT-3_0_0)
       build-depends: network-bsd >= 2.8.1 && <2.9,
-                     network >= 3.0 && <3.1
+                     network >= 3.0 && <3.2
     else
       build-depends: network >= 2.6 && <2.9
 
     if !os(windows)
       Build-Depends: unix >= 2.4.2 && < 2.8
 
+    if !impl(ghc >= 7.6)
+      build-depends: ghc-prim
 
 test-suite runtests
     type: exitcode-stdio-1.0
diff --git a/src/System/Log.hs b/src/System/Log.hs
--- a/src/System/Log.hs
+++ b/src/System/Log.hs
@@ -1,3 +1,10 @@
+{-# LANGUAGE CPP #-}
+{-# LANGUAGE DeriveDataTypeable #-}
+
+#if __GLASGOW_HASKELL__ >= 702
+{-# LANGUAGE DeriveGeneric #-}
+#endif
+
 {- |
    Module     : System.Log
    Copyright  : Copyright (C) 2004-2011 John Goerzen
@@ -25,6 +32,12 @@
 
     where
 
+import Control.DeepSeq (NFData(rnf))
+import Data.Data (Data, Typeable)
+#if __GLASGOW_HASKELL__ >= 702
+import  GHC.Generics (Generic)
+#endif
+
 {- | Priorities are used to define how important a log message is.
 Users can filter log messages based on priorities.
 
@@ -33,7 +46,7 @@
 like.  They are listed here in ascending importance order.
 -}
 
-data Priority = 
+data Priority =
             DEBUG                   -- ^ Debug messages
           | INFO                    -- ^ Information
           | NOTICE                  -- ^ Normal runtime conditions
@@ -42,9 +55,15 @@
           | CRITICAL                -- ^ Severe situations
           | ALERT                   -- ^ Take immediate action
           | EMERGENCY               -- ^ System is unusable
-                    deriving (Eq, Ord, Enum, Bounded, Show, Read)
+#if __GLASGOW_HASKELL__ >= 702
+          deriving (Eq, Ord, Enum, Bounded, Show, Read, Data, Typeable, Generic)
+#else
+          deriving (Eq, Ord, Enum, Bounded, Show, Read, Data, Typeable)
+#endif
 
+-- | @since 1.3.1.0
+instance NFData Priority where rnf = (`seq` ())
+
 {- | Internal type of log records -}
 
 type LogRecord = (Priority, String)
-
diff --git a/src/System/Log/Handler/Simple.hs b/src/System/Log/Handler/Simple.hs
--- a/src/System/Log/Handler/Simple.hs
+++ b/src/System/Log/Handler/Simple.hs
@@ -16,6 +16,7 @@
     where
 
 import Control.Exception (tryJust)
+import Control.DeepSeq
 import Data.Char (ord)
 
 import System.Log
@@ -51,6 +52,7 @@
 streamHandler h pri =
     do lock <- newMVar ()
        let mywritefunc hdl msg =
+               msg `deepseq`
                withMVar lock (\_ -> do writeToHandle hdl msg
                                        hFlush hdl
                              )