diff --git a/examples/serversession-example-yesod-persistent/test/TestImport.hs b/examples/serversession-example-yesod-persistent/test/TestImport.hs
index 69b18ed..6e0ca2b 100644
--- a/examples/serversession-example-yesod-persistent/test/TestImport.hs
+++ b/examples/serversession-example-yesod-persistent/test/TestImport.hs
@@ -10,7 +10,6 @@ import Database.Persist.Sql  (SqlPersistM, SqlBackend, runSqlPersistMPool, rawEx
 import Foundation            as X
 import Model                 as X
 import Test.Hspec            as X
-import Text.Shakespeare.Text (st)
 import Yesod.Default.Config2 (ignoreEnv, loadAppSettings)
 import Yesod.Test            as X
 
diff --git a/serversession-backend-acid-state/serversession-backend-acid-state.cabal b/serversession-backend-acid-state/serversession-backend-acid-state.cabal
index 8772ae5..95ea560 100644
--- a/serversession-backend-acid-state/serversession-backend-acid-state.cabal
+++ b/serversession-backend-acid-state/serversession-backend-acid-state.cabal
@@ -13,6 +13,10 @@ homepage:        https://github.com/yesodweb/serversession
 description:     API docs and the README are available at <http://www.stackage.org/package/serversession-backend-acid-state>
 extra-source-files: README.md
 
+flag lib-Werror
+  default: False
+  manual: True
+
 library
   hs-source-dirs: src
   build-depends:
@@ -35,6 +39,8 @@ library
     TypeFamilies
     UndecidableInstances
   ghc-options:     -Wall
+  if flag(lib-Werror)
+    ghc-options: -Werror
 
 
 test-suite tests
@@ -51,6 +57,8 @@ test-suite tests
   extensions:
     CPP
   ghc-options:     -Wall -threaded "-with-rtsopts=-N -s -M1G -c" -rtsopts
+  if flag(lib-Werror)
+    ghc-options: -Werror
 
 
 source-repository head
diff --git a/serversession-backend-acid-state/src/Web/ServerSession/Backend/Acid/Internal.hs b/serversession-backend-acid-state/src/Web/ServerSession/Backend/Acid/Internal.hs
index 0708805..4b353db 100644
--- a/serversession-backend-acid-state/src/Web/ServerSession/Backend/Acid/Internal.hs
+++ b/serversession-backend-acid-state/src/Web/ServerSession/Backend/Acid/Internal.hs
@@ -1,3 +1,4 @@
+{-# OPTIONS_GHC -fno-warn-orphans #-}
 -- | Internal module exposing the guts of the package.  Use at
 -- your own risk.  No API stability guarantees apply.
 module Web.ServerSession.Backend.Acid.Internal
@@ -23,7 +24,7 @@ module Web.ServerSession.Backend.Acid.Internal
   , AcidStorage(..)
   ) where
 
-import Control.Applicative ((<$>), (<*>))
+import Control.Applicative as A
 import Control.Monad.Reader (ask)
 import Control.Monad.State (get, modify, put)
 import Data.Acid
@@ -103,7 +104,7 @@ insertSessionForAuthId sid = maybe id (flip (HM.insertWith S.union) (S.singleton
 -- @safeCopy@ doesn't contain instances for @HashMap@ as of now.
 instance SafeCopy SS.SessionMap where
   putCopy = contain . safePut . HM.toList . SS.unSessionMap
-  getCopy = contain $ SS.SessionMap . HM.fromList <$> safeGet
+  getCopy = contain $ SS.SessionMap . HM.fromList A.<$> safeGet
 
 
 -- | We can't @deriveSafeCopy 0 'base ''SS.SessionId@ as
diff --git a/serversession-backend-acid-state/tests/Main.hs b/serversession-backend-acid-state/tests/Main.hs
index 884f83b..8307f1c 100644
--- a/serversession-backend-acid-state/tests/Main.hs
+++ b/serversession-backend-acid-state/tests/Main.hs
@@ -1,6 +1,6 @@
 module Main (main) where
 
-import Control.Applicative ((<$>))
+import Control.Applicative as A
 import Data.Acid.Local (openLocalState, createCheckpointAndClose)
 import Data.Acid.Memory (openMemoryState)
 import Test.Hspec
@@ -15,7 +15,7 @@ main =
     (AcidStorage <$> openLocalState emptyState)
     (createCheckpointAndClose . acidState) $
     \acidLocal -> hspec $ do
-      acidMem <- runIO $ AcidStorage <$> openMemoryState emptyState
+      acidMem <- runIO $ AcidStorage A.<$> openMemoryState emptyState
       describe "AcidStorage on memory only" $
         allStorageTests acidMem it runIO parallel shouldBe shouldReturn shouldThrow
       describe "AcidStorage on local storage" $
diff --git a/serversession-backend-persistent/serversession-backend-persistent.cabal b/serversession-backend-persistent/serversession-backend-persistent.cabal
index 1fe7921..ec2f901 100644
--- a/serversession-backend-persistent/serversession-backend-persistent.cabal
+++ b/serversession-backend-persistent/serversession-backend-persistent.cabal
@@ -13,6 +13,10 @@ homepage:        https://github.com/yesodweb/serversession
 description:     API docs and the README are available at <http://www.stackage.org/package/serversession-backend-persistent>
 extra-source-files: README.md
 
+flag lib-Werror
+  default: False
+  manual: True
+
 library
   hs-source-dirs: src
   build-depends:
@@ -51,6 +55,9 @@ library
     TypeFamilies
     UndecidableInstances
   ghc-options:     -Wall
+  if flag(lib-Werror)
+    ghc-options: -Werror
+
 
 
 test-suite tests
@@ -76,6 +83,9 @@ test-suite tests
     TemplateHaskell
   main-is:         Main.hs
   ghc-options:     -Wall -threaded "-with-rtsopts=-N -s -M1G -c" -rtsopts
+  if flag(lib-Werror)
+    ghc-options: -Werror
+
 
 
 source-repository head
diff --git a/serversession-backend-persistent/src/Web/ServerSession/Backend/Persistent/Internal/Impl.hs b/serversession-backend-persistent/src/Web/ServerSession/Backend/Persistent/Internal/Impl.hs
index 70afadb..de17d0f 100644
--- a/serversession-backend-persistent/src/Web/ServerSession/Backend/Persistent/Internal/Impl.hs
+++ b/serversession-backend-persistent/src/Web/ServerSession/Backend/Persistent/Internal/Impl.hs
@@ -12,10 +12,10 @@ module Web.ServerSession.Backend.Persistent.Internal.Impl
   , throwSS
   ) where
 
-import Control.Applicative ((<$>), (<*>))
+import Control.Applicative as A
 import Control.Monad (void)
 import Control.Monad.IO.Class (liftIO)
-import Data.Monoid (mempty)
+import Data.Monoid as M
 import Data.Proxy (Proxy(..))
 import Data.Time (UTCTime)
 import Data.Typeable (Typeable)
@@ -94,7 +94,7 @@ instance forall sess. P.PersistFieldSql (Decomposed sess) => P.PersistEntity (Pe
         []
         []
         ["Eq", "Ord", "Show", "Typeable"]
-        mempty
+        M.mempty
         False
     where
       pfd :: P.EntityField (PersistentSession sess) typ -> P.FieldDef
@@ -109,7 +109,7 @@ instance forall sess. P.PersistFieldSql (Decomposed sess) => P.PersistEntity (Pe
 
   fromPersistValues [a, b, c, d, e] =
     PersistentSession
-      <$> err "key"        (P.fromPersistValue a)
+      A.<$> err "key"        (P.fromPersistValue a)
       <*> err "authId"     (P.fromPersistValue b)
       <*> err "session"    (P.fromPersistValue c)
       <*> err "createdAt"  (P.fromPersistValue d)
diff --git a/serversession-backend-persistent/src/Web/ServerSession/Backend/Persistent/Internal/Types.hs b/serversession-backend-persistent/src/Web/ServerSession/Backend/Persistent/Internal/Types.hs
index b54b396..130c89e 100644
--- a/serversession-backend-persistent/src/Web/ServerSession/Backend/Persistent/Internal/Types.hs
+++ b/serversession-backend-persistent/src/Web/ServerSession/Backend/Persistent/Internal/Types.hs
@@ -12,7 +12,7 @@ module Web.ServerSession.Backend.Persistent.Internal.Types
     -- $orphanSessionMap
   ) where
 
-import Control.Applicative ((<$>))
+import Control.Applicative as A
 import Control.Arrow (first)
 import Control.Monad ((>=>), mzero)
 import Data.ByteString (ByteString)
@@ -103,7 +103,7 @@ instance PersistFieldSql SessionMap where
 
 instance S.Serialize SessionMap where
   put = S.put . map (first TE.encodeUtf8) . HM.toList . unSessionMap
-  get = SessionMap . HM.fromList . map (first TE.decodeUtf8) <$> S.get
+  get = SessionMap . HM.fromList . map (first TE.decodeUtf8) A.<$> S.get
 
 instance A.FromJSON SessionMap where
   parseJSON = fmap fixup . A.parseJSON
diff --git a/serversession-backend-redis/serversession-backend-redis.cabal b/serversession-backend-redis/serversession-backend-redis.cabal
index cb62c50..ae7f0aa 100644
--- a/serversession-backend-redis/serversession-backend-redis.cabal
+++ b/serversession-backend-redis/serversession-backend-redis.cabal
@@ -17,6 +17,10 @@ flag old-locale
   description: Use time-1.4 and old-locale (GHC 7.8).
   default:     False
 
+flag lib-Werror
+  default: False
+  manual: True
+
 library
   hs-source-dirs: src
   build-depends:
@@ -47,6 +51,9 @@ library
     ScopedTypeVariables
     TypeFamilies
   ghc-options:     -Wall
+  if flag(lib-Werror)
+    ghc-options: -Werror
+
 
 
 test-suite tests
@@ -62,6 +69,9 @@ test-suite tests
     , serversession-backend-redis
   main-is:         Main.hs
   ghc-options:     -Wall -threaded "-with-rtsopts=-N -s -M1G -c" -rtsopts
+  if flag(lib-Werror)
+    ghc-options: -Werror
+
 
 
 source-repository head
diff --git a/serversession-backend-redis/src/Web/ServerSession/Backend/Redis/Internal.hs b/serversession-backend-redis/src/Web/ServerSession/Backend/Redis/Internal.hs
index 5ac28d3..b0cdfc0 100644
--- a/serversession-backend-redis/src/Web/ServerSession/Backend/Redis/Internal.hs
+++ b/serversession-backend-redis/src/Web/ServerSession/Backend/Redis/Internal.hs
@@ -26,7 +26,7 @@ module Web.ServerSession.Backend.Redis.Internal
   , throwRS
   ) where
 
-import Control.Applicative ((<$), (<$>))
+import Control.Applicative as A
 import Control.Arrow (first)
 import Control.Monad (void, when)
 import Control.Monad.IO.Class (liftIO)
@@ -230,7 +230,7 @@ batched f xs =
 
 -- | Get the session for the given session ID.
 getSessionImpl :: RedisSession sess => SessionId sess -> R.Redis (Maybe (Session sess))
-getSessionImpl sid = parseSession sid <$> unwrap (R.hgetall $ rSessionKey sid)
+getSessionImpl sid = parseSession sid A.<$> unwrap (R.hgetall $ rSessionKey sid)
 
 
 -- | Delete the session with given session ID.
diff --git a/serversession-backend-redis/tests/Main.hs b/serversession-backend-redis/tests/Main.hs
index 2258c9f..64c15df 100644
--- a/serversession-backend-redis/tests/Main.hs
+++ b/serversession-backend-redis/tests/Main.hs
@@ -3,7 +3,6 @@ module Main (main) where
 import Database.Redis (connect, defaultConnectInfo)
 import Test.Hspec
 import Web.ServerSession.Backend.Redis
-import Web.ServerSession.Core
 import Web.ServerSession.Core.StorageTests
 
 main :: IO ()
diff --git a/serversession-frontend-snap/serversession-frontend-snap.cabal b/serversession-frontend-snap/serversession-frontend-snap.cabal
index 92fd32c..61ee43f 100644
--- a/serversession-frontend-snap/serversession-frontend-snap.cabal
+++ b/serversession-frontend-snap/serversession-frontend-snap.cabal
@@ -13,6 +13,11 @@ homepage:        https://github.com/yesodweb/serversession
 description:     API docs and the README are available at <http://www.stackage.org/package/serversession-frontend-snap>
 extra-source-files: README.md
 
+flag lib-Werror
+  default: False
+  manual: True
+
+
 library
   hs-source-dirs: src
   build-depends:
@@ -38,6 +43,9 @@ library
     TypeFamilies
     UndecidableInstances
   ghc-options:     -Wall
+  if flag(lib-Werror)
+    ghc-options: -Werror
+
 
 source-repository head
   type:     git
diff --git a/serversession-frontend-snap/src/Web/ServerSession/Frontend/Snap/Internal.hs b/serversession-frontend-snap/src/Web/ServerSession/Frontend/Snap/Internal.hs
index 84c1874..1527653 100644
--- a/serversession-frontend-snap/src/Web/ServerSession/Frontend/Snap/Internal.hs
+++ b/serversession-frontend-snap/src/Web/ServerSession/Frontend/Snap/Internal.hs
@@ -12,7 +12,7 @@ module Web.ServerSession.Frontend.Snap.Internal
   , forceInvalidate
   ) where
 
-import Control.Applicative ((<$>))
+import Control.Applicative as A
 import Control.Arrow (first, second)
 import Control.Monad.IO.Class (liftIO)
 import Data.ByteString (ByteString)
@@ -137,7 +137,7 @@ instance ( Storage sto
     mcookie <- S.getCookie (cookieName ssm)
     -- Load session from storage backend.
     (data1, saveSessionToken) <-
-      liftIO $ loadSession (state ssm) (S.cookieValue <$> mcookie)
+      liftIO $ loadSession (state ssm) (S.cookieValue A.<$> mcookie)
     -- Add CSRF token if needed.
     data2 <-
       maybe
diff --git a/serversession-frontend-wai/serversession-frontend-wai.cabal b/serversession-frontend-wai/serversession-frontend-wai.cabal
index 5889446..044e670 100644
--- a/serversession-frontend-wai/serversession-frontend-wai.cabal
+++ b/serversession-frontend-wai/serversession-frontend-wai.cabal
@@ -13,6 +13,10 @@ homepage:        https://github.com/yesodweb/serversession
 description:     API docs and the README are available at <http://www.stackage.org/package/serversession-frontend-wai>
 extra-source-files: README.md
 
+flag lib-Werror
+  default: False
+  manual: True
+
 library
   hs-source-dirs: src
   build-depends:
@@ -38,6 +42,8 @@ library
     OverloadedStrings
     TypeFamilies
   ghc-options:     -Wall
+  if flag(lib-Werror)
+    ghc-options: -Werror
 
 source-repository head
   type:     git
diff --git a/serversession-frontend-wai/src/Web/ServerSession/Frontend/Wai/Internal.hs b/serversession-frontend-wai/src/Web/ServerSession/Frontend/Wai/Internal.hs
index 320d26c..3b722ac 100644
--- a/serversession-frontend-wai/src/Web/ServerSession/Frontend/Wai/Internal.hs
+++ b/serversession-frontend-wai/src/Web/ServerSession/Frontend/Wai/Internal.hs
@@ -10,7 +10,7 @@ module Web.ServerSession.Frontend.Wai.Internal
   , forceInvalidate
   ) where
 
-import Control.Applicative ((<$>))
+import Control.Applicative as A
 import Control.Monad (guard)
 import Control.Monad.IO.Class (MonadIO(..))
 import Data.ByteString (ByteString)
@@ -80,7 +80,7 @@ mkSession sessionRef =
   -- We need to use atomicModifyIORef instead of readIORef
   -- because latter may be reordered (cf. "Memory Model" on
   -- Data.IORef's documentation).
-  ( \k   -> kvLookup k <$> liftIO (I.atomicModifyIORef' sessionRef $ \a -> (a, a))
+  ( \k   -> kvLookup k A.<$> liftIO (I.atomicModifyIORef' sessionRef $ \a -> (a, a))
   , \k v -> liftIO (I.atomicModifyIORef' sessionRef $ flip (,) () . kvInsert k v)
   )
 
diff --git a/serversession-frontend-yesod/serversession-frontend-yesod.cabal b/serversession-frontend-yesod/serversession-frontend-yesod.cabal
index 95becb7..6d69a92 100644
--- a/serversession-frontend-yesod/serversession-frontend-yesod.cabal
+++ b/serversession-frontend-yesod/serversession-frontend-yesod.cabal
@@ -13,6 +13,10 @@ homepage:        https://github.com/yesodweb/serversession
 description:     API docs and the README are available at <http://www.stackage.org/package/serversession-frontend-yesod>
 extra-source-files: README.md
 
+flag lib-Werror
+  default: False
+  manual: True
+
 library
   hs-source-dirs: src
   build-depends:
@@ -38,6 +42,8 @@ library
     OverloadedStrings
     TypeFamilies
   ghc-options:     -Wall
+  if flag(lib-Werror)
+    ghc-options: -Werror
 
 source-repository head
   type:     git
diff --git a/serversession/serversession.cabal b/serversession/serversession.cabal
index 7589d6d..29ac882 100644
--- a/serversession/serversession.cabal
+++ b/serversession/serversession.cabal
@@ -13,6 +13,10 @@ homepage:        https://github.com/yesodweb/serversession
 description:     API docs and the README are available at <http://www.stackage.org/package/serversession>
 extra-source-files: README.md
 
+flag lib-Werror
+  default: False
+  manual: True
+
 library
   hs-source-dirs: src
   build-depends:
@@ -42,6 +46,9 @@ library
     TypeFamilies
     UndecidableInstances
   ghc-options:     -Wall
+  if flag(lib-Werror)
+    ghc-options: -Werror
+
 
 
 test-suite tests
@@ -66,6 +73,9 @@ test-suite tests
     UndecidableInstances
   main-is:         Main.hs
   ghc-options:     -Wall -threaded "-with-rtsopts=-N -s -M1G -c" -rtsopts
+  if flag(lib-Werror)
+    ghc-options: -Werror
+
 
 
 source-repository head
diff --git a/serversession/src/Web/ServerSession/Core/Internal.hs b/serversession/src/Web/ServerSession/Core/Internal.hs
index 26b4f13..c8d3b10 100644
--- a/serversession/src/Web/ServerSession/Core/Internal.hs
+++ b/serversession/src/Web/ServerSession/Core/Internal.hs
@@ -45,7 +45,7 @@ module Web.ServerSession.Core.Internal
   , ForceInvalidate(..)
   ) where
 
-import Control.Applicative ((<$>), (<*>))
+import Control.Applicative as A
 import Control.Monad (guard, when)
 import Control.Monad.IO.Class (MonadIO(..))
 import Data.ByteString (ByteString)
@@ -730,7 +730,7 @@ saveSessionOnDb state now maybeInput DecomposedSession {..} = do
   (saveToDb, key, createdAt) <-
     case maybeInput of
       Nothing -> liftIO $
-        (,,) <$> return (insertSession $ storage state)
+        (,,) A.<$> return (insertSession $ storage state)
              <*> generateSessionId (generator state)
              <*> return now
       Just Session {..} ->
diff --git a/serversession/src/Web/ServerSession/Core/StorageTests.hs b/serversession/src/Web/ServerSession/Core/StorageTests.hs
index 3591a5c..875d1b1 100644
--- a/serversession/src/Web/ServerSession/Core/StorageTests.hs
+++ b/serversession/src/Web/ServerSession/Core/StorageTests.hs
@@ -7,7 +7,7 @@ module Web.ServerSession.Core.StorageTests
   ( allStorageTests
   ) where
 
-import Control.Applicative ((<$), (<$>), (<*>))
+import Control.Applicative as A
 import Control.Exception (Exception)
 import Control.Monad
 import Web.ServerSession.Core.Internal
@@ -95,7 +95,7 @@ allStorageTests storage it runIO parallel _shouldBe shouldReturn shouldThrow = d
         master <- generateSession gen HasAuthId
         let Just authId = sessionAuthId master
         preslaves <-
-          (++) <$> replicateM 100 (generateSession gen HasAuthId)
+          (++) A.<$> replicateM 100 (generateSession gen HasAuthId)
                <*> replicateM 100 (generateSession gen NoAuthId)
         let slaves = (\s -> s { sessionAuthId = Just authId }) <$> preslaves
         others <-
diff --git a/serversession/tests/Main.hs b/serversession/tests/Main.hs
index e9bd33e..eaa07df 100644
--- a/serversession/tests/Main.hs
+++ b/serversession/tests/Main.hs
@@ -1,6 +1,6 @@
 module Main (main) where
 
-import Control.Applicative ((<$), (<$>), (<*>))
+import Control.Applicative as A
 import Control.Arrow
 import Control.Monad
 import Data.Maybe
@@ -49,7 +49,7 @@ main = hspec $ parallel $ do
       -- The probability of a given character not appearing on
       -- this test is (63/64)^(24*reps), so it's extremely
       -- unlikely for this test to fail on correct code.
-      let observed = S.fromList $ concat $ T.unpack . unS <$> sids
+      let observed = S.fromList $ concat $ T.unpack . unS A.<$> sids
           expected = S.fromList $ ['a'..'z'] ++ ['A'..'Z'] ++ ['0'..'9'] ++ "-_"
       observed `shouldBe` expected
 
diff --git a/stack.yaml b/stack.yaml
index a9c2e78..7968892 100644
--- a/stack.yaml
+++ b/stack.yaml
@@ -8,6 +8,21 @@ packages:
   - serversession-frontend-wai
   - serversession-frontend-yesod
   - examples/serversession-example-yesod-persistent
+flags:
+  serversession:
+    lib-Werror: true
+  serversession-backend-acid-state:
+    lib-Werror: true
+  serversession-backend-persistent:
+    lib-Werror: true
+  serversession-backend-redis:
+    lib-Werror: true
+  serversession-frontend-snap:
+    lib-Werror: true
+  serversession-frontend-wai:
+    lib-Werror: true
+  serversession-frontend-yesod:
+    lib-Werror: true
 extra-deps:
   - acid-state-0.14.0
   - nonce-1.0.2