diff --git a/Control/Applicative/Perm.hs b/Control/Applicative/Perm.hs
--- a/Control/Applicative/Perm.hs
+++ b/Control/Applicative/Perm.hs
@@ -1,18 +1,18 @@
-{- |
-Copyright: Andy Sonnenburg (c) 2012
-License: BSD-style (see the file LICENSE)
-Maintainer: Andy Sonnenburg <andy22286@gmail.com>
-Stability: experimental
-Portability: non-portable
--}
-module Control.Applicative.Perm
-       ( Perm
-       , runPerm
-       , liftPerm
-       , hoistPerm
-       ) where
-
-import Control.Monad.Perm.Internal (Perm,
-                                    hoistPerm,
-                                    liftPerm,
-                                    runPerm)
+{- |
+Copyright: Andy Sonnenburg (c) 2012
+License: BSD-style (see the file LICENSE)
+Maintainer: Andy Sonnenburg <andy22286@gmail.com>
+Stability: experimental
+Portability: non-portable
+-}
+module Control.Applicative.Perm
+       ( Perm
+       , runPerm
+       , liftPerm
+       , hoistPerm
+       ) where
+
+import Control.Monad.Perm.Internal (Perm,
+                                    hoistPerm,
+                                    liftPerm,
+                                    runPerm)
diff --git a/Control/Monad/Perm.hs b/Control/Monad/Perm.hs
--- a/Control/Monad/Perm.hs
+++ b/Control/Monad/Perm.hs
@@ -1,18 +1,18 @@
-{- |
-Copyright: Andy Sonnenburg (c) 2012
-License: BSD-style (see the file LICENSE)
-Maintainer: Andy Sonnenburg <andy22286@gmail.com>
-Stability: experimental
-Portability: non-portable
--}
-module Control.Monad.Perm
-       ( PermT
-       , runPermT
-       , liftPerm
-       , hoistPerm
-       ) where
-
-import Control.Monad.Perm.Internal (PermT,
-                                    liftPerm,
-                                    hoistPerm,
-                                    runPermT)
+{- |
+Copyright: Andy Sonnenburg (c) 2012
+License: BSD-style (see the file LICENSE)
+Maintainer: Andy Sonnenburg <andy22286@gmail.com>
+Stability: experimental
+Portability: non-portable
+-}
+module Control.Monad.Perm
+       ( PermT
+       , runPermT
+       , liftPerm
+       , hoistPerm
+       ) where
+
+import Control.Monad.Perm.Internal (PermT,
+                                    liftPerm,
+                                    hoistPerm,
+                                    runPermT)
diff --git a/Control/Monad/Perm/Internal.hs b/Control/Monad/Perm/Internal.hs
--- a/Control/Monad/Perm/Internal.hs
+++ b/Control/Monad/Perm/Internal.hs
@@ -23,23 +23,31 @@
 
 import Control.Applicative
 import Control.Monad
-import Control.Monad.Catch.Class
-import Control.Monad.IO.Class
-import Control.Monad.Reader.Class
-import Control.Monad.State.Class
+#if LANGUAGE_DefaultSignatures
+import Control.Monad.Catch.Class (MonadThrow)
+#else
+import Control.Monad.Catch.Class (MonadThrow (throw))
+#endif
+import Control.Monad.IO.Class (MonadIO (liftIO))
+#if MIN_VERSION_mtl(2, 1, 0)
+import Control.Monad.Reader.Class (MonadReader (ask, local, reader))
+import Control.Monad.State.Class (MonadState (get, put, state))
+#else
+import Control.Monad.Reader.Class (MonadReader (ask, local))
+import Control.Monad.State.Class (MonadState (get, put))
+#endif
 import Control.Monad.Trans.Class (MonadTrans (lift))
 
 import Data.Foldable (foldr)
-import Data.Monoid (mempty)
-#if defined(__GLASGOW_HASKELL__) && __GLASGOW_HASKELL__ >= 704
-import Data.Monoid ((<>))
+#if MIN_VERSION_base(4, 5, 0)
+import Data.Monoid ((<>), mempty)
 #else
-import Data.Monoid (Monoid, mappend)
+import Data.Monoid (Monoid, mappend, mempty)
 #endif
 
 import Prelude (Maybe (..), ($), (.), const, flip, fst, id, map, maybe)
 
-#if !defined(__GLASGOW_HASKELL__) || __GLASGOW_HASKELL__ < 704
+#if !MIN_VERSION_base(4, 5, 0)
 (<>) :: Monoid m => m -> m -> m
 (<>) = mappend
 {-# INLINE (<>) #-}
@@ -57,16 +65,25 @@
 
 instance Functor (PermT m) where
   fmap f (Choice a xs) = Choice (f <$> a) (fmap f <$> xs)
+#if MIN_VERSION_base(4, 2, 0)
+  a <$ Choice b xs = Choice (a <$ b) (fmap (a <$) xs)
+#endif
 
 instance Functor (Branch m) where
   fmap f (Ap perm m) = Ap (fmap (f .) perm) m
   fmap f (Bind k m) = Bind (fmap f . k) m
+#if MIN_VERSION_base(4, 2, 0)
+  a <$ Ap perm m = Ap (const a <$ perm) m
+  a <$ Bind k m = Bind ((a <$) . k) m
+#endif
 
 instance Applicative (PermT m) where
   pure a = Choice (pure a) mempty
   f@(Choice f' fs) <*> a@(Choice a' as) =
     Choice (f' <*> a') (fmap (`apB` a) fs <> fmap (f `apP`) as)
+#if MIN_VERSION_base(4, 2, 0)
   (*>) = liftThen (*>)
+#endif
 
 apP :: PermT m (a -> b) -> Branch m a -> Branch m b
 f `apP` Ap perm m = (f .@ perm) `Ap` m
@@ -92,7 +109,7 @@
   Choice (Just a) xs >>= k = case k a of
     Choice a' xs' -> Choice a' (map (bindP k) xs <> xs')
   (>>) = liftThen (>>)
-  fail _ = Choice mzero mempty
+  fail s = Choice (fail s) mempty
 
 bindP :: Monad m => (a -> PermT m b) -> Branch m a -> Branch m b
 bindP k (Ap perm m) = Bind (\ a -> k . ($ a) =<< perm) m
@@ -111,6 +128,9 @@
 instance MonadReader r m => MonadReader r (PermT m) where
   ask = lift ask
   local f (Choice a xs) = Choice a (map (localBranch f) xs)
+#if MIN_VERSION_mtl(2, 1, 0)
+  reader = lift . reader
+#endif
 
 localBranch :: MonadReader r m => (r -> r) -> Branch m a -> Branch m a
 localBranch f (Ap perm m) = Ap (local f perm) (local f m)
@@ -119,6 +139,9 @@
 instance MonadState s m => MonadState s (PermT m) where
   get = lift get
   put = lift . put
+#if MIN_VERSION_mtl(2, 1, 0)
+  state = lift . state
+#endif
 
 #ifdef LANGUAGE_DefaultSignatures
 instance MonadThrow e m => MonadThrow e (PermT m)
@@ -171,8 +194,8 @@
 liftBranch = Ap (Choice (pure id) mempty)
 
 {- |
-Lift a natural transformation from @m@ to @n@ into a natural transformation
-from @'PermT'' c m@ to @'PermT'' c n@.
+Lift a monad homomorphism from @m@ to @n@ into a monad homomorphism from
+@'PermT' m@ to @'PermT' n@.
 -}
 hoistPerm :: Monad n => (forall a . m a -> n a) -> PermT m b -> PermT n b
 hoistPerm f (Choice a xs) = Choice a (hoistBranch f <$> xs)
diff --git a/LICENSE b/LICENSE
--- a/LICENSE
+++ b/LICENSE
@@ -1,30 +1,30 @@
-Copyright (c) 2012, Andy Sonnenburg
-
-All rights reserved.
-
-Redistribution and use in source and binary forms, with or without
-modification, are permitted provided that the following conditions are met:
-
-    * Redistributions of source code must retain the above copyright
-      notice, this list of conditions and the following disclaimer.
-
-    * Redistributions in binary form must reproduce the above
-      copyright notice, this list of conditions and the following
-      disclaimer in the documentation and/or other materials provided
-      with the distribution.
-
-    * Neither the name of Andy Sonnenburg nor the names of other
-      contributors may be used to endorse or promote products derived
-      from this software without specific prior written permission.
-
-THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
-"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
-LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
-A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
-OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
-SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
-LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
-DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
-THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
-(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
-OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+Copyright (c) 2012, Andy Sonnenburg
+
+All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are met:
+
+    * Redistributions of source code must retain the above copyright
+      notice, this list of conditions and the following disclaimer.
+
+    * Redistributions in binary form must reproduce the above
+      copyright notice, this list of conditions and the following
+      disclaimer in the documentation and/or other materials provided
+      with the distribution.
+
+    * Neither the name of Andy Sonnenburg nor the names of other
+      contributors may be used to endorse or promote products derived
+      from this software without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
diff --git a/Setup.lhs b/Setup.lhs
--- a/Setup.lhs
+++ b/Setup.lhs
@@ -1,7 +1,7 @@
-#!/usr/bin/env runhaskell
-> module Main (main) where
-
-> import Distribution.Simple (defaultMain)
-
-> main :: IO ()
-> main = defaultMain
+#!/usr/bin/env runhaskell
+> module Main (main) where
+
+> import Distribution.Simple (defaultMain)
+
+> main :: IO ()
+> main = defaultMain
diff --git a/perm.cabal b/perm.cabal
--- a/perm.cabal
+++ b/perm.cabal
@@ -1,5 +1,5 @@
 name:          perm
-version:       0.2.0.0
+version:       0.2.0.1
 cabal-version: >= 1.10
 synopsis:      permutation Applicative and Monad with many mtl instances
 description: