diff --git a/Control/Monad/Trans/RWS/CPS.hs b/Control/Monad/Trans/RWS/CPS.hs
--- a/Control/Monad/Trans/RWS/CPS.hs
+++ b/Control/Monad/Trans/RWS/CPS.hs
@@ -5,6 +5,24 @@
 #if __GLASGOW_HASKELL__ >= 710
 {-# LANGUAGE AutoDeriveTypeable #-}
 #endif
+-----------------------------------------------------------------------------
+-- |
+-- Module      :  Control.Monad.Trans.RWS.CPS
+-- Copyright   :  (c) Daniel Mendler 2016,
+--                (c) Andy Gill 2001,
+--                (c) Oregon Graduate Institute of Science and Technology, 2001
+-- License     :  BSD-style (see the file LICENSE)
+--
+-- Maintainer  :  mail@daniel-mendler.de
+-- Stability   :  experimental
+-- Portability :  portable
+--
+-- A monad transformer that combines 'ReaderT', 'WriterT' and 'StateT'.
+-- This version uses continuation-passing-style for the writer part
+-- to achieve constant space usage. This transformer can be used as a
+-- drop-in replacement for Control.Monad.Trans.RWS.Strict/Lazy.
+-----------------------------------------------------------------------------
+
 module Control.Monad.Trans.RWS.CPS (
   -- * The RWS monda
   RWS,
diff --git a/Control/Monad/Trans/Writer/CPS.hs b/Control/Monad/Trans/Writer/CPS.hs
--- a/Control/Monad/Trans/Writer/CPS.hs
+++ b/Control/Monad/Trans/Writer/CPS.hs
@@ -5,6 +5,30 @@
 #if __GLASGOW_HASKELL__ >= 710
 {-# LANGUAGE AutoDeriveTypeable #-}
 #endif
+-----------------------------------------------------------------------------
+-- |
+-- Module      :  Control.Monad.Trans.Writer.CPS
+-- Copyright   :  (c) Daniel Mendler 2016,
+--                (c) Andy Gill 2001,
+--                (c) Oregon Graduate Institute of Science and Technology, 2001
+-- License     :  BSD-style (see the file LICENSE)
+--
+-- Maintainer  :  mail@daniel-mendler.de
+-- Stability   :  experimental
+-- Portability :  portable
+--
+-- The strict 'WriterT' monad transformer, which adds collection of
+-- outputs (such as a count or string output) to a given monad.
+--
+-- This monad transformer provides only limited access to the output
+-- during the computation. For more general access, use
+-- "Control.Monad.Trans.State" instead.
+--
+-- This version builds its output strictly and uses continuation-passing-style
+-- to achieve constant space usage. This transformer can be used as a
+-- drop-in replacement for Control.Monad.Trans.Writer.Strict/Lazy.
+-----------------------------------------------------------------------------
+
 module Control.Monad.Trans.Writer.CPS (
   -- * The Writer monad
   Writer,
@@ -109,7 +133,7 @@
 {-# INLINE mapWriterT #-}
 
 instance Functor m => Functor (WriterT w m) where
-  fmap f m = WriterT $ \w -> (\(a, w) -> (f a, w)) <$> unWriterT m w
+  fmap f m = WriterT $ \w -> (\(a, w') -> (f a, w')) <$> unWriterT m w
   {-# INLINE fmap #-}
 
 instance (Functor m, Monad m) => Applicative (WriterT w m) where
diff --git a/writer-cps-transformers.cabal b/writer-cps-transformers.cabal
--- a/writer-cps-transformers.cabal
+++ b/writer-cps-transformers.cabal
@@ -3,8 +3,8 @@
 -- see: https://github.com/sol/hpack
 
 name:           writer-cps-transformers
-version:        0.1.0.1
-license:        MIT
+version:        0.1.0.2
+license:        BSD3
 license-file:   LICENSE
 tested-with:    GHC == 7.0.4, GHC == 7.2.2, GHC == 7.4.2, GHC == 7.6.3, GHC == 7.8.4, GHC == 7.10.3, GHC == 8.0.1
 author:         Andy Gill, Ross Paterson, Daniel Mendler
@@ -14,7 +14,7 @@
 category:       Control
 homepage:       https://github.com/minad/writer-cps-transformers#readme
 synopsis:       WriteT and RWST monad transformers
-description:    The WriterT and RWST monad transformers provided by writer-cps-transformers are written in continuation passing style and avoid the space-leak problem of the traditional Control.Monad.Trans.Writer.Strict/Lazy.
+description:    The WriterT and RWST monad transformers provided by writer-cps-transformers are written in continuation passing style and avoid the space-leak problem of the traditional Control.Monad.Trans.Writer.Strict/Lazy. The corresponding MTL class instances are in the package writer-cps-mtl (<https://hackage.haskell.org/package/writer-cps-mtl>).
 build-type:     Simple
 cabal-version:  >= 1.10