diff --git a/LICENSE b/LICENSE
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--- /dev/null
+++ b/LICENSE
@@ -0,0 +1,30 @@
+Copyright (c) 2014, Jasper Van der Jeugt <m@jaspervdj.be>
+
+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 Jasper Van der Jeugt <m@jaspervdj.be> 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.hs b/Setup.hs
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--- /dev/null
+++ b/Setup.hs
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+import Distribution.Simple
+main = defaultMain
diff --git a/acme-cofunctor.cabal b/acme-cofunctor.cabal
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--- /dev/null
+++ b/acme-cofunctor.cabal
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+Name:          acme-cofunctor
+Version:       0.1.0.0
+Synopsis:      A Cofunctor is a structure from category theory dual to Functor
+License:       BSD3
+License-file:  LICENSE
+Author:        Jasper Van der Jeugt <m@jaspervdj.be>
+Maintainer:    Jasper Van der Jeugt <m@jaspervdj.be>
+Copyright:     2014 Jasper Van der Jeugt
+Category:      Acme
+Build-type:    Simple
+Cabal-version: >= 1.10
+Homepage:      https://github.com/jaspervdj/acme-cofunctor
+
+Description:
+  A 'Cofunctor' is a structure from category theory dual to 'Functor'.
+
+  .
+
+  A 'Functor' is defined by the operation 'fmap':
+
+  .
+
+  > fmap :: (a -> b) -> (f a -> f b)
+
+  .
+
+  This means that its dual must be defined by the following operation:
+
+  .
+
+  > cofmap :: (b -> a) -> (f b -> f a)
+
+  .
+
+  Since beginning his investigations, the author of this package has discovered
+  that this pattern is /at least/ as commonly used as 'Functor'. In fact, many
+  ubiquitous Haskell types (e.g. @[]@, 'Maybe', @((->) a)@ turn out to have a
+  'Cofunctor' instance.
+
+Library
+  Exposed-modules:  Data.Cofunctor
+  default-language: Haskell98
+  Ghc-options:      -Wall
+  Hs-source-dirs:   src
+
+  Build-depends:
+    base >= 4 && < 5
+
+Source-repository head
+  Type:     git
+  Location: https://github.com/jaspervdj/acme-cofunctor
diff --git a/src/Data/Cofunctor.hs b/src/Data/Cofunctor.hs
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--- /dev/null
+++ b/src/Data/Cofunctor.hs
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+-- | 'Cofunctor' is a structure from category theory dual to 'Functor'
+--
+-- A 'Functor' is defined by the operation 'fmap':
+--
+-- > fmap :: (a -> b) -> (f a -> f b)
+--
+-- This means that its dual must be defined by the following operation:
+--
+-- > cofmap :: (b -> a) -> (f b -> f a)
+--
+-- Since beginning his investigations, the author of this package has discovered
+-- that this pattern is /at least/ as commonly used as 'Functor'. In fact, many
+-- ubiquitous Haskell types (e.g. @[]@, 'Maybe', @((->) a)@ turn out to have a
+-- 'Cofunctor' instance.
+module Data.Cofunctor
+    ( Cofunctor (..)
+    ) where
+
+import Control.Monad (liftM)
+
+-- | 'Cofunctor' is a structure from category theory dual to 'Functor'
+class Cofunctor f where
+    cofmap :: (b -> a) -> f b -> f a
+
+instance Cofunctor [] where
+    cofmap _ []       = []
+    cofmap f (x : xs) = f x : cofmap f xs
+
+instance Cofunctor Maybe where
+    cofmap _ Nothing  = Nothing
+    cofmap f (Just x) = Just (f x)
+
+instance Cofunctor (Either e) where
+    cofmap _ (Left e)  = Left e
+    cofmap f (Right x) = Right (f x)
+
+instance Cofunctor ((->) a) where
+    cofmap = (.)
+
+instance Cofunctor ((,) a) where
+    cofmap f (e, x) = (e, f x)
+
+instance Cofunctor IO where
+    cofmap = liftM