diff --git a/Control/Category/Dual.hs b/Control/Category/Dual.hs
new file mode 100644
--- /dev/null
+++ b/Control/Category/Dual.hs
@@ -0,0 +1,12 @@
+module Control.Category.Dual where
+
+import Control.Category.Groupoid
+
+newtype Dual k a b = Dual { dual :: k b a } deriving (Data, Typeable)
+
+instance Category k => Category (Dual k) where
+    id = Dual id
+    Dual f . Dual g = Dual (g . f)
+
+instance Groupoid k => Groupoid (Dual k) where
+    invert = Dual . invert . dual
diff --git a/Control/Category/Groupoid.hs b/Control/Category/Groupoid.hs
new file mode 100644
--- /dev/null
+++ b/Control/Category/Groupoid.hs
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+{-# LANGUAGE GADTs #-}
+
+module Control.Category.Groupoid where
+
+class Category k => Groupoid k where
+    invert :: k a b -> k b a
+
+instance Groupoid (:~:) where
+    invert Refl = Refl
diff --git a/LICENSE b/LICENSE
new file mode 100644
--- /dev/null
+++ b/LICENSE
@@ -0,0 +1,30 @@
+Copyright (c) 2017, M Farkas-Dyck
+
+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 M Farkas-Dyck 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/Prelude.hs b/Prelude.hs
new file mode 100644
--- /dev/null
+++ b/Prelude.hs
@@ -0,0 +1,11 @@
+module Prelude (module Control.Category,
+                module Data.Data,
+                module Data.Either,
+                module Data.Type.Equality,
+                module Data.Typeable) where
+
+import Control.Category
+import Data.Data
+import Data.Either
+import Data.Type.Equality
+import Data.Typeable
diff --git a/Setup.hs b/Setup.hs
new file mode 100644
--- /dev/null
+++ b/Setup.hs
@@ -0,0 +1,2 @@
+import Distribution.Simple
+main = defaultMain
diff --git a/category.cabal b/category.cabal
new file mode 100644
--- /dev/null
+++ b/category.cabal
@@ -0,0 +1,22 @@
+name:                category
+version:             0.1.0.0
+synopsis:            Categorical types and classes
+-- description:         
+license:             BSD3
+license-file:        LICENSE
+author:              M Farkas-Dyck
+maintainer:          strake888@gmail.com
+-- copyright:           
+category:            Control
+build-type:          Simple
+cabal-version:       >=1.10
+
+library
+  exposed-modules:     Control.Category.Dual
+                     , Control.Category.Groupoid
+  other-modules:       Prelude
+  build-depends:       base >=4.10 && <5
+  default-language:    Haskell2010
+  default-extensions:  LambdaCase
+                     , DeriveDataTypeable
+  ghc-options:         -Wall