diff --git a/Control/Category/Const2.hs b/Control/Category/Const2.hs
--- a/Control/Category/Const2.hs
+++ b/Control/Category/Const2.hs
@@ -6,9 +6,9 @@
 newtype Const2 a b c = Const2 a
   deriving (Semigroup, Monoid, Group)
 
-instance Monoid a => Category (Const2 a) where
+instance (Semigroup a, Monoid a) => Category (Const2 a) where
     id = Const2 mempty
     Const2 a . Const2 b = Const2 (a <> b)
 
-instance Group a => Groupoid (Const2 a) where
+instance (Semigroup a, Group a) => Groupoid (Const2 a) where
     invert (Const2 a) = Const2 (A.invert a)
diff --git a/Data/Morphism/Endo.hs b/Data/Morphism/Endo.hs
--- a/Data/Morphism/Endo.hs
+++ b/Data/Morphism/Endo.hs
@@ -9,6 +9,7 @@
     Endo f <> Endo g = Endo (f . g)
 
 instance Category s => Monoid (Endo s a) where
+    mappend = (<>)
     mempty = Endo id
 
 instance Groupoid s => Group (Endo s a) where
diff --git a/Data/Morphism/Iso.hs b/Data/Morphism/Iso.hs
--- a/Data/Morphism/Iso.hs
+++ b/Data/Morphism/Iso.hs
@@ -10,10 +10,13 @@
 instance (Semigroup (s a b), Semigroup (s b a)) => Semigroup (Iso s a b) where
     Iso f f' <> Iso g g' = Iso (f <> g) (f' <> g')
 
-instance (Monoid (s a b), Monoid (s b a)) => Monoid (Iso s a b) where
+instance (Semigroup (s a b), Semigroup (s b a),
+          Monoid (s a b), Monoid (s b a)) => Monoid (Iso s a b) where
+    mappend = (<>)
     mempty = Iso mempty mempty
 
-instance (Group (s a b), Group (s b a)) => Group (Iso s a b) where
+instance (Semigroup (s a b), Semigroup (s b a),
+          Group (s a b), Group (s b a)) => Group (Iso s a b) where
     invert (Iso f f') = Iso (A.invert f) (A.invert f')
 
 instance Category s => Category (Iso s) where
diff --git a/category.cabal b/category.cabal
--- a/category.cabal
+++ b/category.cabal
@@ -1,5 +1,5 @@
 name:                category
-version:             0.2.0.0
+version:             0.2.0.1
 synopsis:            Categorical types and classes
 -- description:         
 license:             BSD3
@@ -10,6 +10,8 @@
 category:            Control, Math
 build-type:          Simple
 cabal-version:       >=1.10
+bug-reports:         http://github.com/strake/category.hs/issues
+tested-with:         GHC ==8.2.2
 
 library
   exposed-modules:     Control.Categorical.Functor
@@ -31,3 +33,7 @@
                      , FlexibleInstances
                      , GeneralizedNewtypeDeriving
   ghc-options:         -Wall
+
+source-repository head
+  type: git
+  location: http://github.com/strake/category.hs.git