[project @ 2005-11-17 15:54:17 by ross]

add a couple of Boolean instances

diff --git a/Data/Monoid.hs b/Data/Monoid.hs
index e57b909..cba6993 100644 (file)
--- a/Data/Monoid.hs
+++ b/Data/Monoid.hs
@@ -20,8 +20,10 @@
 
 module Data.Monoid (
        Monoid(..),
-       Endo(..),
        Dual(..),
+       Endo(..),
+       All(..),
+       Any(..),
        Sum(..),
        Product(..)
   ) where
@@ -93,6 +95,13 @@ instance Monoid Ordering where
        EQ `mappend` y = y
        GT `mappend` _ = GT
 
+-- | The dual of a monoid, obtained by swapping the arguments of 'mappend'.
+newtype Dual a = Dual { getDual :: a }
+
+instance Monoid a => Monoid (Dual a) where
+       mempty = Dual mempty
+       Dual x `mappend` Dual y = Dual (y `mappend` x)
+
 -- | The monoid of endomorphisms under composition.
 newtype Endo a = Endo { appEndo :: a -> a }
 
@@ -100,12 +109,19 @@ instance Monoid (Endo a) where
        mempty = Endo id
        Endo f `mappend` Endo g = Endo (f . g)
 
--- | The dual of a monoid, obtained by swapping the arguments of 'mappend'.
-newtype Dual a = Dual { getDual :: a }
+-- | Boolean monoid under conjunction.
+newtype All = All { getAll :: Bool }
 
-instance Monoid a => Monoid (Dual a) where
-       mempty = Dual mempty
-       Dual x `mappend` Dual y = Dual (y `mappend` x)
+instance Monoid All where
+       mempty = All True
+       All x `mappend` All y = All (x && y)
+
+-- | Boolean monoid under disjunction.
+newtype Any = Any { getAny :: Bool }
+
+instance Monoid Any where
+       mempty = Any False
+       Any x `mappend` Any y = Any (x || y)
 
 -- | Monoid under addition.
 newtype Sum a = Sum { getSum :: a }