Added a new Set monoid

monoid-extras.cabal

@@ -33,6 +33,7 @@ library
                      Data.Monoid.Inf,
                      Data.Monoid.MList,
                      Data.Monoid.Recommend,
+                     Data.Monoid.Set,
                      Data.Monoid.Split,
                      Data.Monoid.WithSemigroup
 

src/Data/Monoid/Set.hs

@@ -0,0 +1,51 @@
+{-# LANGUAGE DeriveDataTypeable #-}
+{-# LANGUAGE DeriveFoldable     #-}
+{-# LANGUAGE DeriveFunctor      #-}
+{-# LANGUAGE DeriveTraversable  #-}
+{-# LANGUAGE MagicHash          #-}
+{-# OPTIONS_GHC -fno-warn-unused-imports #-}
+
+module Data.Monoid.Set where
+
+import Data.Data
+import Data.Semigroup
+import Data.Foldable
+import Data.Traversable
+
+import GHC.Exts (isTrue#, dataToTag#)
+import Unsafe.Coerce (unsafeCoerce)
+
+----
+
+-- | @Set@ is like @Maybe@, but the value can either be
+--   unspecified with @Unset@, or explicitly cleared with @Clear@
+data Set a
+   = Unset
+   | Set a
+   | Clear
+   deriving (Data, Typeable, Show, Read, Functor, Foldable, Traversable)
+
+
+-- | The right-hand-side or "newer" value is prefered, unless it
+--   is Unset, in which case the old value is left unchanged
+instance Semigroup (Set a) where
+
+  l <> Unset = l
+  _ <> r     = r
+
+  stimes = stimesIdempotentMonoid
+
+instance Monoid (Set a) where
+  mempty = Unset
+
+
+isSet :: Set a -> Bool
+isSet s = isTrue# (dataToTag# s)
+
+maybeToSet :: Maybe a -> Set a
+maybeToSet = unsafeCoerce
+
+setToMaybe :: Set a -> Maybe a
+setToMaybe (Set a) = Just a
+setToMaybe _       = Nothing
+