From 748fe299428529ac76b809c5e0fbf4dc5fd81670 Mon Sep 17 00:00:00 2001
From: BlackCapCoder <mailhph@gmail.com>
Date: Thu, 24 Oct 2024 04:25:59 +0200
Subject: [PATCH 1/2] Added Set

---
 monoid-extras.cabal    |  1 +
 src/Data/Monoid/Set.hs | 53 ++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 54 insertions(+)
 create mode 100644 src/Data/Monoid/Set.hs

diff --git a/monoid-extras.cabal b/monoid-extras.cabal
index 6a5d738..93dd6c2 100644
--- a/monoid-extras.cabal
+++ b/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
 
diff --git a/src/Data/Monoid/Set.hs b/src/Data/Monoid/Set.hs
new file mode 100644
index 0000000..721f406
--- /dev/null
+++ b/src/Data/Monoid/Set.hs
@@ -0,0 +1,53 @@
+{-# 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 n s
+    | n < 1 = Unset
+    | let   = s
+
+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
+

From 0bba66cffb36e96c042b0af01916ece5b3bfc461 Mon Sep 17 00:00:00 2001
From: BlackCapCoder <mailhph@gmail.com>
Date: Thu, 24 Oct 2024 05:18:09 +0200
Subject: [PATCH 2/2] prefer stimesIdempotentMonoid

---
 src/Data/Monoid/Set.hs | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/src/Data/Monoid/Set.hs b/src/Data/Monoid/Set.hs
index 721f406..f4ba34a 100644
--- a/src/Data/Monoid/Set.hs
+++ b/src/Data/Monoid/Set.hs
@@ -33,9 +33,7 @@ instance Semigroup (Set a) where
   l <> Unset = l
   _ <> r     = r
 
-  stimes n s
-    | n < 1 = Unset
-    | let   = s
+  stimes = stimesIdempotentMonoid
 
 instance Monoid (Set a) where
   mempty = Unset