Fix instances for DecidablyEmpty for Sum and Product; add instances for Patch

Author: ryantrinkle

src/Data/Monoid/DecidablyEmpty.hs

@@ -61,8 +61,10 @@ instance
 #endif
   => DecidablyEmpty (Maybe a) where
   isEmpty = isNothing
-deriving instance (Num a, DecidablyEmpty a) => DecidablyEmpty (Product a)
-deriving instance (DecidablyEmpty a, Num a) => DecidablyEmpty (Sum a)
+instance (Num a, Eq a) => DecidablyEmpty (Product a) where
+  isEmpty = (== 1)
+instance (Num a, Eq a) => DecidablyEmpty (Sum a) where
+  isEmpty = (== 0)
 deriving instance DecidablyEmpty a => DecidablyEmpty (Dual a)
 instance DecidablyEmpty (First a) where
   isEmpty (First a) = isNothing a

src/Data/Patch/Class.hs

@@ -12,9 +12,13 @@ module Data.Patch.Class where
 import Data.Functor.Identity
 import Data.Kind (Type)
 import Data.Maybe
+import Data.Semigroup
+  ( Sum (..)
+  , Product (..)
 #if !MIN_VERSION_base(4,11,0)
-import Data.Semigroup (Semigroup(..))
+  , Semigroup(..)
 #endif
+  )
 import Data.Proxy
 
 -- | A 'Patch' type represents a kind of change made to a datastructure.
@@ -41,6 +45,14 @@ instance forall (a :: Type). Patch (Proxy a) where
   type PatchTarget (Proxy a) = a
   apply ~Proxy _ = Nothing
 
+instance (Num a, Eq a) => Patch (Sum a) where
+  type PatchTarget (Sum a) = a
+  apply (Sum a) b = if a == 0 then Nothing else Just $ a + b
+
+instance (Num a, Eq a) => Patch (Product a) where
+  type PatchTarget (Product a) = a
+  apply (Product a) b = if a == 1 then Nothing else Just $ a * b
+
 -- | Like '(.)', but composes functions that return patches rather than
 -- functions that return new values.  The Semigroup instance for patches must
 -- apply patches right-to-left, like '(.)'.