Change Replace semantics

src/Data/Patch/Class.hs

@@ -192,11 +192,27 @@ instance Eq a => Composable (Replace a) where
   composable (Replace _ n) (Replace o _) =
     check (n == o) "Composing two @Replace@ patches with a different inner value"
 
+{- Two semantics are possible for the `Replace old new` patch:
+   (1) The patch is assumed to only be used on a source which is known
+       to be equal to `old`.
+   (2) The patch only replaces `old` by `new` when the source is equal to `old` and
+       leaves it unchanged otherwise.
+
+   Here we choose (2).
+-}
+instance Eq a => Action (Replace a) a where
+  act Keep a = a
+  act (Replace old new) source
+    | source == old = new
+    | otherwise     = source
+
+{- PRODUCTION instance for semantics (1)
 instance Action (Replace a) a where
   act Keep a = a
   act (Replace _ n) _ = n
+-}
 
-{- DEBUGGING instance
+{- DEBUGGING instance for semantics (1)
 instance (Show a, Eq a) => Action (Replace a) a where
   act Keep a = a
   act p@(Replace expected n) found