@@ -46,7 +46,13 @@ instance DecidablyEmpty All
4646-- instance DecidablyEmpty Event
4747instance DecidablyEmpty [a] where
4848 isEmpty = null
49−instance Semigroup a => DecidablyEmpty (Maybe a) where
49+instance
50+#if MIN_VERSION_base(4,10,0)
51+ Semigroup a
52+#else
53+ Monoid a
54+#endif
55+ => DecidablyEmpty (Maybe a) where
5056 isEmpty = isNothing
5157deriving instance (Num a, DecidablyEmpty a) => DecidablyEmpty (Product a)
5258deriving instance (DecidablyEmpty a, Num a) => DecidablyEmpty (Sum a)