Add default method for DecidablyEmpty.isEmpty

Author: Ericson2314

src/Data/Monoid/DecidablyEmpty.hs

@@ -1,5 +1,9 @@
+{-# LANGUAGE DefaultSignatures #-}
+
 -- TODO upstream somwhere else?
 module Data.Monoid.DecidablyEmpty where
 
 class Monoid a => DecidablyEmpty a where
   isEmpty :: a -> Bool
+  default isEmpty :: Eq a => a -> Bool
+  isEmpty = (==) mempty