1 files changed, 9 insertions, 1 deletions
diff --git a/compiler/utils/Bag.hs b/compiler/utils/Bag.hs
index 5b2c6d56f6..cf7bd123b3 100644
--- a/compiler/utils/Bag.hs
+++ b/compiler/utils/Bag.hs
@@ -6,7 +6,7 @@
 Bag: an unordered collection with duplicates
 -}
 
-{-# LANGUAGE ScopedTypeVariables, CPP #-}
+{-# LANGUAGE ScopedTypeVariables, CPP, BangPatterns #-}
 
 module Bag (
         Bag, -- abstract type
@@ -33,6 +33,7 @@ import Util
 import MonadUtils
 import Control.Monad
 import Data.Data
+import Data.Semigroup
 import Data.Maybe( mapMaybe )
 import Data.List ( partition, mapAccumL )
 import qualified Data.Foldable as Foldable
@@ -46,6 +47,13 @@ data Bag a
   | TwoBags (Bag a) (Bag a) -- INVARIANT: neither branch is empty
   | ListBag [a]             -- INVARIANT: the list is non-empty
 
+instance Semigroup (Bag a) where
+    (<>) = unionBags
+
+instance Monoid (Bag a) where
+    mappend = (Data.Semigroup.<>)
+    mempty = emptyBag
+
 emptyBag :: Bag a
 emptyBag = EmptyBag