1 files changed, 2 insertions, 4 deletions
diff --git a/compiler/main/TidyPgm.hs b/compiler/main/TidyPgm.hs
index d0e813a403..4f9c8c856f 100644
--- a/compiler/main/TidyPgm.hs
+++ b/compiler/main/TidyPgm.hs
@@ -4,7 +4,7 @@
 \section{Tidying up Core}
 -}
 
-{-# LANGUAGE CPP, ViewPatterns #-}
+{-# LANGUAGE CPP, DeriveFunctor, ViewPatterns #-}
 
 module TidyPgm (
        mkBootModDetailsTc, tidyProgram, globaliseAndTidyId
@@ -751,9 +751,7 @@ newtype DFFV a
                               -- we don't want to record these as free vars
       -> (VarSet, [Var])      -- Input State: (set, list) of free vars so far
       -> ((VarSet,[Var]),a))  -- Output state
-
-instance Functor DFFV where
-    fmap = liftM
+    deriving (Functor)
 
 instance Applicative DFFV where
     pure a = DFFV $ \_ st -> (st, a)