diff --git a/Syntax.hs b/Syntax.hs
index e443c4f3a..fc128b5c0 100644
--- a/Syntax.hs
+++ b/Syntax.hs
@@ -1,5 +1,7 @@
 module Syntax where
 
+{-# LANGUAGE DeriveFunctor #-}
+
 import Data.Map
 
 data Syntax f a =
@@ -7,3 +9,4 @@ data Syntax f a =
 	| Indexed [f]
 	| Fixed [f]
 	| Keyed (Map String f)
+	deriving Functor