diff --git a/2023/day09/day09.hs b/2023/day09/day09.hs
index 23d7960..245eb4b 100644
--- a/2023/day09/day09.hs
+++ b/2023/day09/day09.hs
@@ -13,7 +13,5 @@ day09 =
   (sum *** sum) .
   unzip .
   map
-    ((f (+) last &&& f (-) head) .
+    ((sum . map last &&& foldr1 (-) . map head) .
      takeWhile (not . null) . iterate (zipWith (-) =<< tail))
-  where
-    f f1 f2 = foldr1 f1 . map f2