[exercise] 14.1

Author: Dev-XYS

chapter14-Foldables-and-friends/1.hs

@@ -0,0 +1,9 @@
+-- This is a hypothetical program which does not run in GHCi.
+
+import Data.Monoid
+
+instance (Monoid a, Monoid b) => Monoid (a, b) where
+    -- mempty :: (a, b)
+    mempty = (mempty, mempty)
+    -- mappend :: (a, b) -> (a, b) -> (a, b)
+    (x1, y1) `mappend` (x2, y2) = (x1 `mappend` x2, y1 `mappend` y2)