From: Ramakrishnan Muthukrishnan <ram@rkrishnan.org>
Date: Tue, 23 Dec 2014 17:06:44 +0000 (+0530)
Subject: play more with monoids
X-Git-Url: https://git.rkrishnan.org/pf/content/en/seg/biz/rgr-080307.php?a=commitdiff_plain;h=981151b9dc55ce9ed50fe7640018fe776a3d81c5;p=yorgey.git

play more with monoids
---

diff --git a/misc/monoids.hs b/misc/monoids.hs
index 848dab9..f15c89f 100644
--- a/misc/monoids.hs
+++ b/misc/monoids.hs
@@ -1,3 +1,4 @@
+{-# LANGUAGE GeneralizedNewtypeDeriving #-}
 module Monoids where
 
 import Data.Monoid
@@ -46,3 +47,29 @@ treeDepth' = treeFold 0 (\l _ r -> 1 + max l r)
 flatten' :: Tree a -> [a]
 flatten' = treeFold [] (\l x r -> l ++ [x] ++ r)
 
+-- monoids
+newtype Sum' a = Sum' a
+    deriving (Eq, Num, Ord, Show)
+
+getSum :: Sum' a -> a
+getSum (Sum' x) = x
+
+instance Num a => Monoid (Sum' a) where
+  mempty = 0
+  mappend = (+)
+
+newtype Prod' a = Prod' a
+    deriving (Eq, Num, Ord, Show)
+
+getProd :: Prod' a -> a
+getProd (Prod' x) = x
+
+instance Num a => Monoid (Prod' a) where
+  mempty = 1
+  mappend = (*)
+
+lst :: [Integer]
+lst = [1,5,8,23,423,99]
+
+prod :: Integer
+prod = getProd $ mconcat $ map Prod' lst