From 423693811904a6038a1a7532700726799e05fade Mon Sep 17 00:00:00 2001
From: Joe Zhao <ztuowen@gmail.com>
Date: Fri, 8 Aug 2014 09:20:42 +0800
Subject: add difflist

---
 difflist.hs | 27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)
 create mode 100644 difflist.hs

diff --git a/difflist.hs b/difflist.hs
new file mode 100644
index 0000000..f206351
--- /dev/null
+++ b/difflist.hs
@@ -0,0 +1,27 @@
+import Control.Monad.Writer
+
+newtype DiffList a = DiffList { getDiffList :: [a] -> [a] }
+
+toDiffList :: [a] -> DiffList a
+toDiffList xs = DiffList (xs++)
+
+fromDiffList :: DiffList a -> [a]
+fromDiffList (DiffList f) = f []
+
+instance Monoid (DiffList a) where
+    mempty = DiffList (\xs -> [] ++ xs)
+    (DiffList f) `mappend` (DiffList g) = DiffList (\xs -> f (g xs))
+
+diffCountDown :: Int -> Writer (DiffList String) ()
+diffCountDown 0 = do
+    tell (toDiffList ["0"])
+diffCountDown x = do
+    diffCountDown (x-1)
+    tell (toDiffList [show x])
+
+normCountDown :: Int -> Writer [String] ()
+normCountDown 0 = do
+    tell ["0"]
+normCountDown x = do
+    normCountDown (x-1)
+    tell [show x]
-- 
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