From e285e81c2519dfe9b000d0f297b72c6c9909662d Mon Sep 17 00:00:00 2001
From: Joe Zhao <ztuowen@gmail.com>
Date: Fri, 8 Aug 2014 14:37:06 +0800
Subject: Last few chapters in LYHGG

---
 solveRPN.hs | 15 +++++++++++++++
 1 file changed, 15 insertions(+)
 create mode 100644 solveRPN.hs

(limited to 'solveRPN.hs')

diff --git a/solveRPN.hs b/solveRPN.hs
new file mode 100644
index 0000000..051999d
--- /dev/null
+++ b/solveRPN.hs
@@ -0,0 +1,15 @@
+import Data.List
+import Control.Monad
+import Text.Read
+
+foldingFunction :: [Double] -> String -> Maybe [Double]
+foldingFunction (x:y:ys) "*" = return ((y * x):ys)
+foldingFunction (x:y:ys) "/" = return ((y / x):ys)
+foldingFunction (x:y:ys) "+" = return ((y + x):ys)
+foldingFunction (x:y:ys) "-" = return ((y - x):ys)
+foldingFunction xs numberString = liftM (:xs) (readMaybe numberString)
+
+solveRPN :: String -> Maybe Double
+solveRPN st = do
+    [result] <- foldM foldingFunction [] (words st)
+    return result
-- 
cgit v1.2.3-70-g09d2