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

---
 addStuff.hs | 12 ++++++++++++
 powerset.hs |  4 ++++
 solveRPN.hs | 15 +++++++++++++++
 stateful.hs | 15 +++++++++++++++
 4 files changed, 46 insertions(+)
 create mode 100644 addStuff.hs
 create mode 100644 powerset.hs
 create mode 100644 solveRPN.hs
 create mode 100644 stateful.hs

diff --git a/addStuff.hs b/addStuff.hs
new file mode 100644
index 0000000..d440f7e
--- /dev/null
+++ b/addStuff.hs
@@ -0,0 +1,12 @@
+import Control.Monad.Instances
+
+addStuff :: Int -> Int
+addStuff = do
+    a <- (*2)
+    b <- (+10)
+    return (a+b)
+
+complexOp = do
+    a <- sqrt
+    b <- log . (**2)
+    return (a+b)
diff --git a/powerset.hs b/powerset.hs
new file mode 100644
index 0000000..39d53ab
--- /dev/null
+++ b/powerset.hs
@@ -0,0 +1,4 @@
+import Control.Monad
+
+powerset :: [a] -> [[a]]
+powerset xs = filterM (\x -> [True,False]) xs
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
diff --git a/stateful.hs b/stateful.hs
new file mode 100644
index 0000000..63548d3
--- /dev/null
+++ b/stateful.hs
@@ -0,0 +1,15 @@
+import Control.Monad.State
+
+type Stack = [Int]
+
+pop :: State Stack Int
+pop = state $ \(x:xs) -> (x,xs)
+
+push :: Int -> State Stack ()
+push a = state $ \xs -> ((), a:xs)
+
+stackManip :: State Stack Int
+stackManip = do
+    push 3
+    pop
+    pop
-- 
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