From dbd943b89ff481e0971f86c2271223cfddee7a02 Mon Sep 17 00:00:00 2001
From: Vivian McPhail <haskell.vivian.mcphail@gmail.com>
Date: Sun, 26 Sep 2010 21:03:59 +0000
Subject: add note about strictness

---
 examples/monadic.hs | 90 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 90 insertions(+)
 create mode 100644 examples/monadic.hs

(limited to 'examples')

diff --git a/examples/monadic.hs b/examples/monadic.hs
new file mode 100644
index 0000000..b933de9
--- /dev/null
+++ b/examples/monadic.hs
@@ -0,0 +1,90 @@
+-- monadic computations
+-- (contributed by Vivian McPhail)
+
+import Numeric.Container
+import Control.Monad.State
+import Control.Monad.Maybe
+
+-------------------------------------------
+
+-- an instance of MonadIO, a monad transformer
+type VectorMonadT = StateT Int IO
+
+v :: Vector Int
+v = 10 |> [0..]
+
+test1 :: Vector Int -> IO (Vector Int)
+test1 = mapVectorM $ \x -> do
+    putStr $ (show x) ++ " "
+    return (x + 1)
+
+-- we can have an arbitrary monad AND do IO
+addInitialM :: Vector Int -> VectorMonadT ()
+addInitialM = mapVectorM_ $ \x -> do
+    i <- get
+    liftIO $ putStr $ (show $ x + i) ++ " "
+    put $ x + i
+
+-- sum the values of the even indiced elements
+sumEvens :: Vector Int -> Int
+sumEvens = foldVectorWithIndex (\x a b -> if x `mod` 2 == 0 then a + b else b) 0
+
+-- sum and print running total of evens
+sumEvensAndPrint :: Vector Int -> VectorMonadT ()
+sumEvensAndPrint = mapVectorWithIndexM_ $ \ i x -> do
+    when (i `mod` 2 == 0) $ do
+        v <- get
+        put $ v + x
+        v' <- get
+        liftIO $ putStr $ (show v') ++ " "
+
+
+indexPlusSum :: Vector Int -> VectorMonadT ()
+indexPlusSum v' = do
+    let f i x = do
+            s <- get
+            let inc = x+s
+            liftIO $ putStr $ show (i,inc) ++ " "
+            put inc
+            return inc
+    v <- mapVectorWithIndexM f v'
+    liftIO $ do
+        putStrLn ""
+        putStrLn $ show v
+
+-------------------------------------------
+
+-- short circuit
+monoStep :: Double -> MaybeT (State Double) ()
+monoStep d = do
+    dp <- get
+    when (d < dp) (fail "negative difference")
+    put d
+{-# INLINE monoStep #-}
+
+isMonotoneIncreasing :: Vector Double -> Bool
+isMonotoneIncreasing v =
+    let res = evalState (runMaybeT $ (mapVectorM_ monoStep v)) (v @> 0)
+     in case res of
+        Nothing -> False
+        Just _  -> True
+
+w = fromList ([1..10]++[10,9..1]) :: Vector Double
+
+-------------------------------------------
+
+main = do
+    v' <- test1 v
+    putStrLn ""
+    putStrLn $ show v'
+    evalStateT (addInitialM v) 0
+    putStrLn ""
+    putStrLn $ show (sumEvens v)
+    evalStateT (sumEvensAndPrint v) 0
+    putStrLn ""
+    evalStateT (indexPlusSum v) 0
+    putStrLn "-----------------------"
+    print $ isMonotoneIncreasing w
+    print $ isMonotoneIncreasing (subVector 0 7 w)
+    print $ successive_ (>) v
+    print $ successive_ (>) w
-- 
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