2016 Update

Task/Averages-Simple-moving-average/Haskell/averages-simple-moving-average-1.hs

@@ -1,26 +1,25 @@
+{-# LANGUAGE BangPatterns #-}
+
 import Control.Monad
 import Data.List
 import Data.IORef
 
+data Pair a b = Pair !a !b
+
 mean :: Fractional a => [a] -> a
-mean xs = sum xs / (genericLength xs)
+mean = divl . foldl' (\(Pair s l) x -> Pair (s+x) (l+1)) (Pair 0.0 0)
+  where divl (_,0) = 0.0
+        divl (s,l) = s / fromIntegral l
 
 series = [1,2,3,4,5,5,4,3,2,1]
 
-simple_moving_averager period = do
-  numsRef <- newIORef []
-  return (\x -> do
-            nums <- readIORef numsRef
-            let xs = take period (x:nums)
-            writeIORef numsRef xs
-            return $ mean xs
-         )
+mkSMA :: Int -> IO (Double -> IO Double)
+mkSMA period = avgr <$> newIORef []
+  where avgr nsref x = readIORef nsref >>= (\ns ->
+            let xs = take period (x:ns)
+            in writeIORef nsref xs $> mean xs)
 
-main = do
-  sma3 <- simple_moving_averager 3
-  sma5 <- simple_moving_averager 5
-  forM_ series (\n -> do
-                mm3 <- sma3 n
-                mm5 <- sma5 n
-                putStrLn $ "Next number = " ++ (show n) ++ ", SMA_3 = " ++ (show mm3) ++ ", SMA_5 = " ++ (show mm5)
-               )
+main = mkSMA 3 >>= (\sma3 -> mkSMA 5 >>= (\sma5 ->
+    mapM_ (str <$> pure n <*> sma3 <*> sma5) series))
+  where str n mm3 mm5 =
+    concat ["Next number = ",show n,", SMA_3 = ",show mm3,", SMA_5 = ",show mm5]

Task/Averages-Simple-moving-average/Haskell/averages-simple-moving-average-3.hs

@@ -23,6 +23,4 @@ demostrateSMA :: State SMAState [Float]
 demostrateSMA = mapM computeSMA [1, 2, 3, 4, 5, 5, 4, 3, 2, 1]
 
 main :: IO ()
-main = putStrLn $ (show result)
-        where
-         (result, _) = runState demostrateSMA []
+main = print $ evalState demostrateSMA []