September 2017 Update

Task/Statistics-Basic/Haskell/statistics-basic.hs

@@ -1,48 +1,69 @@
-{-# LANGUAGE BangPatterns #-}
-
-import Data.Foldable
-import System.Random
+import Data.Foldable (foldl') --'
+import System.Random (randomRs, newStdGen)
+import Control.Monad (zipWithM_)
 import System.Environment (getArgs)
 
-intervals :: [(Double,Double)]
-intervals = map conv [0..9]
-    where xs = [0.0,0.1,0.2,0.3,0.4,0.5,0.6,0.7,0.8,0.9,1.0]
-          conv s = let { [h,l] = take 2 $ drop s xs } in (h,l)
+intervals :: [(Double, Double)]
+intervals = map conv [0 .. 9]
+  where
+    xs = [0.0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0]
+    conv s =
+      let [h, l] = take 2 $ drop s xs
+      in (h, l)
 
 count :: [Double] -> [Int]
-count rands = map (\iv -> foldl' (loop iv) 0 rands) intervals
-   where loop :: (Double,Double) -> Int -> Double -> Int
-         loop (lo,hi) n x | lo <= x && x < hi = n+1
-                          | otherwise         = n
-         -- ^ fuses length and filter within (lo,hi)
+count rands = map (\iv -> foldl'' (loop iv) 0 rands) intervals
+  where
+    loop :: (Double, Double) -> Int -> Double -> Int
+    loop (lo, hi) n x
+      | lo <= x && x < hi = n + 1
+      | otherwise = n
 
-data Pair a b = Pair !a !b
+-- ^ fuses length and filter within (lo,hi)
+data Pair a b =
+  Pair !a
+       !b
 
 -- accumulate sum and length in one fold
 sumLen :: [Double] -> Pair Double Double
-sumLen = fion2 . foldl' (\(Pair s l) x -> Pair (s+x) (l+1)) (Pair 0.0 0)
-  where fion2 :: Pair Double Int -> Pair Double Double
-        fion2 (Pair s l) = Pair s (fromIntegral l)
+sumLen = fion2 . foldl'' (\(Pair s l) x -> Pair (s + x) (l + 1)) (Pair 0.0 0)
+  where
+    fion2 :: Pair Double Int -> Pair Double Double
+    fion2 (Pair s l) = Pair s (fromIntegral l)
 
 -- safe division on pairs
 divl :: Pair Double Double -> Double
 divl (Pair _ 0.0) = 0.0
-divl (Pair s   l) = s / l
+divl (Pair s l) = s / l
 
 -- sumLen and divl are separate for stddev below
 mean :: [Double] -> Double
 mean = divl . sumLen
 
 stddev :: [Double] -> Double
-stddev xs = sqrt $ foldl' (\s x -> s+(x-m)^2) 0 xs / l
-   where p@(Pair s l) = sumLen xs
-         m = divl p
+stddev xs = sqrt $ foldl'' (\s x -> s + (x - m) ^ 2) 0 xs / l
+  where
+    p@(Pair s l) = sumLen xs
+    m = divl p
 
-main = do nr    <- read.head <$> getArgs
-          rands <- take nr . randomRs (0.0,1.0) <$> newStdGen
-          putStrLn $ "The mean is " ++ show (mean rands) ++ " !"
-          putStrLn $ "The standard deviation is " ++ show (stddev rands) ++ " !"
-          zipWithM_ (\iv fq -> putStrLn $ ivstr iv ++ ": " ++ fqstr fq) intervals (count rands)
-    where
-        fqstr i = replicate (if i > 50 then div i (div i 50) else i) '*'
-        ivstr (lo,hi) = show lo ++ " - " ++ show hi
+main = do
+  nr <- read . head <$> getArgs
+  -- or in code, e.g.  let nr = 1000
+  rands <- take nr . randomRs (0.0, 1.0) <$> newStdGen
+  putStrLn $ "The mean is " ++ show (mean rands) ++ " !"
+  putStrLn $ "The standard deviation is " ++ show (stddev rands) ++ " !"
+  zipWithM_
+    (\iv fq -> putStrLn $ ivstr iv ++ ": " ++ fqstr fq)
+    intervals
+    (count rands)
+  where
+    fqstr i =
+      replicate
+        (if i > 50
+           then div i (div i 50)
+           else i)
+        '*'
+    ivstr (lo, hi) = show lo ++ " - " ++ show hi
+
+-- To avoid Wiki formatting issue
+foldl'' = foldl'