Try an immutable MWC generator. Refactor the timing code a little.

tests/MWC.hs

@@ -0,0 +1,78 @@
+{-# LANGUAGE MagicHash, TypeOperators #-}
+
+module MWC (MWC, init, next) where
+
+import Data.Array.Vector
+--import Data.Bits
+import Data.IntMap
+import Data.Word
+import Prelude hiding (init)
+import GHC.Word
+import GHC.Base (Int(..))
+
+data MWC = MWC {-# UNPACK #-} !Word32 {-# UNPACK #-} !Word8 !(IntMap Word32)
+
+init :: Word32 -> MWC
+init k = MWC 362436 255 (insert 0 k seed)
+{-# INLINE init #-}
+
+-- | Unchecked 64-bit right shift.
+shiftR :: Word64 -> Int -> Word64
+shiftR (W64# x#) (I# i#) = W64# (x# `uncheckedShiftRL64#` i#)
+
+next :: MWC -> Word32 :*: MWC
+next (MWC c i m) = ti :*: MWC (fromIntegral (t `shiftR` 32))
+                       j
+                       (insert ji ti m)
+    where t = a * fromIntegral (m!ji) + fromIntegral c
+          a = 809430660 :: Word64
+          j = i + 1
+          ji = fromIntegral j
+          ti = fromIntegral t
+{-# INLINE next #-}
+
+seed = fromDistinctAscList $ zip [0..] [
+  0x7042e8b3, 0x06f7f4c5, 0x789ea382, 0x6fb15ad8, 0x54f7a879, 0x0474b184,
+  0xb3f8f692, 0x4114ea35, 0xb6af0230, 0xebb457d2, 0x47693630, 0x15bc0433,
+  0x2e1e5b18, 0xbe91129c, 0xcc0815a0, 0xb1260436, 0xd6f605b1, 0xeaadd777,
+  0x8f59f791, 0xe7149ed9, 0x72d49dd5, 0xd68d9ded, 0xe2a13153, 0x67648eab,
+  0x48d6a1a1, 0xa69ab6d7, 0x236f34ec, 0x4e717a21, 0x9d07553d, 0x6683a701,
+  0x19004315, 0x7b6429c5, 0x84964f99, 0x982eb292, 0x3a8be83e, 0xc1df1845,
+  0x3cf7b527, 0xb66a7d3f, 0xf93f6838, 0x736b1c85, 0x5f0825c1, 0x37e9904b,
+  0x724cd7b3, 0xfdcb7a46, 0xfdd39f52, 0x715506d5, 0xbd1b6637, 0xadabc0c0,
+  0x219037fc, 0x9d71b317, 0x3bec717b, 0xd4501d20, 0xd95ea1c9, 0xbe717202,
+  0xa254bd61, 0xd78a6c5b, 0x043a5b16, 0x0f447a25, 0xf4862a00, 0x48a48b75,
+  0x1e580143, 0xd5b6a11b, 0x6fb5b0a4, 0x5aaf27f9, 0x668bcd0e, 0x3fdf18fd,
+  0x8fdcec4a, 0x5255ce87, 0xa1b24dbf, 0x3ee4c2e1, 0x9087eea2, 0xa4131b26,
+  0x694531a5, 0xa143d867, 0xd9f77c03, 0xf0085918, 0x1e85071c, 0x164d1aba,
+  0xe61abab5, 0xb8b0c124, 0x84899697, 0xea022359, 0x0cc7fa0c, 0xd6499adf,
+  0x746da638, 0xd9e5d200, 0xefb3360b, 0x9426716a, 0xabddf8c2, 0xdd1ed9e4,
+  0x17e1d567, 0xa9a65000, 0x2f37dbc5, 0x9a4b8fd5, 0xaeb22492, 0x0ebe8845,
+  0xd89dd090, 0xcfbb88c6, 0xb1325561, 0x6d811d90, 0x03aa86f4, 0xbddba397,
+  0x0986b9ed, 0x6f4cfc69, 0xc02b43bc, 0xee916274, 0xde7d9659, 0x7d3afd93,
+  0xf52a7095, 0xf21a009c, 0xfd3f795e, 0x98cef25b, 0x6cb3af61, 0x6fa0e310,
+  0x0196d036, 0xbc198bca, 0x15b0412d, 0xde454349, 0x5719472b, 0x8244ebce,
+  0xee61afc6, 0xa60c9cb5, 0x1f4d1fd0, 0xe4fb3059, 0xab9ec0f9, 0x8d8b0255,
+  0x4e7430bf, 0x3a22aa6b, 0x27de22d3, 0x60c4b6e6, 0x0cf61eb3, 0x469a87df,
+  0xa4da1388, 0xf650f6aa, 0x3db87d68, 0xcdb6964c, 0xb2649b6c, 0x6a880fa9,
+  0x1b0c845b, 0xe0af2f28, 0xfc1d5da9, 0xf64878a6, 0x667ca525, 0x2114b1ce,
+  0x2d119ae3, 0x8d29d3bf, 0x1a1b4922, 0x3132980e, 0xd59e4385, 0x4dbd49b8,
+  0x2de0bb05, 0xd6c96598, 0xb4c527c3, 0xb5562afc, 0x61eeb602, 0x05aa192a,
+  0x7d127e77, 0xc719222d, 0xde7cf8db, 0x2de439b8, 0x250b5f1a, 0xd7b21053,
+  0xef6c14a1, 0x2041f80f, 0xc287332e, 0xbb1dbfd3, 0x783bb979, 0x9a2e6327,
+  0x6eb03027, 0x0225fa2f, 0xa319bc89, 0x864112d4, 0xfe990445, 0xe5e2e07c,
+  0xf7c6acb8, 0x1bc92142, 0x12e9b40e, 0x2979282d, 0x05278e70, 0xe160ba4c,
+  0xc1de0909, 0x458b9bf4, 0xbfce9c94, 0xa276f72a, 0x8441597d, 0x67adc2da,
+  0x6162b854, 0x7f9b2f4a, 0x0d995b6b, 0x193b643d, 0x399362b3, 0x8b653a4b,
+  0x1028d2db, 0x2b3df842, 0x6eecafaf, 0x261667e9, 0x9c7e8cda, 0x46063eab,
+  0x7ce7a3a1, 0xadc899c9, 0x017291c4, 0x528d1a93, 0x9a1ee498, 0xbb7d4d43,
+  0x7837f0ed, 0x34a230cc, 0x614a628d, 0xb03f93b8, 0xd72e3b08, 0x604c98db,
+  0x3cfacb79, 0x8b81646a, 0xc0f082fa, 0xd1f92388, 0xe5a91e39, 0xf95c756d,
+  0x1177742f, 0xf8819323, 0x5c060b80, 0x96c1cd8f, 0x47d7b440, 0xbbb84197,
+  0x35f749cc, 0x95b0e132, 0x8d90ad54, 0x5c3f9423, 0x4994005b, 0xb58f53b9,
+  0x32df7348, 0x60f61c29, 0x9eae2f32, 0x85a3d398, 0x3b995dd4, 0x94c5e460,
+  0x8e54b9f3, 0x87bc6e2a, 0x90bbf1ea, 0x55d44719, 0x2cbbfe6e, 0x439d82f0,
+  0x4eb3782d, 0xc3f1e669, 0x61ff8d9e, 0x0909238d, 0xef406165, 0x09c1d762,
+  0x705d184f, 0x188f2cc4, 0x9c5aa12a, 0xc7a5d70e, 0xbc78cb1b, 0x1d26ae62,
+  0x23f96ae3, 0xd456bf32, 0xe4654f55, 0x31462bd8
+  ]

tests/T.hs

@@ -11,21 +11,26 @@ import Data.Array.Vector
 import Statistics.RandomVariate
 import qualified System.Random as R
 import qualified System.Random.Mersenne as M
+import Text.Printf
+import System.IO
+import MWC as MWC
+
+type T = Word32
 
 getTime :: IO Double
 getTime = (fromRational . toRational) `fmap` getPOSIXTime
 
-time :: IO a -> IO (Double, a)
-time act = do
+time :: String -> IO (a,Int) -> IO ()
+time desc act = do
+  putStr (desc ++ ": ")
   s <- getTime
-  ret <- act
-  e <- ret `seq` getTime
-  return (e-s, ret)
+  (ret,n) <- act
+  e <- getTime
+  let t = e-s
+  hPrintf stdout "%.3g secs (%.3g M/sec)\n" t (fromIntegral n/t/1e6)
 
 count = 100 * 1000000
 
-type T = Double
-
 --summ :: [T] -> T
 --summ = foldl' (+) 0
 
@@ -37,42 +42,52 @@ loop n act = go 0 0
       d <- act
       go (i+1) (s+d)
 
-system :: IO Double
+system :: IO (Double, Int)
 system = do
     gen <- R.getStdGen
     let a :*: g = loop 0 0 gen
     R.setStdGen g
-    return a
+    a `seq` return (a, k)
   where loop !i !a !g | i == k = a :*: g
                       | otherwise = let (b,h) = R.random g
                                     in loop (i+1) (a+b) h
         k = count `div` 100
 
+immut :: Word32 -> IO (Word32, Int)
+immut k = let r = loop (MWC.init k) 0 0
+          in r `seq` return (r,n)
+  where loop !g !i !a | i == n = a
+                      | otherwise = let (b:*:h) = MWC.next g
+                                    in loop h (i+1) (a+b)
+        n = count `div` 10
+
 
-mwc :: Word32 -> IO T
-mwc k = return $! runST go where
-  go = do
-    gen <- initialize (singletonU k)
-    loop count (uniform gen)
+mwc :: Word32 -> IO (T, Int)
+mwc k = let r = runST go
+        in r `seq` return (r,count)
+  where
+    go = do
+      gen <- initialize (singletonU k)
+      loop count (uniform gen)
 
-mwca :: Word32 -> IO T
-mwca k = return $! runST go where
-  go = do
-    gen <- initialize (singletonU k)
-    sumU `fmap` uniformArray gen count
+mwca :: Word32 -> IO (T, Int)
+mwca k = let r = runST go
+         in r `seq` return (r,count)
+  where
+    go = do
+      gen <- initialize (singletonU k)
+      sumU `fmap` uniformArray gen count
 
-mersenne :: IO T
+mersenne :: IO (T, Int)
 mersenne = do
   gen <- M.getStdGen
-  loop count (M.random gen)
+  r <- loop count (M.random gen)
+  return (r,count)
 
 main = do
   forM_ [1..3] $ \n -> do
-    putStr "system: "
-    print =<< time system
-    putStr "mwc: "
-    print =<< time (mwc n)
-    putStr "mwca: "
-    print =<< time (mwca n)
-    putStr "mersenne: "
-    print =<< time mersenne
+    time "system" system
+    time "immut" (immut n)
+    time "mwc" (mwc n)
+    time "mwca" (mwca n)
+    time "mersenne" mersenne