haskell - 为 MWC (Haskell) 编写一个 MonadRandom 实例

Question

我希望能够使用 MWC random number generator与 random-fu，因此需要实现 MonadRandom 的实例。我的尝试是将 Reader (Gen (PrimState m)) v 视为 MonadRandom v。但是，以下失败:

$(monadRandom [d|
  instance (PrimMonad m) => MonadRandom (Reader (MWC.Gen (PrimState m))) where
    getRandomWord16 = ask >>= MWC.uniform
    getRandomWord32 = ask >>= MWC.uniform
    getRandomWord64 = ask >>= MWC.uniform
  |])

错误:

Illegal type synonym family application in instance:
  Reader (MWC.Gen (PrimState m_ahwD))
In the instance declaration for
  ‘MonadRandom (Reader (MWC.Gen (PrimState m_ahwD)))’

我做错了什么？

Answer 1

基于其他贡献者已经说过的话

{-# LANGUAGE TemplateHaskell   #-}
{-# LANGUAGE GADTs             #-}
{-# LANGUAGE FlexibleInstances #-}

import Data.Random
import Data.Random.Source
import qualified System.Random.MWC as MWC
import Control.Monad.Reader
import Control.Monad.Primitive

$(monadRandom [d|
  instance (PrimMonad m, s ~ PrimState m) => MonadRandom (ReaderT (MWC.Gen s) m) where
    getRandomWord16 = ask >>= lift . MWC.uniform
    getRandomWord32 = ask >>= lift . MWC.uniform
    getRandomWord64 = ask >>= lift . MWC.uniform
  |])

testUniform :: MonadRandom m => Int -> m [Double]
testUniform n = replicateM (fromIntegral n) (sample stdUniform)

n :: Int
n = 10^7

main :: IO ()
main = do
    seed <- MWC.create
    xs <- runReaderT (testUniform n) seed
    print (sum xs / fromIntegral n)

但是效果不是很好

./RandomFuMWC +RTS -s
0.5000432391067587
   3,286,220,896 bytes allocated in the heap
   2,427,475,880 bytes copied during GC
     600,186,048 bytes maximum residency (12 sample(s))
     100,510,656 bytes maximum slop
            1249 MB total memory in use (0 MB lost due to fragmentation)

                                    Tot time (elapsed)  Avg pause  Max pause
  Gen  0      5942 colls,     0 par    1.06s    1.13s     0.0002s    0.0013s
  Gen  1        12 colls,     0 par    0.82s    1.23s     0.1024s    0.5787s

  INIT    time    0.00s  (  0.00s elapsed)
  MUT     time    1.47s  (  1.39s elapsed)
  GC      time    1.87s  (  2.36s elapsed)
  EXIT    time    0.01s  (  0.09s elapsed)
  Total   time    3.35s  (  3.84s elapsed)

  %GC     time      56.0%  (61.3% elapsed)

  Alloc rate    2,242,365,923 bytes per MUT second

  Productivity  44.0% of total user, 38.3% of total elapsed