奇怪的表现与evalRandIO

Question

我有一些奇怪的evalRandIO性能问题。这里是有问题的代码：

import Control.Monad.Random 

inf :: (RandomGen g, Random a) => Rand g [a] 
inf = sequence $ repeat $ getRandom 

many :: (RandomGen g, Random a) => Int -> Rand g [a] 
many n = sequence $ replicate n $ getRandom 

main = do 
    m <- evalRandIO $ many 1000000 :: IO [Bool] 
    i <- evalRandIO $ inf :: IO [Bool] 
    putStrLn $ show $ take 5 m 
    putStrLn $ show $ take 5 i 

这段代码将打印5个随机布尔变量，然后溢出堆栈。但是，如果我注释掉evalRandIO声明，如下所示：

main = do 
    --m <- evalRandIO $ many 1000000 :: IO [Bool] 
    i <- evalRandIO $ inf :: IO [Bool] 
    --putStrLn $ show $ take 5 m 
    putStrLn $ show $ take 5 i 

代码运行良好。发生什么事？

我ghci的输出：

strontium:movie andrew$ ghci rtest.hs 
GHCi, version 6.12.3: http://www.haskell.org/ghc/ :? for help 
Loading package ghc-prim ... linking ... done. 
Loading package integer-gmp ... linking ... done. 
Loading package base ... linking ... done. 
Loading package ffi-1.0 ... linking ... done. 
Ok, modules loaded: Main. 
Prelude Main> main 
Loading package syb-0.1.0.2 ... linking ... done. 
Loading package base-3.0.3.2 ... linking ... done. 
Loading package mtl-1.1.0.2 ... linking ... done. 
Loading package old-locale-1.0.0.2 ... linking ... done. 
Loading package time-1.1.4 ... linking ... done. 
Loading package random-1.0.0.3 ... linking ... done. 
Loading package MonadRandom-0.1.6 ... linking ... done. 
[True,True,True,True,True] 
[^CInterrupted. 

Answer 1

答案比看起来更简单。当你写：

m <- evalRandIO $ many 1000000 :: IO [Bool] 

evalRandIO的下一次调用将需要评估1000000操作后的种子值。现在需要很多堆栈空间。

因此，当调用inf时，首先必须评估当前的种子值，然后继续进行自己的计算。如果在运行之前INF改变种子你的程序将立即完成：你newStdGen使用当前种子值，所以你必须

import Control.Monad.Random 

inf :: (RandomGen g, Random a) => Rand g [a] 
inf = sequence $ repeat $ getRandom 

many :: (RandomGen g, Random a) => Int -> Rand g [a] 
many n = sequence $ replicate n $ getRandom 

main = do 
    newGen' <- newStdGen 
    m <- evalRandIO $ many 1000000 :: IO [Bool] 
    setStdGen newGen' 
    i <- evalRandIO $ inf :: IO [Bool] 
    putStrLn $ show $ take 5 m 
    putStrLn $ show $ take 5 i 

注意之前呼吁“多”运行。

Answer 2

无法复制在MacOS X 10.6.4：

botanix:~ stolz$ ghci rand.hs 
GHCi, version 6.12.3: http://www.haskell.org/ghc/ :? for help 
Loading package ghc-prim ... linking ... done. 
Loading package integer-gmp ... linking ... done. 
Loading package base ... linking ... done. 
Loading package ffi-1.0 ... linking ... done. 
[1 of 1] Compiling Main    (rand.hs, interpreted) 
Ok, modules loaded: Main. 
*Main> main 
Loading package old-locale-1.0.0.2 ... linking ... done. 
Loading package time-1.1.4 ... linking ... done. 
Loading package random-1.0.0.2 ... linking ... done. 
Loading package transformers-0.2.2.0 ... linking ... done. 
Loading package mtl-2.0.1.0 ... linking ... done. 
Loading package MonadRandom-0.1.6 ... linking ... done. 
[True,False,False,True,True] 
[False,True,False,True,False] 

更新：D'哦，在编译时的表现不同，所以我吃我字：

botanix:~ stolz$ ./a.out 
[False,True,True,False,True] 
Stack space overflow: current size 8388608 bytes.