奇怪的表現與evalRandIO

Question

我有一些奇怪的evalRandIO性能問題。這裏是有問題的代碼：

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 

這段代碼將打印5個隨機布爾變量，然後溢出堆棧。但是，如果我註釋掉evalRandIO聲明，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 

代碼運行良好。發生什麼事？

我ghci的輸出：

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

答案比看起來更簡單。當你寫：

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

evalRandIO的下一次調用將需要評估1000000操作後的種子值。現在需要很多堆棧空間。

因此，當調用inf時，首先必須評估當前的種子值，然後繼續進行自己的計算。如果在運行之前INF改變種子你的程序將立即完成：你newStdGen使用當前種子值，所以你必須

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 

注意之前呼籲「多」運行。

Answer 2

無法複製在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，在編譯時的表現不同，所以我吃我字：

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