haskell将IO整数转换为Int System.Random.MWC [英] haskell Convert IO Int to Int System.Random.MWC

问题描述

我想使用 unsafePerformIO 将 IO Int 从 System.Random.MWC 转换为 Int .它确实可以在ghci中工作:

I would like to convert an IO Int to Int from System.Random.MWC, using unsafePerformIO. It does work in ghci:

Prelude System.Random.MWC System.IO.Unsafe> let p = unsafePerformIO(uniformR (0, 30) gen :: IO Int)

Prelude System.Random.MWC System.IO.Unsafe> p

11

Prelude System.Random.MWC System.IO.Unsafe> :t p

p :: Int

但是在GHC中

import System.Random.MWC
import System.IO.Unsafe  

main :: IO()
main = do
        gen <-createSystemRandom
        print $! s 30 gen

s :: Int-> GenIO -> Int
s !k g = unsafePerformIO(uniformR (0, k - 1) g)

它返回

ghc: panic! (the 'impossible' happened)
(GHC version 7.6.3 for i386-unknown-linux):
make_exp (App _ (Coercion _))

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

推荐答案

这里真的不需要 unsafePerformIO .只需更改 s 的类型以返回 IO Int ，然后使用do-notation或bind运算符将结果提供给 print .

There's really no need for unsafePerformIO here. Just change the type of s to return IO Int and use do-notation or the bind operator to feed the result to print.

s :: Int -> GenIO -> IO Int
s k g = uniformR (0, k - 1) g

main :: IO ()
main = do
    gen <- createSystemRandom
    x <- s 30 gen
    print x

或

main = do
    gen <- createSystemRandom
    print =<< s 30 gen

或

main = print =<< s 30 =<< createSystemRandom

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