haskell将IO Int转换为Int System.Random.MWC

Question

我想使用IO Int从Int将System.Random.MWC转换为unsafePerformIO。它在ghci中起作用：

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

然而在GHC

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

Answer 1

这里真的不需要unsafePerformIO。只需将s的类型更改为返回IO Int，然后使用do-notation或bind运算符将结果提供给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