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

Question

我想一个转换IO Int到Int从System.Random.MWC使用unsafePerformIO.它在ghci中有效:

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

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

这里真的没有必要unsafePerformIO.只需更改s要返回的类型IO Int并使用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

要么

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

要么

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