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

Question

我想使用 unsafePerformIO 将IO Int转换为Int System.Random.MWC .它确实在ghci中工作：

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

然而在全健康康

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的类型以返回IO Int，并使用do表示法或绑定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