Haskell 使用randomR生成随机值的元组

我正在研究，在一个介绍性示例中，我们遇到了以下代码（本文中称为

clumsyRollDice

）

…无法编译（如下所示）

我错过了什么？自示例发布以来，

System.Random

中的函数/类之一的实现是否发生了变化

statemonad.hs:15:5:
    No instance for (Random t0)
      arising from the ambiguity check for ‘g’
    The type variable ‘t0’ is ambiguous
    When checking that ‘g’ has the inferred type ‘StdGen’
    Probable cause: the inferred type is ambiguous
    In an equation for ‘rollDice3’:
        rollDice3
          = (n, m)
          where
              (n, g) = randomR (1, 6) (mkStdGen 0)
              (m, _) = randomR (1, 6) g

statemonad.hs:16:13:
    Could not deduce (RandomGen t0) arising from a use of ‘randomR’
    from the context (Random t, Num t)
      bound by the inferred type of m :: (Random t, Num t) => t
      at statemonad.hs:16:5-27
    The type variable ‘t0’ is ambiguous
    Note: there is a potential instance available:
      instance RandomGen StdGen -- Defined in ‘System.Random’
    In the expression: randomR (1, 6) g
    In a pattern binding: (m, _) = randomR (1, 6) g
    In an equation for ‘rollDice3’:
        rollDice3
          = (n, m)
          where
              (n, g) = randomR (1, 6) (mkStdGen 0)
              (m, _) = randomR (1, 6) g
Failed, modules loaded: none.
(0.00 secs, 0 bytes)

更新：
下面修复了代码，但我仍然很好奇：在运行代码的逐字版本时，是什么更改（即在编译器/etc中）导致了错误：

rollDice3 :: (Int, Int)
rollDice3 = (n,m)
  where
    (n,g) = randomR ((1::Int),6) (mkStdGen 0)
    (m,_) = randomR (1,6) g

---

考虑此文件，另存为

test.hs

module Main (main, clumsyRollDice) where
import System.Random
clumsyRollDice :: (Int, Int)
clumsyRollDice = (n, m)
        where
        (n, g) = randomR (1,6) (mkStdGen 0)
        (m, _) = randomR (1,6) g

main = putStrLn (show clumsyRollDice)

此文件在

ghc test.hs

下编译，但在

ghc test.hs-XNoMonomorphismRestriction

下失败，错误与您报告的非常类似。您是否在ghci中设置了

NoMonomonomorphismRestriction

此邮件列表线程中讨论了一个非常类似的案例：

wiki上详细讨论了单态限制：

---

自2010年起，randomR的实施依赖于randomIvalIntegral，所以“最近”没有任何变化哈哈，公平的说，我在说“最近”时应该更小心是的，这正是错误的原因。我确实打开了单态限制。（事实上，我正在运行GHC7.8.3，默认情况下它是打开的。）谢谢

module Main (main, clumsyRollDice) where
import System.Random
clumsyRollDice :: (Int, Int)
clumsyRollDice = (n, m)
        where
        (n, g) = randomR (1,6) (mkStdGen 0)
        (m, _) = randomR (1,6) g

main = putStrLn (show clumsyRollDice)