从自我声明的数据类型的值列表中选取一个随机值

Question

我正在尝试编写一个函数,从所有可以选择的卡中挑选一张随机卡.卡片属于我声明的名为rank的类型.我的代码看起来像这样:

data Rank = Ace | Two | Three | Four | Five | Six | Seven | Eight | Nine | Ten
            | Jack | Queen | King
            deriving(Eq, Ord, Bounded, Enum, Show, Read)

pickCard :: Rank
pickCard = error "How should I pick a random card here? :("

当然我想要这个函数做的是从列表中选择一个随机值[Ace .. King]我该怎么做？请记住,我是Haskell的初学者和一般的函数式编程.

Answer 1

在Haskell中,函数是引用透明的,这意味着如果你将相同的东西放入一个函数中,你会得到同样的东西.所以没有像这样的功能

random :: () -> Int

每次使用它都会返回不同的结果.有几种方法可以解决这个问题,最简单的方法是使用现有的随机设施.

import System.Random -- requires the random package from Hackage

instance Random Rank where -- Describes how to choose random card
  randomR (a,b) = over toEnum . randomR (fromEnum a, fromEnum b)
    where over f (a, b) = (f a, b)
  random = randomR (minBound, maxBound)

pickCard :: IO Rank
pickCard = randomIO

-- Or we can pass in a source of randomness and pass out the new one
pickCard :: RandomGen g => g -> (Rank, g)
pickCard = random