在 Haskell,是否有用于生成 Random
的“标准”库/包?/Arbitrary
枚举?
我编写了以下代码,但我不敢相信我是第一个有这种需求或解决它的人(而且我不确定我的解决方案是否完全正确)。
另外,我希望现有的解决方案具有其他不错的功能。
这是一对从 Enum 类型中选择随机值的函数:
enumRandomR :: (RandomGen g, Enum e) => (e, e) -> g -> (e, g)
enumRandomR (lo,hi) gen =
let (int, gen') = randomR (fromEnum lo, fromEnum hi) gen in (toEnum int, gen')
enumRandom :: (RandomGen g, Enum e) => g -> (e, g)
enumRandom gen =
let (int, gen') = random gen in (toEnum int, gen')
这里是
System.Random.Random
的实例和 Test.QuickCheck.Arbitrary
{-# LANGUAGE FlexibleInstances, UndecidableInstances, OverlappingInstances #-}
instance (Enum a, Bounded a) => Random a where
random = enumRandom
randomR = enumRandomR
instance (Enum a, Bounded a) => Arbitrary a where
arbitrary = choose (minBound, maxBound)
这是一个示例
Bounded
, Enum
类型data Dir = N | E | S | W
deriving (Show, Enum, Bounded)
这是随机/任意方法的测试
> import Test.QuickCheck
> sample (arbitrary:: Gen Dir)
N
E
N
S
N
E
W
N
N
W
W
我不高兴我的解决方案依赖于这些扩展:
{-# LANGUAGE FlexibleInstances, UndecidableInstances, OverlappingInstances #-}"
因为:
- Constraint is no smaller than the instance head
in the constraint: Enum a
(Use -XUndecidableInstances to permit this)
,
- Overlapping instances for Random Int
arising from a use of `randomR'
Matching instances:
instance Random Int -- Defined in System.Random
instance (Enum a, Bounded a) => Random a
, 和
- Illegal instance declaration for `Random a'
(All instance types must be of the form (T a1 ... an)
where a1 ... an are *distinct type variables*,
and each type variable appears at most once in the instance head.
Use -XFlexibleInstances if you want to disable this.)
有没有更好的办法?我的解决方案对于某些(更“异国情调”)有界枚举类型是否比我的简单示例失败?
最佳答案
在这种情况下的标准解决方法是创建一个 newtype
包装器并为此提供实例。
{-# LANGUAGE GeneralizedNewtypeDeriving #-} -- avoid some boilerplate
newtype Enum' a = Enum' a
deriving (Bounded, Enum, Show)
instance (Enum a, Bounded a) => Random (Enum' a) where
random = enumRandom
randomR = enumRandomR
instance (Enum a, Bounded a) => Arbitrary (Enum' a) where
arbitrary = choose (minBound, maxBound)
当然,这种方法在使用新类型时需要一些额外的包装和展开,但是对于 QuickCheck 使用,这应该不会太糟糕,因为您通常只需要对每个属性进行一次模式匹配:
prop_foo (Enum' x) = ... -- use x as before here
关于 haskell : "instance (Enum a, Bounded a) => Random a"和 "=> Arbitrary a",我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/9188409/