[英]Polymorphic type family instance in Haskell
我正在尝试编写代码以模拟随机变量,并且我想让事情尽可能保持多态。 这可能涉及使用类型家族,这对我来说是全新的。
这是我的代码的简化版本:
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE InstanceSigs #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE UndecidableInstances #-}
data TrivialDist a = Trivial a
getVal :: TrivialDist a -> a
getVal (Trivial x) = x
class JointDist d a where
toTrivialDist :: d a -> TrivialDist [a]
data TrivialJointDist a = TrivialJoint [a]
instance JointDist TrivialJointDist a where
toTrivialDist :: TrivialJointDist a -> TrivialDist [a]
toTrivialDist (TrivialJoint xs) = Trivial xs
class Simulable s a where
type Samp s a :: *
-- generates infinite stream of random samples from a distribution
samples :: s a -> IO [Samp s a]
-- generates a single random sample from a distribution
sample :: s a -> IO (Samp s a)
sample = fmap head . samples
instance Simulable TrivialDist a where
type Samp TrivialDist a = a
samples :: TrivialDist a -> IO [a]
samples (Trivial x) = return $ repeat x
instance (JointDist d a) => Simulable d a where
type Samp d a = [a]
samples :: d a -> IO [[a]]
samples = samples . toTrivialDist
将其加载到ghci中时,出现以下错误:
test.hs:30:10: error:
Conflicting family instance declarations:
Samp TrivialDist a = a -- Defined at test.hs:30:10
Samp d a = [a] -- Defined at test.hs:40:10
|
30 | type Samp TrivialDist a = a
| ^^^^^^^^^^^^^^^^^^^^^^
Failed, 0 modules loaded.
这个问题似乎与此处发现的问题相似(解释似乎是GHC无法根据约束条件来区分类型),但未提出解决方案。
如果将最后一个实例声明更改为以下代码,则可以编译该代码:
instance Simulable TrivialJointDist a where
type Samp TrivialJointDist a = [a]
samples :: TrivialJointDist a -> IO [[a]]
samples = samples . toTrivialDist
但是,我然后失去了多态的优势,因为我需要为JointDist da
每个子类型分别进行实例声明。
任何帮助深表感谢!
经过一段时间的摸索,我认为我找到了一个使用相等约束而不是显式类型族的解决方案:
class Simulable s a b where
-- generates infinite stream of random samples from a distribution
samples :: s a -> IO [b]
-- generates a single random sample from a distribution
sample :: s a -> IO b
sample = fmap head . samples
instance (b ~ a) => Simulable TrivialDist a b where
samples :: TrivialDist a -> IO [a]
samples (Trivial x) = return $ repeat x
instance {-# OVERLAPPABLE #-} (JointDist d a, b ~ [a]) => Simulable d a b where
samples :: d a -> IO [[a]]
samples = samples . toTrivialDist
{-# OVERLAPPABLE #-}
编译指示GHC允许TrivialDist ab
实例与dab
实例重叠(否则,我们将得到错误)。
声明:本站的技术帖子网页,遵循CC BY-SA 4.0协议,如果您需要转载,请注明本站网址或者原文地址。任何问题请咨询:yoyou2525@163.com.