类型项限制的类型项列表

Question

我正在尝试编码一个项目列表,这些项目的类型仅限于某些类型类的实例:

{-# LANGUAGE RankNTypes, TypeSynonymInstances, LiberalTypeSynonyms #-}
module Test where

class Someable a where
  some :: a -> String

data Some = Some String

type SomeGroup = forall a. Someable a => [a]

instance Someable Some where
  some (Some v) = v

instance Someable SomeGroup where
  some (x:xs) = (some x) ++ ", " ++ (some xs)

main = do
  putStrLn $ show.some [Some "A", [Some "B", Some "C"]]

但编译失败并出现错误:

Test.hs:14:10:
    Illegal polymorphic or qualified type: SomeGroup
    In the instance declaration for `Someable SomeGroup'

似乎我甚至没能为类型同义定义实例...

我知道异构集合 wiki文章,但想知道为什么我的方法不起作用 - 我似乎很自然地通过限制集合来定义类型以包含具有类型的项目,这些类型是某种类型的实例.

Answer 1

如果我理解正确,就需要有一个数据类型包装存在主义,以便有一个存储类型类字典和每个元素的地方.

添加一些包装器使它工作:

{-# LANGUAGE ExistentialQuantification, TypeSynonymInstances #-}

module Test where

class Someable a where
  some :: a -> String

data Some = Some String

data SomeWrapper = forall a. Someable a => SomeWrapper a

type SomeGroup = [SomeWrapper]

instance Someable Some where
  some (Some v) = v

instance Someable SomeWrapper where
  some (SomeWrapper v) = some v

instance Someable SomeGroup where
  some (x:xs) = (some x) ++ ", " ++ (some xs)

main = do
  putStrLn $ some [SomeWrapper (Some "A"), SomeWrapper [SomeWrapper (Some "B"), SomeWrapper (Some "C")]]

当然,这有点难看.不幸的是,我不知道有任何更好的方法.