Mutable Hastable：实例中的非法类型同义词族应用程序

Question

我正在尝试使用此库中的 Mutable BasicHashTable：https://github.com/gregorycollins/hashtables

{-# LANGUAGE GeneralizedNewtypeDeriving #-}

import qualified Data.HashTable.IO as H
import Control.Monad.State.Strict
import Control.Monad.IO.Class (MonadIO)

type A  =  H.BasicHashTable Int String

newtype MyWrapper a = MyWrapper { runM :: StateT A IO a  }
  deriving (Functor, Applicative, Monad, MonadIO, MonadState A )

编译器抱怨我试图在类型类实例中使用 A：

 error:
    • Illegal type synonym family application ‘Control.Monad.Primitive.PrimState
                                                 IO’ in instance:
        MonadState A MyWrapper
    • In the newtype declaration for ‘MyWrapper’
   |
10 |   deriving (Functor, Applicative, Monad, MonadIO, MonadState A )
   |                                                   ^^^^^^^^^^^^

Answer 1

我认为它很奇怪，因为 PrimState 是一个类型家族。试试这个：

import Control.Monad.ST (RealWorld)
import qualified Data.HashTable.ST.Basic as B
type A = B.HashTable Int String RealWorld

你得到的编译错误告诉我们它不能处理类型族。如果您查看哈希 table 类型的定义，您会发现它抱怨的 PrimState 用法：

import qualified Data.HashTable.ST.Basic as B
type BasicHashTable k v = IOHashTable (B.HashTable) k v
type IOHashTable tabletype k v = tabletype (PrimState IO) k v

所以你可以自己直接使用，because:

type instance PrimState IO = RealWorld

事实上。我什至会在上游提交带有修复的 PR：

- type IOHashTable tabletype k v = tabletype (PrimState IO) k v
+ type IOHashTable tabletype k v = tabletype RealWorld k v

因为没有充分的理由按照现在的方式定义它

Mutable Hastable：实例中的非法类型同义词族应用程序

Mutable Hastable : Illegal type synonym family application in instance

haskell

hashtable

type-families

type-synonyms