实现“mixins”时GADT，TypeFamilies类型推断失败

Question

我试图用可组合逻辑创建复杂的数据结构。 也就是说，数据结构具有通用格式（实质上是具有某些类型可以更改的字段的记录）和一些通用函数。 具体结构具有通用功能的具体实现。

我试过两种方法。 一种是使用类型系统（具有类型类，类型族，功能依赖性等）。 另一个是创建我自己的“vtable”并使用GADT。 两种方法都以类似的方式失败 - 这里似乎有一些基本的东西。 或者，或许，有更好的Haskell-ish方法来做到这一点？

这是失败的“打字”代码：

{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TypeSynonymInstances #-}

module Typed where

import Control.Monad.State
import Data.Lens.Lazy
import Data.Lens.Template

-- Generic Block.
data Block state ports = Block { _blockState :: state, _blockPorts :: ports }

-- For the logic we want to use, we need some state and ports.
data LogicState = LogicState { _field :: Bool }
data LogicPorts incoming outgoing =
  LogicPorts { _input :: incoming, _output :: outgoing }

makeLenses [ ''Block, ''LogicState, ''LogicPorts ]

-- We need to describe how to reach the needed state and ports,
-- and provide a piece of the logic.
class LogicBlock block incoming outgoing | block -> incoming, block -> outgoing where
  logicState :: block ~ Block state ports => Lens state LogicState
  logicPorts :: block ~ Block state ports => Lens ports (LogicPorts incoming outgoing)
  convert :: block ~ Block state ports => incoming -> State block outgoing
  runLogic :: State block outgoing
  runLogic = do
    state <- access $ blockState
    let myField = state ^. logicState ^. field
    if myField
    then do
      ports <- access blockPorts
      let inputMessage = ports ^. logicPorts ^. input
      convert inputMessage
    else
      error "Sorry"

-- My block uses the generic logic, and also maintains additional state
-- and ports.
data MyState = MyState { _myLogicState :: LogicState, _myMoreState :: Bool }
data MyPorts = MyPorts { _myLogicPorts :: LogicPorts Int Bool, _myMorePorts :: Int }

makeLenses [ ''MyState, ''MyPorts ]

type MyBlock = Block MyState MyPorts

instance LogicBlock MyBlock Int Bool where
  logicState = myLogicState
  logicPorts = myLogicPorts
  convert x = return $ x > 0

-- All this work to write:
testMyBlock :: State MyBlock Bool
testMyBlock = runLogic

它会导致以下错误：

Typed.hs:39:7:
    Could not deduce (block ~ Block state1 ports1)
    from the context (LogicBlock block incoming outgoing)
      bound by the class declaration for `LogicBlock'
      at Typed.hs:(27,1)-(41,19)
      `block' is a rigid type variable bound by
              the class declaration for `LogicBlock' at Typed.hs:26:18
    Expected type: StateT block Data.Functor.Identity.Identity outgoing
      Actual type: State (Block state1 ports1) outgoing
    In the return type of a call of `convert'
    In a stmt of a 'do' block: convert inputMessage

这是失败的“vtable”代码：

{-# LANGUAGE GADTs #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE RecordWildCards #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TemplateHaskell #-}

module VTable where

import Control.Monad.State
import Data.Lens.Lazy
import Data.Lens.Template

-- Generic Block.
data Block state ports = Block { _blockState :: state, _blockPorts :: ports }

-- For the logic we want to use, we need some state and ports.
data LogicState = LogicState { _field :: Bool }
data LogicPorts incoming outgoing =
  LogicPorts { _input :: incoming, _output :: outgoing }

makeLenses [ ''Block, ''LogicState, ''LogicPorts ]

-- We need to describe how to reach the needed state and ports,
-- and provide a piece of the logic.
data BlockLogic block incoming outgoing where
  BlockLogic :: { logicState :: Lens state LogicState
                , logicPorts :: Lens ports (LogicPorts incoming outgoing)
                , convert :: incoming -> State block outgoing
                }
             -> BlockLogic (Block state ports) incoming outgoing

-- | The generic piece of logic.
runLogic :: forall block state ports incoming outgoing
          . block ~ Block state ports
         => BlockLogic block incoming outgoing
         -> State block outgoing
runLogic BlockLogic { .. } = do
  state <- access $ blockState
  let myField = state ^. logicState ^. field
  if myField
  then do
    ports <- access blockPorts
    let inputMessage = ports ^. logicPorts ^. input
    convert inputMessage
  else
    error "Sorry"

-- My block uses the generic logic, and also maintains additional state and ports.
data MyState = MyState { _myLogicState :: LogicState, _myMoreState :: Bool }
data MyPorts = MyPorts { _myLogicPorts :: LogicPorts Int Bool, _myMorePorts :: Int }

makeLenses [ ''MyState, ''MyPorts ]

type MyBlock = Block MyState MyPorts

-- All this work to write:
testMyBlock :: State MyBlock Bool
testMyBlock = runLogic $ BlockLogic
                         { logicState = myLogicState
                         , logicPorts = myLogicPorts
                         , convert = \x -> return $ x > 0
                         }

它会导致以下错误：

VTable.hs:44:5:
    Could not deduce (block1 ~ Block state1 ports1)
    from the context (block ~ Block state ports)
      bound by the type signature for
                 runLogic :: block ~ Block state ports =>
                             BlockLogic block incoming outgoing -> State block outgoing
      at VTable.hs:(37,1)-(46,17)
    or from (block ~ Block state1 ports1)
      bound by a pattern with constructor
                 BlockLogic :: forall incoming outgoing state ports block.
                               Lens state LogicState
                               -> Lens ports (LogicPorts incoming outgoing)
                               -> (incoming -> State block outgoing)
                               -> BlockLogic (Block state ports) incoming outgoing,
               in an equation for `runLogic'
      at VTable.hs:37:10-26
      `block1' is a rigid type variable bound by
               a pattern with constructor
                 BlockLogic :: forall incoming outgoing state ports block.
                               Lens state LogicState
                               -> Lens ports (LogicPorts incoming outgoing)
                               -> (incoming -> State block outgoing)
                               -> BlockLogic (Block state ports) incoming outgoing,
               in an equation for `runLogic'
               at VTable.hs:37:10
    Expected type: block1
      Actual type: block
    Expected type: StateT
                     block1 Data.Functor.Identity.Identity outgoing
      Actual type: State block outgoing
    In the return type of a call of `convert'
    In a stmt of a 'do' block: convert inputMessage

当整个事情明确地在ScopedTypeVariables和“forall block”下时，我不明白为什么GHC会选择“block1”。

编辑＃1：添加了功能依赖，感谢Chris Kuklewicz指出这一点。 问题仍然存在。

编辑＃2：正如克里斯指出的那样，在VTable解决方案中，摆脱所有“阻塞〜阻塞状态端口”，而不是在任何地方编写“阻塞状态端口”解决问题。

编辑＃3：好的，所以问题似乎是对于每个单独的函数，GHC在参数中需要足够的类型信息来推断所有类型，即使对于根本没有使用的类型。 因此，在（例如）上面的logicState的情况下，参数只给我们状态，这不足以知道端口以及传入和传出类型是什么。 没关系，它对logicState函数并不重要; GHC想知道，但不能，所以编译失败。 如果这确实是核心原因，那么如果GHC在编译logicState decleration时直接抱怨会更好 - 它似乎有足够的信息来检测那里的问题; 如果我在该位置看到“端口类型未被使用/确定”的问题，那就更清楚了。

编辑＃4：我仍然不清楚为什么（阻塞〜阻塞状态端口）不起作用; 我想我是出于意想不到的目的使用它？ 看起来应该有用。 我同意克里斯的观点，即使用CPP来解决这个问题是令人厌恶的; 但写“B trpe”（在我的真实代码中有更多的paraneters）也不是一个好的解决方案。

Answer 1

我有一个针对您的VTable代码的一行修复：

            , convert :: incoming -> State block outgoing

变

            , convert :: incoming -> State (Block state ports) outgoing

然后你应该简化runLogic的类型

runLogic :: BlockLogic (Block state ports) incoming outgoing
         -> State (Block state ports) outgoing

PS：更多细节回答以下评论。

消除“阻止〜”不是解决方案的一部分。 通常只有在instance a~b => ... where情况下才需要“〜”。

以前如果我给一个函数一个xxx :: BlockLogic (Block state ports) incoming outgoing那么它可以解包convert xxx :: State block outgoing 。 但是新block与(Block state ports)完全没有关系，它是一种新的不可知类型。 编译器在名称的末尾附加一个数字以生成block1 ，然后出现在错误消息中。

原始代码（两个版本）都存在编译器可以从给定上下文推断出哪些类型的问题。

至于详细程度，请尝试type 。 不要使用CPP和DEFINE。

type B s p = BlockLogic (Block s p)

runLogic :: B s p i o -> State (Block s p) o

PPS：进一步解释类版本的问题。 如果我替换（阻止sp）阻止并添加你提到的功能依赖：

class LogicBlock state ports incoming outgoing | state ports -> incoming outgoing where
  logicState :: Lens state LogicState
  logicPorts :: Lens ports (LogicPorts incoming outgoing)
  convert :: incoming -> State (Block state ports) outgoing

使用logicState钉住state但留下ports未知，使ports#

使用logicPorts钉在ports但留下state未知，使ports#

编译runLogic在端口，端口0，端口1和状态，状态0，状态1之间运行许多类型不匹配错误。

这些操作似乎不适合同一类型类。 您可以将它们分解为单独的类型类，或者可以将“，state-> ports，ports-> state”函数依赖项添加到类声明中。