Haskell 如何解决反应性问题中的t矩类型问题？

我试着写一个这样的函数：

module Main where

import Reactive.Banana
import Reactive.Banana.Frameworks


main = putStrLn "hello world"

type MIDIMessage = (Int, Int, Double)

startRBMidi f = do
    (addHandler, fire) <- newAddHandler :: IO (AddHandler MIDIMessage, Handler MIDIMessage)
    let
        networkDesc = do
            emidi <- fromAddHandler (addHandler :: AddHandler MIDIMessage)
            f emidi
    network <- compile networkDesc
    actuate network
    -- add fire to midi callbacks

modulemain其中
进口香蕉
导入Reactive.Banana.Frameworks
main=putStrLn“你好，世界”
键入MIDIMessage=（Int，Int，Double）
startRBMidi f=do
（addHandler，火灾）力矩t1（）
在测试时。hs:17:16-34
预期类型：力矩t1（）
实际类型：力矩t（）
相关绑定包括
networkDesc:：矩t（）（在test.hs:14:9处绑定）
f:：Event t MIDIMessage->Moment t（）（绑定在test.hs:11:13）
startRBMidi:：（事件t MIDIMessage->矩t（））->IO（）
（在测试时绑定。hs:11:1）
在“compile”的第一个参数中，即“networkDesc”
在“do”块的stmt中：网络时刻t（））->IO（）
startRBMidi f=do
（addHandler，fire）IO（）
startRBMidi f=do
（addHandler，火灾）力矩t（）
networkDesc=do
emidi您需要为您的
startRBMidi
函数指定一个显式类型签名，因为它有一个rank-2类型：

startRBMidi :: (forall t. Event t MIDIMessage -> Moment t ()) -> IO ()

这与编译函数的类型类似

本质上，这意味着参数函数
f
需要在任何开始时间
t
工作。您还需要向
networkDesc
添加类型签名，因为否则，GHC将为本地
let
绑定推断单态类型签名，这不是您想要的。同样，
networkDesc
需要在所有开始时间都工作
t
。太好了，成功了！我必须说，对于不熟悉RANK2类型和幻影类型的人来说，要理解如何正确使用幻影参数t几乎是不可能的。对于那些不使用动态事件切换的人来说，拥有一个更简单版本的库是很有用的，尽管这会导致更多的工作和维护。谢谢你的帮助！很公平。由于这些问题，我打算离开类型参数
t
。看见
{-# LANGUAGE Rank2Types #-}
module Main where
import Reactive.Banana
import Reactive.Banana.Frameworks

main = putStrLn "hello world"

type MIDIMessage = (Int, Int, Double)

startRBMidi :: (forall t. Event t MIDIMessage -> Moment t ()) -> IO ()
startRBMidi f = do
    (addHandler, fire) <- newAddHandler :: IO (AddHandler MIDIMessage, Handler MIDIMessage)
    let
        networkDesc = do
            emidi <- fromAddHandler (addHandler :: AddHandler MIDIMessage)
            f emidi
    network <- compile networkDesc
    actuate network

test.hs:18:22:
No instance for (Frameworks t0)
  arising from a use of ‘fromAddHandler’
The type variable ‘t0’ is ambiguous
Relevant bindings include
  networkDesc :: Moment t0 () (bound at test.hs:17:9)
Note: there is a potential instance available:
  instance Frameworks
             (reactive-banana-0.8.0.4:Reactive.Banana.Internal.Phantom.FrameworksD,
              t)
    -- Defined in ‘reactive-banana-0.8.0.4:Reactive.Banana.Internal.Phantom’
In a stmt of a 'do' block:
  emidi <- fromAddHandler (addHandler :: AddHandler MIDIMessage)
In the expression:
  do { emidi <- fromAddHandler
                  (addHandler :: AddHandler MIDIMessage);
       f emidi }
In an equation for ‘networkDesc’:
    networkDesc
      = do { emidi <- fromAddHandler
                        (addHandler :: AddHandler MIDIMessage);
             f emidi }

test.hs:20:24:
Couldn't match type ‘t0’ with ‘t’
  because type variable ‘t’ would escape its scope
This (rigid, skolem) type variable is bound by
  a type expected by the context: Frameworks t => Moment t ()
  at test.hs:20:16-34
Expected type: Moment t ()
  Actual type: Moment t0 ()
Relevant bindings include
  networkDesc :: Moment t0 () (bound at test.hs:17:9)
In the first argument of ‘compile’, namely ‘networkDesc’
In a stmt of a 'do' block: network <- compile networkDesc

{-# LANGUAGE Rank2Types #-}
module Main where

import Reactive.Banana
import Reactive.Banana.Frameworks

main = putStrLn "hello world"

type MIDIMessage = (Int, Int, Double)

startRBMidi :: (forall t. Event t MIDIMessage -> Moment t ()) -> IO ()
startRBMidi f = do
    (addHandler, fire) <- newAddHandler :: IO (AddHandler MIDIMessage, Handler MIDIMessage)
    let
        networkDesc :: forall t. Frameworks t => Moment t ()
        networkDesc = do
            emidi <- fromAddHandler (addHandler :: AddHandler MIDIMessage)
            f emidi
    network <- compile networkDesc
    actuate network

startRBMidi :: (forall t. Event t MIDIMessage -> Moment t ()) -> IO ()