免费Monad中的多态教学

Question

我正在试着编译这小块代码。

module Sodium where

import Prelude
import Control.Monad.Free
import Data.Coyoneda
import Data.Tuple

data ReactiveF more
  = RFNewEvent (forall a. (Tuple (Event a) (a -> Reactive Unit) -> more))

type Reactive a = FreeC ReactiveF a

data Event a = Event a

newEvent :: forall a. Reactive (Tuple (Event a) (a -> Reactive Unit))
newEvent = liftFC $ RFNewEvent id

如果我在RFNewEvent中使用"Number“而不是"a”，那么一切都编译得很好。但就在我“追求一切”的那一刻。并将“数字”改为"a“，不再编译。

我收到以下错误消息

Cannot unify type
  a1
with type
  a0

有人知道怎么做吗？

我用的是0.5.0版的免费纯文本。

编辑

如果我使用以下方法

data NewEventData = NewEventData forall a. Tuple (Event a) (a -> Reactive Unit)

并将其替换为RFNewEvent，然后进行编译。但是我最终得到了一个不想要的newEvent类型签名。

newEvent :: Reactive NewEventData
newEvent = liftFC $ RFNewEvent id

它允许我创建一个事件，但允许我将不同的事件值拍摄到事件流，而不是相同类型的值。(失踪的a.现在newEvent上)

我可能犯了个错误。

总体目标是模拟SodiumFRP的界面使用一个自由Monad。然后插入一个现有的JavaScript玻璃钢库，工作类似于钠通过FFI解释自由Monad。

这个是可能的吗？

Answer 1

下面的代码现在编译"newEvent“并具有所需的类型签名

module FRP.Sodium where

import Prelude
import Control.Monad.Free
import Data.Coyoneda
import Data.Tuple

data ReactiveF more
  = RFNewEvent (NewEventData -> more)

type Reactive a = FreeC ReactiveF a

data NewEventData = NewEventData forall a. Tuple (Event a) (a -> Reactive Unit)

data Event a
  = ENever
  | EMerge (Event a) (Event a)
  | EFilterJust (Event (Maybe a))
  | ECoalesce (a -> a -> a) (Event a)
  | EOnce (Event a)
  | ESplit (Event (Array a))
  | EVar Int

data Behaviour a = BVar Int

extractNewEventData :: forall a. NewEventData -> (Tuple (Event a) (a -> Reactive Unit))
extractNewEventData (NewEventData x) = x

newEvent :: forall a. Reactive (Tuple (Event a) (a -> Reactive Unit))
newEvent = map extractNewEventData $ liftFC $ RFNewEvent id

编辑

也尝试了纯文本-存在。使定义“样本”成为可能

RFSample被添加到ReactiveF ..。

.
.
.
data ReactiveF more
  = RFNewEvent (NewEventData -> more)
  | RFSample (SampleData more)
.
.
.
data SampleDataF more a = SampleDataF (Behaviour a) (a -> more)
type SampleData more = Exists (SampleDataF more)

sample :: forall a. Behaviour a -> Reactive a
sample beh = liftFC $ RFSample $ mkExists $ SampleDataF beh id

谢谢菲尔·弗里曼的评论。