purescript - Purescript中的免费Monad中的多态指令

我正在尝试编译此小段代码。

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

如果我改为在RFNewEvent中使用“ Number”而不是“ a”，那么一切都可以编译。但是那一刻我走到了“永远”。并将“ Number”替换为不再编译的“ a”。

我收到以下错误消息

Cannot unify type
  a1
with type
  a0

有人知道如何进行这项工作吗？

我正在使用无purescript的0.5.0版本。

编辑

如果我使用以下

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

并将其替换为RFNewEvent，然后将进行编译。但是我最终得到了newEvent不需要的类型签名。

newEvent :: Reactive NewEventData
newEvent = liftFC $ RFNewEvent id

这使我可以创建事件，但可以让我向事件流拍摄不同的事件值，而不是相同类型的值。（现在在newEvent上完全丢失a。）

我可能会犯错。

总体目标是使用Free Monad模拟SodiumFRP的界面。然后，在解释Free Monad时，通过FFI插入一个与Sodium类似的现有JavaScript FRP库。

这可能吗？

最佳答案

现在，以下代码可以编译并具有“ newEvent”的所需类型签名

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

还尝试了purescript-exists。可以定义“样本”

将RFSample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Phil Freeman的评论。

关于purescript - Purescript中的免费Monad中的多态指令，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/33318973/