F# 异步工作流/任务与免费 monad 相结合

Question

我正在尝试使用免费的 monad 模式构建用于消息处理的管道，我的代码如下所示：

module PipeMonad =
type PipeInstruction<'msgIn, 'msgOut, 'a> =
    | HandleAsync of 'msgIn * (Async<'msgOut> -> 'a)
    | SendOutAsync of 'msgOut * (Async -> 'a)

let private mapInstruction f = function
    | HandleAsync (x, next) -> HandleAsync (x, next >> f)
    | SendOutAsync (x, next) -> SendOutAsync (x, next >> f)

type PipeProgram<'msgIn, 'msgOut, 'a> =
    | Act of PipeInstruction<'msgIn, 'msgOut, PipeProgram<'msgIn, 'msgOut, 'a>>
    | Stop of 'a

let rec bind f = function
    | Act x -> x |> mapInstruction (bind f) |> Act
    | Stop x -> f x

type PipeBuilder() =
    member __.Bind (x, f) = bind f x
    member __.Return x = Stop x
    member __.Zero () = Stop ()
    member __.ReturnFrom x = x

let pipe = PipeBuilder()
let handleAsync msgIn = Act (HandleAsync (msgIn, Stop))
let sendOutAsync msgOut = Act (SendOutAsync (msgOut, Stop))

这是我根据this article

写的

然而，让这些方法异步对我来说很重要（Task 最好，但 Async 是可以接受的），但是当我为我的 pipeline 创建构建器时，我可以'不知道如何使用它 - 我怎样才能等待 Task<'msgOut> 或 Async<'msgOut> 以便我可以发送它并等待这个 "send" 任务？

现在我有了这段代码：

let pipeline log msgIn =
    pipe {
        let! msgOut = handleAsync msgIn
        let result = async {
            let! msgOut = msgOut
            log msgOut
            return sendOutAsync msgOut
        }
        return result
    }

哪个returnsPipeProgram<'b, 'a, Async<PipeProgram<'c, 'a, Async>>>

Answer 1

首先，我认为在 F# 中使用自由 monad 非常接近于反模式。这是一个非常抽象的结构，不适合惯用的 F# 风格 - 但这是一个偏好问题，如果您（和您的团队）发现这种编写代码的方式可读且易于理解，那么您当然可以朝这个方向。

出于好奇，我花了一些时间研究您的示例 - 虽然我还没有完全弄清楚如何完全修复您的示例，但我希望以下内容可能有助于引导您朝着正确的方向前进。总结是，我认为您需要将 Async 集成到您的 PipeProgram 中，以便管道程序本质上是异步的：

type PipeInstruction<'msgIn, 'msgOut, 'a> =
    | HandleAsync of 'msgIn * (Async<'msgOut> -> 'a)
    | SendOutAsync of 'msgOut * (Async<unit> -> 'a)
    | Continue of 'a 

type PipeProgram<'msgIn, 'msgOut, 'a> =
    | Act of Async<PipeInstruction<'msgIn, 'msgOut, PipeProgram<'msgIn, 'msgOut, 'a>>>
    | Stop of Async<'a>

请注意，我必须添加 Continue 才能对我的函数进行类型检查，但我认为这可能是一个错误的 hack，您可能需要对其进行远程处理。使用这些定义，您可以执行以下操作：

let private mapInstruction f = function
    | HandleAsync (x, next) -> HandleAsync (x, next >> f)
    | SendOutAsync (x, next) -> SendOutAsync (x, next >> f)
    | Continue v -> Continue v

let rec bind (f:'a -> PipeProgram<_, _, _>) = function
    | Act x -> 
        let w = async { 
          let! x = x 
          return mapInstruction (bind f) x }
        Act w
    | Stop x -> 
        let w = async {
          let! x = x
          let pg = f x
          return Continue pg
        }
        Act w

type PipeBuilder() =
    member __.Bind (x, f) = bind f x
    member __.Return x = Stop x
    member __.Zero () = Stop (async.Return())
    member __.ReturnFrom x = x

let pipe = PipeBuilder()
let handleAsync msgIn = Act (async.Return(HandleAsync (msgIn, Stop)))
let sendOutAsync msgOut = Act (async.Return(SendOutAsync (msgOut, Stop)))

let pipeline log msgIn =
    pipe {
        let! msgOut = handleAsync msgIn
        log msgOut
        return! sendOutAsync msgOut
    }

pipeline ignore 0

现在这只是简单的PipeProgram<int, unit, unit>，您应该能够通过对命令起作用的递归异步函数来评估它。

Answer 2

根据我的理解，免费 monad 的全部意义在于您不会公开像 Async 这样的效果，所以我认为它们不应该用于 PipeInstruction 类型。解释器是添加效果的地方。

此外，Free Monad 只在 Haskell 中才有意义，您需要做的就是定义一个仿函数，然后自动获得其余的实现。在 F# 中，您还必须编写其余代码，因此与更传统的解释器模式相比，使用 Free 并没有太多好处。您链接到的 TurtleProgram 代码只是一个实验——我根本不建议将 Free 用于实际代码。

最后，如果您已经知道要使用的效果，并且不会有不止一种解释，那么使用这种方法就没有意义。只有当收益大于复杂性时才有意义。

无论如何，如果你确实想写一个解释器版本（而不是免费的），我会这样做：

首先，定义指令没有任何影响。

/// The abstract instruction set
module PipeProgram =

    type PipeInstruction<'msgIn, 'msgOut,'state> =
        | Handle of 'msgIn * ('msgOut -> PipeInstruction<'msgIn, 'msgOut,'state>)
        | SendOut of 'msgOut * (unit -> PipeInstruction<'msgIn, 'msgOut,'state>)
        | Stop of 'state

那么你可以为它写一个计算表达式：

/// A computation expression for a PipeProgram
module PipeProgramCE =
    open PipeProgram

    let rec bind f instruction =
        match instruction with
        | Handle (x,next) ->  Handle (x, (next >> bind f))
        | SendOut (x, next) -> SendOut (x, (next >> bind f))
        | Stop x -> f x

    type PipeBuilder() =
        member __.Bind (x, f) = bind f x
        member __.Return x = Stop x
        member __.Zero () = Stop ()
        member __.ReturnFrom x = x

let pipe = PipeProgramCE.PipeBuilder()

然后你就可以开始写你的计算表达式了。这将有助于在开始解释器之前清除设计。

// helper functions for CE
let stop x = PipeProgram.Stop x
let handle x = PipeProgram.Handle (x,stop)
let sendOut x  = PipeProgram.SendOut (x, stop)

let exampleProgram : PipeProgram.PipeInstruction<string,string,string> = pipe {
    let! msgOut1 = handle "In1"
    do! sendOut msgOut1
    let! msgOut2 = handle "In2"
    do! sendOut msgOut2
    return msgOut2
    }

描述完指令后，您就可以编写解释器了。正如我所说，如果您不编写多个解释器，那么也许您根本不需要这样做。

这是一个非异步版本的解释器（"Id monad"）：

module PipeInterpreterSync =
    open PipeProgram

    let handle msgIn =
        printfn "In: %A"  msgIn
        let msgOut = System.Console.ReadLine()
        msgOut

    let sendOut msgOut =
        printfn "Out: %A"  msgOut
        ()

    let rec interpret instruction =
        match instruction with
        | Handle (x, next) ->
            let result = handle x
            result |> next |> interpret
        | SendOut (x, next) ->
            let result = sendOut x
            result |> next |> interpret
        | Stop x ->
            x

这是异步版本：

module PipeInterpreterAsync =
    open PipeProgram

    /// Implementation of "handle" uses async/IO
    let handleAsync msgIn = async {
        printfn "In: %A"  msgIn
        let msgOut = System.Console.ReadLine()
        return msgOut
        }

    /// Implementation of "sendOut" uses async/IO
    let sendOutAsync msgOut = async {
        printfn "Out: %A"  msgOut
        return ()
        }

    let rec interpret instruction =
        match instruction with
        | Handle (x, next) -> async {
            let! result = handleAsync x
            return! result |> next |> interpret
            }
        | SendOut (x, next) -> async {
            do! sendOutAsync x
            return! () |> next |> interpret
            }
        | Stop x -> x

F# 异步工作流/任务与免费 monad 相结合

F# async workflow / tasks combined with free monad

f#

asynchronous

computation-expression

async-await

free-monad