如何在F#中为自定义计算操作定义Yield和For

Question

我正在为我的应用程序开发一些DSL，下面是我如何定义计算类型和生成器：

// expression type
type Action<'a,'b> = Action of ('a -> Async<'b>)
let runAction (Action r) ctx = r ctx
let returnF a = Action (fun _ -> async {return a})
let bind m f = Action (fun r -> async {
    let! a = runAction m r in return! runAction (f a) r
    })
let bindA ac f = Action (fun r -> async {
    let! a = ac in return! runAction (f a) r
    })
type ActionBuilder<'x>() =
  member this.Return(c) = returnF c
  member this.Zero()    = returnF ()
  member this.Delay(f)  = bind (returnF ()) f
  // binds both monadic and for async computations
  member this.Bind(m, f) = bind m f
  member this.Bind(m, f) = bindA m f
  member this.Combine(r1, r2) = bind r1 (fun () -> r2)
  member this.For(s:seq<_>, f)  = Action (fun x -> async {
    for i in s do runAction (f i) x |> ignore
    })
  // here's the attempt to implement 'need' operations
  [<CustomOperation("need")>]
  member this.Need(Action a,  targets: string list) =
    Action (fun x ->
      let r = a x
      printfn "need(%A, [%A])" a targets
      r)
   member this.For(a, f)  = bindA a f
   member this.Yield(()) =
    returnF ()
let action = ActionBuilder<string>()
/////////////////////////////////////////////////////////////
// other functions for Action
/// Gets action context
let getCtx = Action (fun ctx -> async {return ctx})

let needFn res = action {
    let! ctx = getCtx
    printfn "need([%A]) in %A" res ctx
  }

生成的代码应该是：

let program1 = fun filename -> action {
  let! a = async {return 123}
  let f = a+1
  // need ["def"; "dd"]
  do! needFn ["def"; "dd"]
  printfn "after need"
  for i in [0..10] do
    do! Async.Sleep (1)
    printfn "i: %A" i
  let! d = async {return f}
  let! ctx = getCtx
  printfn "ctx: %A, %A" ctx f
}
Async.RunSynchronously(runAction (program1 "m.c") "abc")

现在我想把CCD_ 1语法改成一个更好的语法；需要"；自定义操作，但从编译器收到了各种抱怨。这是正确的方法还是我滥用了计算表达式？

另一个问题是，如果在循环体内部使用do!，则for不起作用。

Answer 1

阅读论文后，通过反复试验的方法，我得出了以下for实现（不需要Yield构建器方法）：

    let forF (e: seq<_>) prog =
    usingF (e.GetEnumerator()) (fun e ->
        whileF
            (fun () -> e.MoveNext())
            ((fun () -> prog e.Current) |> delayF)
    )

在目标项目中可以找到计算表达式生成器的完整源代码。整个项目是Fake构建系统的变体。

注意：操作已重命名为Recipe。need运算符根本无法实现。