[英]F# rewrite computation expression
我正在研究延續,因為我想對協同程序進行一些有趣的使用......無論如何,我想更好地理解我發現的一個實現。
為此我想重寫實現而不使用計算表達式(continuation Monad),但我不能完成它。
我有這個:
type K<'T,'r> = (('T -> 'r) -> 'r)
let returnK x = (fun k -> k x)
let bindK m f = (fun k -> m (fun a -> f a k))
let runK (c:K<_,_>) cont = c cont
let callcK (f: ('T -> K<'b,'r>) -> K<'T,'r>) : K<'T,'r> =
fun cont -> runK (f (fun a -> (fun _ -> cont a))) cont
type ContinuationBuilder() =
member __.Return(x) = returnK x
member __.ReturnFrom(x) = x
member __.Bind(m,f) = bindK m f
member this.Zero () = this.Return ()
let K = new ContinuationBuilder()
/// The coroutine type from http://fssnip.net/7M
type Coroutine() =
let tasks = new System.Collections.Generic.Queue<K<unit,unit>>()
member this.Put(task) =
let withYield = K {
do! callcK (fun exit ->
task (fun () ->
callcK (fun c ->
tasks.Enqueue(c())
exit ())))
if tasks.Count <> 0 then
do! tasks.Dequeue() }
tasks.Enqueue(withYield)
member this.Run() =
runK (tasks.Dequeue()) ignore
// from FSharpx tests
let ``When running a coroutine it should yield elements in turn``() =
// This test comes from the sample on http://fssnip.net/7M
let actual = System.Text.StringBuilder()
let coroutine = Coroutine()
coroutine.Put(fun yield' -> K {
actual.Append("A") |> ignore
do! yield' ()
actual.Append("B") |> ignore
do! yield' ()
actual.Append("C") |> ignore
do! yield' ()
})
coroutine.Put(fun yield' -> K {
actual.Append("1") |> ignore
do! yield' ()
actual.Append("2") |> ignore
do! yield' ()
})
coroutine.Run()
actual.ToString() = "A1B2C"
``When running a coroutine it should yield elements in turn``()
所以,我想在不使用計算表達式K
情況下重寫Coroutine類的Put
成員。
我當然讀過這 本以及其他幾篇關於catamorphisms的文章,但是重寫這個延續monand並不是很容易,因為它是重寫Write Monad的例子......
我嘗試了幾種方法,這是其中之一:
member this.Put(task) =
let withYield =
bindK
(callcK (fun exit ->
task (fun () ->
callcK (fun c ->
tasks.Enqueue(c())
exit ()))))
(fun () ->
if tasks.Count <> 0
then tasks.Dequeue()
else returnK ())
tasks.Enqueue(withYield)
當然它不起作用:(
(順便說一句:有一些關於編譯器應用於在普通F#中重寫計算的所有規則的大量文檔?)
您的Put
版本幾乎是正確的。 但有兩個問題:
bindK
函數正在向后使用,需要交換參數。 task
應該傳遞給Cont<_,_> -> Cont<_,_>
,而不是unit -> Cont<_,_> -> Cont<_,_>
。 修復這些問題可能如下所示:
member this.Put(task) =
let withYield =
bindK
(fun () ->
if tasks.Count <> 0
then tasks.Dequeue()
else returnK ())
(callcK (fun exit ->
task (
callcK (fun c ->
tasks.Enqueue(c())
exit ()))))
tasks.Enqueue(withYield)
當然它不是太優雅。 使用bind
,最好聲明一個operator >>=
:
let (>>=) c f = bindK f c
那樣
do!
轉換為放置>>= fun () ->
之后 let! a =
let! a =
轉換為放置>>= fun a ->
after 然后你的代碼看起來會更好一些:
member this.Put2(task) =
let withYield =
callcK( fun exit ->
task( callcK (fun c ->
tasks.Enqueue(c())
exit())
)
) >>= fun () ->
if tasks.Count <> 0 then
tasks.Dequeue()
else returnK ()
tasks.Enqueue withYield
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