Question

继续探讨 F# workflow Buildingers, I 决定对建筑商进行例外处理,特别是在“for.in.do”建筑中。

这一试验的终极目标是工作流程建设者,即使在出现例外情况时,也将继续处理顺序。

The below, for example, doesn t work, because the For method of the builder does not appear to get called until after the point where the exception occurs (at the first Directory.EnumerateFiles(dir) call)

type failSafeSeq() =
    member this.For(seq1, mapFunction) =
        try
            seq1 |> Seq.collect mapFunction
        with
            ex -> Console.WriteLine ex
                  Seq.empty
    member this.Yield(yieldExpr) = yieldExpr |> Seq.singleton
    member this.YieldFrom(yieldBang) = yieldBang
    member this.Combine(a, b) = Seq.append a b
    member this.Delay(delayFun) = delayFun()
    member this.Zero() = Seq.empty 

let failSafe = new failSafeSeq();

let rec allFilesSeq dir =
    failSafe { for file in Directory.EnumerateFiles(dir) do yield file
               for subdir in Directory.EnumerateDirectories dir do yield! (allFilesSeq subdir) }

[<EntryPoint>]
let main args =
    allFilesSeq "C:\System Volume Information\" //almost guaranteed to cause an UnauthorizedAccessException on Windows systems at the first Directory.EnumerateFiles(dir) call.
    |> Seq.iter Console.WriteLine
    0

利用F#工作流程是可能的?

Answer 1

页: 1

... for pat in expr do cexpr ...

翻译

... b.For(expr, fun pat -> cexpr) ...

可在<代码>之前评价<代码>。 ,如果投了弃权票,则上至。不适用。

顺便说一句,您的战略也不会发挥作为人的作用,因为seqs are lazy, and e.gSeq.org。通常不会抛弃(此外,要求它和评估顺序的法典可能会导致法典管理这种 throw。

还可以采取其他一些方式,以恢复<代码>以下...... 我认为,工作流程或许是错误的做法,相反,你只想就<条码>以下的制作和绘图做一些总结。

真正的最终目标是什么? http://www.un.org/Depts/DGACM/index_spanish.htm 似乎对我不有用,你打算如何适用?

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