Question

在纯函数式编程中，执行顺序无关紧要，因此未确定（即由编译器决定）。如果您有副作用，执行顺序很重要。那怎样才能在F＃中定义？

我有一个递归删除给定路径的所有空子文件夹的函数。此外，如果名称在给定列表中，它将删除其中包含的一些文件。

算法很简单：

删除列表中的所有文件。
对子文件夹进行递归调用。
删除文件夹（如果该文件夹为空）。这必须是最后一步。

此外，该函数返回已删除元素的数量作为元组（已删除文件夹的数量，已删除文件的数量）。

这是我的代码：

let rec DeleteEmptyFolders path filenames =
    // Deletes a file or folder using the given function.
    // Returns 1 if the file could be deleted, otherwise 0.
    let Delete delete name =
        try
            delete name;
            1
        with
            | _ -> 0

    // The function result (number of deleted folders and files).
    let deletedFolders (a, _) = a
    let deletedFiles   (_, a) = a

    let accumulator a b = 
        ( deletedFolders a + deletedFolders b,
          deletedFiles a + deletedFiles b )

    // Deletes the given files and returns the number of deleted elements.
    let DeleteFiles folder names =
        names
        |> Seq.map (fun n -> Path.Combine (folder, n))
        |> Seq.map (fun n -> Delete File.Delete n)
        |> Seq.reduce (+)

    // Deletes the folder if it is empty
    // (Directory.Delete will fail if it is not empty).
    let DeleteFolder folder = Delete Directory.Delete folder

    // The recursive call
    let DeleteEmptySubFolders folder files =
        Directory.EnumerateDirectories folder
        |> Seq.map (fun p -> DeleteEmptyFolders p files)
        |> Seq.reduce (accumulator)

    // Three functions are executed: DeleteEmptySubFolders, DeleteFolder and DeleteFiles
    // But it has to be done in the correct order: DeleteFolder must be executed last.
    accumulator (DeleteEmptySubFolders path filenames) (DeleteFolder path, DeleteFiles path filenames)

我可以弄清楚首先执行DeleteEmptySubFolders，然后执行DeleteFolder和DeleteFiles。这是函数在代码中出现的顺序。但我不认为这是F＃的规则，它正是编译器所决定的。它可以是任何其他订单。

当然我可以交换代码最后一行中的元素。编译器会相应地更改执行顺序。但由于这不是语言的规则，它只是运气。

关于这个主题的another question我已经读过，这些值（即没有参数的函数）按照它们被声明的顺序初始化。

let firstCall  = DeleteFiles path filenames
let secondCall = DeleteEmptySubFolders path filenames
let thirdCall  = DeleteFolder path

accumulator (secondCall) (thirdCall, firstCall)

现在调用恰好是正确的顺序。但同样：这是F＃的规则还是编译器的工作原理？（如果未使用这些函数，编译器可能会决定不对值进行初始化）

如果我想告诉F＃执行顺序是否重要以及何时应该进行每次调用，我该如何写最后一行？是否有关键字或特殊语法将函数标记为无副作用？

Answer 1

F＃不是纯函数式编程语言：函数和值将从上到下，从左到右计算。

Answer 2

我认为最好将代码编写为

let firstCall  = DeleteFiles path filenames
let secondCall = DeleteEmptySubFolders path filenames
let thirdCall  = DeleteFolder path

accumulator (secondCall) (thirdCall, firstCall)

如何在F＃中定义执行顺序？

2 个答案: