如何在 F# 中编写一个带有可变数量参数的函数 [英] Howto write a function taking variable number of arguments in F#

问题描述

我在 C# 中有一个函数，我想将它(以及其他一些东西)移植到 F#，只是为了这样做.不幸的是，我刚刚遇到了一个似乎无法在 F# 中表达这一点的案例:使用这个 C# 函数

I've got a function in C#, and I'd like to port it (among some other stuff) over to F#, just for the sake of doing it. Unfortunately, I just hit a case for which there seems to be no way to express this in F#: Take this C# function

public static T Min<T>(params T[] p) where T : IComparable
{
    T m1 = p[0];

    foreach (T v in p)
    {
        m1 = (m1.CompareTo(v) < 0) ? m1 : v;
    }

    return m1;
}

我认为这会很容易，但我不明白如何在 F# 中指定可变参数列表.我试过这个:

I'd thought this would be pretty easy, but I don't understand how I would specify a variable argument list in F#. I have tried this:

let rec Min l =
    match l with
    | [] -> 0 // should throw exception here
    | [v] -> v
    | (h::t) -> min h (Min t)

但是从 C# 调用它需要一个 Microsoft.FSharp.Collections.List.是否有可能得到它期望 params T[]，如果是，如何?

but calling that from C# expects a Microsoft.FSharp.Collections.List. Is it possible to get it expect a params T[], and if so, how?

推荐答案

正如 Jon 所指出的，params 数组只是一个带有属性的数组.在参数前添加属性.

A params array is simply an array with an attribute, as Jon notes. Add the attribute before the parameter.

let test ([<ParamArray>] arr : 'a array) = 
    if arr.Length = 0 then invalid_arg "arr"
    // ....

您不需要指定类型:

let test ([<ParamArray>] arr) = ... // lets type inference do its thing

但是...模式匹配不适用于数组类型.您可以编写一个活动模式来提供帮助.基本上，您必须决定哪个更重要:F# 代码还是 C# 代码.当您设计高阶函数、使用元组、使用有区别的联合等时，同样的权衡将适用. C# 不能表达大多数东西，而且 F# 目前不支持 C# 的一些小糖点(表达树在例如编译器).

But... pattern matching doesn't work on the array type. You could write an active pattern to help. Basically, you have to decide what's more important: the F# code or the C# code. The same tradeoff will apply as you design higher order functions, use tuples, use discriminated unions, etc. C# can't express most things, and F# doesn't currently support some of the little bits of sugar C# has (Expression Tree writing in the compiler, for example).

这篇关于如何在 F# 中编写一个带有可变数量参数的函数的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！