如何在F#度量单位上定义扩展成员? [英] How can I define an extension member on an F# unit of measure?

问题描述

抛开是否应该对无单位概念(例如角度)使用度量单位，假设我在F#中定义了degree和radian单位

Leaving aside whether we should use units of measure for unitless concepts like angles, suppose I have define degree and radian units in F#

type [<Measure>] degree =
    static member ToRadians (d:float<degree>) : float<radian> = d * (Math.PI * 1.<radian>) / 180.0<degree>
and [<Measure>] radian =
    static member ToDegrees (r:float<radian>) : float<degree> = r * 180.0<degree> / (Math.PI * 1.<radian>)

我可以像这样相对容易地使用它们

I can use them relatively easily like

4.0<degree> |> degree.ToRadians

似乎扩展成员会更方便.所以我只能说

It seems like extension members would be even handier. So I could just say

let d = 4.0<degree>
let r = d.ToRadians()

但是我无法以明显的方式定义扩展成员

But I can't define the extension member the obvious way

type float<degree> with
    member degrees.ToRadians() = degree.ToRadians(degrees)

...这给我以下错误

... this gets me the following error

error FS0010: Unexpected identifier in type name. Expected infix operator, quote symbol or other token.

在F#中以扩展为单位的扩展成员是否有语法技巧，或者该功能受支持?

Is there a syntactic trick for extension members on units of measure in F#, or is the feature supported?

推荐答案

F#扩展成员与C#扩展成员的不同之处在于，您无法定义构造的泛型类型的扩展.例如，您可以在seq<'t>上定义扩展名，但不能在seq<int>上定义扩展名.换句话说，扩展成员实际上就像该类型的成员一样，而不是静态方法.这也适用于度量类型，因此您不能在float<degree>上定义扩展名，但是可以在float<[<Measure>]'u>上定义扩展名:

F# extension members are different from C# extension members in that you can't define extensions of constructed generic types. For instance, you can define extensions on seq<'t>, but not seq<int>. In other words, extension members really act like members of the type, rather than static methods. This applies to measure types too, so you can't define an extension on float<degree>, but you can define an extension on float<[<Measure>]'u>:

type float<[<Measure>]'u> with
    member f.Squared() = f * f

[<Measure>]
type m

let area = 2.0<m>.Squared()

但是，我看不出这对您有什么帮助……

However, I don't see how this helps you in your case...

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