如何将测量单位转换并添加到 F# 中的现有值？

Question

我需要将值转换为其他数据类型，因为环境处理米、秒、弧度、度等。我也使用测量单位。我的问题源于对 C# 代码的依赖并需要在我的项目中使用它。

这似乎是一个微不足道的问题，但我一生都无法解决这个问题。如果我可以进去并将单位添加到原始定义中，那不会有问题，但我需要将它们添加到现有类型和现有值中。

这是一个演示 C# 项目。 F# 项目将此作为依赖项。

namespace test
{
    public class Polygon
    {
        public int angle = 3;
        public int length = 1;
    }
}

这里是 F# 项目，其中包含我在转换和添加测量单位时失败的各种方法。

[<Measure>] type m

open test

let x : int = 1;

// == trying to cast int to float and assign unit-of-measure ==
type a = float<m>
let y:a = a x                 // The value or constructor 'a' is not defined
let y:a = (a)x                // The value or constructor 'a' is not defined
let y:a = upcast (float x)    // Type constraint mismatch. The type 'float' is not compatible with type 'a'
                              //    The type 'a' does not have any subtypes and need not be used as the
                              //    target of a static coercion.
let y:a = downcast (float x)  // The type 'float' does not have any proper subtypes and cannot be used
                              //    as the source of a type test or runtime coercion.
let y:a = float<m> x          // Expected type, not unit-of-measure
let y:a = (float x)<m>        // Unexpected token '>' or incomplete expression
let y:a = (float<m>)(float x) // Expected type, not unit-of-measure

// == trying to only assign unit-of-measure ==
type b = int<m>
let y:b = x      // This expression was expected to have type 'b' but here has type 'int' 
let y:b = x<m>   // Unexpected type arguments
let y:b = (x)<m> // Unexpected token '>' or incomplete expression

// == actual usecase ==
let getLength (shape : test.Polygon) : float<m> =
    float<m> shape.length // ???

let getSides (shape : test.Polygon) : float =
    float shape.angle

let usecase (shape : test.Polygon) =
    let fancySide = getLength shape
    let fancySides = getSides shape
    fancySide + fancySides // Needs to give error

Answer 1

您可以使用

FloatWithMeasure

进行此转换：

let y:a = LanguagePrimitives.FloatWithMeasure x

有关详细信息，请参阅 F# 语言指南：

要将无单位值转换为有单位的值，您可以乘以用适当单位注释的 1 或 1.0 值。但是，为了编写互操作性层，还可以使用一些显式函数将无单位值转换为有单位的值。这些位于 FSharp.Core.LanguagePrimitives 模块中。