f# - 动态检查联合案例

Question

当存在值声明时，如何在 F# 中动态匹配联合案例？

非工作代码:

let myShape = Shape.Square
expect myShape Shape.Circle 

type Shape =
   | Circle of int
   | Square of int
   | Rectangle of ( int * int )

let expect someShape someUnionCase =
    if not ( someShape = someUnionCase )
    then failwith ( sprintf "Expected shape %A. Found shape %A" someShape someUnionCase )

let myShape = Shape.Square
expect myShape Shape.Circle // Here I want to compare the value types, not the values

如果我的联合案例没有声明值，这可以使用实例化样本(这不是我想要的):

let myShape = Shape.Square
expect myShape Shape.Circle 

type Shape =
   | Circle
   | Square
   | Rectangle

let expect someShape someUnionCase =
    if not ( someShape = someUnionCase )
    then failwith ( sprintf "Expected shape %A. Found shape %A" someShape someUnionCase )

let myShape = Shape.Square
expect myShape Shape.Circle // Comparing values instead of types

Answer 1

有趣的是，这在 C# 中可以很容易地完成，但 F# 编译器不允许您调用函数 - 这看起来很奇怪。

规范说受歧视的联盟将有(第 8.5.3 节):

One CLI instance property u.Tag for each case C that fetches or computes an integer tag corresponding to the case.

所以我们可以简单地用 C# 编写您的 expect 函数

public bool expect (Shape expected, Shape actual)
{
    expected.Tag == actual.Tag;
}

关于为什么这不能在 F# 代码中完成，这是一个有趣的问题，规范似乎没有给出一个很好的理由。