F# 类型的自定义比较运算符

Question

我想为我的类型(<、>、<=、>= 等)创建自定义比较运算符。我试过:

type MyType() =
    static member (>) (left: MyType, right: int) = true


let foo = new MyType();
let bar = foo > 12;

并收到错误:

The type 'MyType' does not support the 'comparison' constraint. For example, it does not support the 'System.IComparable' interface

(为什么像运算符重载这样的语言特性依赖于来自框架的接口(interface) IComparable？语言和它使用的框架不应该是独立的吗？)所以我尝试:

type MyType() =
    interface IComparable
        member self.CompareTo yobj = true
    interface IComparable<int>
        member self.CompareTo yobj = true
    static member (>) (left: MyType, right: int) = true

我得到:

This expression was expected to have type 'MyType' but here has type 'int'

如何获得预期的行为？

Answer 1

出现这种错误的原因

不知道为什么F#不允许我写这样的代码 C# .

public class MyType  
{
    public int Value {get;set;}

    public static bool operator >(MyType left, int right) 
    {
        return left.Value > right; 
    }

    public static bool operator <(MyType left, int right) 
    {
        return left.Value < right; 
    }
}

即使此类型未实现 IComparable界面，我可以将它与 int 进行比较如下:

t > 2
t < 6

似乎F#对待(>)作为 T' -> T' -> bool :

val ( > ): 
   x: 'T (requires comparison )->
   y: 'T (requires comparison )
   -> bool

这表明:

1. 左右参数类型相同。
2. T'需要比较 ( IComparable )

如果我理解正确，这就是您收到如下错误的原因:

This expression was expected to have type 'MyType' but here has type 'int'

即使您已经实现了 IComparable接口(interface)，标准(>)要求左右参数属于同一Type。

绕行

绕一圈就是创建自定义函数 (>) 接受左 MyType和权利int直接参数:

type MyType(info)  =     member x.Info : int = info 
  
   let inline (>) (left: MyType) (right: int) =     left.Info > rightlet inline (<) (left: MyType) (right: int) =     left.Info < right
  // see https://stackoverflow.com/questions/19682432/global-operator-overloading-in-flet inline (>) (left) (right) =     match (box left, box right) with    | (:? MyType as l, :? int as r ) ->        l.Info > int right    | (:? IComparable as left', :? IComparable  )->        let r = left'.CompareTo right        r > 0    | _ -> failwith "not support type "let inline (<) (left) (right) =     match (box left, box right) with    | (:? MyType as l, :? int as r ) ->        l.Info < int right    | (:? IComparable as left', :? IComparable  )->        let r = left'.CompareTo right        r < 0    | _ -> failwith "not support type "

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