内置“ >比较“不能与“IComparable< T>”工作 [英] Built-in "< > compare" doesn't work with "IComparable<T>"?

问题描述

我有一个歧视联盟，我希望使用内置的运算符> <比较最大。

I have a Discriminated Union, and I hope to use built in operators like > < compare max for it.

[<CustomComparison>]
type SymbolType = 
    | A
    | B
    | C
    | D

    interface IComparable<SymbolType> with
        member x.CompareTo y =
            match x, y with
            | A, A-> 0
            | A, _ -> 1
            | _, A-> -1
            | _, _ -> 0

我理解我可以使用 IComparable 但是我必须做一个 null 检查，更糟的是，我必须像（SymbolType）y

I understand I can use IComparable, but then i have to do a null check, what's worse is that I have to cast it like (SymbolType) y which I assume would be time consuming.

推荐答案

您可以使用薄型包装器实现所需的方法：

You can just implement the required methods with thin wrappers:

[<CustomComparison>] 
[<CustomEquality>] 
type SymbolType = 
    | A
    | B
    | C
    | D
    override x.Equals y =
       match y with
       | :? SymbolType as t -> (((x :> IComparable<_>).CompareTo) t)=0
       | _ -> false
    interface IComparable with
        member x.CompareTo y =
            match y with
            | :? SymbolType as t -> ((x :> IComparable<_>).CompareTo) t
            | _ -> failwith "bad comparison"
    interface IComparable<SymbolType> with
        member x.CompareTo y =
            match x, y with
            | A, A-> 0
            | A, _ -> 1
            | _, A-> -1
            | _, _ -> 0

这样可避免重复输入。

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