f# - 对受歧视的联合实现 F# 比较

Question

我有一个日志级别类型:

type LoggingLevel =
| Trace
| Debug
| Info

我想说一些日志记录级别高于其他日志记录级别。例如，Trace 高于 Info。

所以我这样实现了IComparable:

[<StructuralEqualityAttribute>]
[<CustomComparisonAttribute>]
type LoggingLevel =
| Trace
| Debug
| Info

  interface IComparable<LoggingLevel> with
    override this.CompareTo other =
      let score x =
        match x with
        | Trace -> 0
        | Debug -> 1
        | Info -> 2
      (score this) - (score other)

但是当我尝试使用它时，出现错误:

if a >= b 
then 
  // ...

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

我这里怎么出错了？

---

我设法让它工作了，但是现在类型定义太冗长了!一定有更好的方法……

[<CustomEquality>]
[<CustomComparisonAttribute>]
type LoggingLevel =
  | Trace
  | Debug
  | Info

  override this.Equals (obj) =
    match obj with
    | :? LoggingLevel as other ->
      match (this, other) with
      | (Trace, Trace) -> true
      | (Debug, Debug) -> true
      | (Info, Info) -> true
      | _ -> false
    | _ -> false

  override this.GetHashCode () =
    match this with
    | Trace -> 0
    | Debug -> 1
    | Info -> 2

  interface IComparable<LoggingLevel> with
    member this.CompareTo (other : LoggingLevel) =
      let score x =
        match x with
        | Trace -> 0
        | Debug -> 1
        | Info -> 2
      (score this) - (score other)

  interface IComparable with
    override this.CompareTo other =
      (this :> IComparable<LoggingLevel>).CompareTo (other :?> LoggingLevel)

Answer 1

I would like to say that some logging levels are higher than others. For example, Trace is higher than Info.

您是否需要使用自定义相等和自定义比较？ F# 为可区分联合内置了这些。您只需要在类型定义中按递增顺序编写它们:

type LoggingLevel =
  | Info
  | Debug
  | Trace // Note the order here! 🤔

Trace > Info // true

let levels = [ Trace; Debug; Info; Trace; Debug; Info ]

levels |> List.sort
// [Info; Info; Debug; Debug; Trace; Trace]
// Comparison ✔

levels |> List.countBy id
// [(Trace, 2); (Debug, 2); (Info, 2)]
// Equality ✔

更多信息:https://stackoverflow.com/a/52220335/1256041