"requires" 在 F# 函数签名中是什么意思？

Question

请解释当我将鼠标悬停在 VS Code 中的函数上时出现的以下函数签名。我特别好奇“需要”到底是什么意思，为什么 'b 是 'a。

val handleSingleEvent:
    : Request
    -> 'b (requires :> seq<list<string>>)

通用参数

'b 是 'a

下面是代码

let handleEvents (requests: Request list, reqEventQueue: EventQueue, session: Session)  =
      let rec handleSingleEvent (request: Request) : seq<list<string>> =
        seq {
          
              let eventObj = reqEventQueue.NextEvent()

              match eventObj.Type with
              | Event.EventType.REQUEST_STATUS -> yield processMiscEvents eventObj |> makeJson
              | Event.EventType.ADMIN -> yield processAdminEvent eventObj |> makeJson
              | Event.EventType.AUTHORIZATION_STATUS -> yield processAuthEvent eventObj session |> makeJson
              | Event.EventType.PARTIAL_RESPONSE ->
                yield processReferenceResponseEvent eventObj
                |> makeJson
                yield! handleSingleEvent request
              | Event.EventType.RESPONSE -> yield processReferenceResponseEvent eventObj |> makeJson
              | _ -> yield processMiscEvents eventObj |> makeJson
        } |> ignore
        handleSingleEvent request
      List.map (fun request -> handleSingleEvent request) requests

添加 return 类型注释 seq<list<string>> 后，将鼠标悬停在 VS Code 中的函数上现在将函数签名显示为

val handleSingleEvent:
    : Request
    -> seq<list<string>>

“requires”消失了，“Generic Parameters `b is `a”消失了。

Answer 1

'requires' 指示成员约束，这意味着泛型类型参数被约束以显示该成员。举个简单的例子：

let inline f<'b  when 'b : (member Name : string)> (x: 'b) = x

泛型类型 'b 现在被限制为具有 returns 字符串的成员名称。

https://docs.microsoft.com/en-us/dotnet/fsharp/language-reference/generics/constraints

"requires" 在 F# 函数签名中是什么意思？

What does "requires" mean in F# function signature?

f#

function-signature