F# 函数签名中的“requires”是什么意思？

问：

请解释以下函数签名，当我在 VS Code 中将鼠标悬停在函数上时出现。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

代码如下

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

添加返回类型注解后，将鼠标悬停在 VS Code 中的函数上现在将函数签名显示为seq<list<string>>

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

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