[F#] return a method's object

type A() =
    member this.get () =
        this

type Result<'a>() =
    [<DefaultValue>] val mutable check: 'a


type Check (target, f) = 
    static member (+) (c1, c2) =
        c1,c2    
    //member this.id = counter.next
    //member this.target : Targets = target
    member this.f (target : obj) : obj -> Result<Check> = f
    member this.propagate (target) = 
        let res = f target
        res.check <- this //error
        res

type IResultCallable =
    abstract member ResultCall: unit -> int


type Result() =
    [<DefaultValue>] val mutable check: IResultCallable

    member x.T () =
        x.check.ResultCall()


type Check (target, f) = 

    interface IResultCallable with
        member x.ResultCall() = 2*2

    static member (+) (c1, c2) =
        c1,c2    
    //member this.id = counter.next
    //member this.target : Targets = target
    member this.f (target : obj) : obj -> Result = f
    member this.propagate (target) = 
        let res = f target
        res.check <- this //error
        res

type A () =
  ...

and B () =
  ...