具有多态(在 OO 意义上)返回类型的 F# 函数

Question

是否可以让一个函数返回多个类型，只要它们共享一个公共(public)基类，而不强制转换每个返回值？

例如:

[<AbstractClass>]
type A() = 
  abstract Do : unit -> unit

type B() =
  inherit A()
  override x.Do() = ()

type C() =
  inherit A()
  override x.Do() = ()

let getA i : A =
  if i = 0 then new B() else new C() //Error: This expression was expected to have type A but here has type B

Answer 1

不，你必须投:

let getA i : A =
  if i = 0 then upcast new B() else upcast new C()