Question

I have to use new to create new instances of dynamically created, anonymous object types that are based on existing base types, or interfaces, using object expressions:

let a = { new System.Object() with member x.ToString() = "F#" }

But in other cases, for example computation expressions, I don't have to write new at all:

type MaybeBuilder() =
    member this.Bind(x, f) = match x with
                             | None -> None
                             | Some a -> f a
    member this.Return(x) = Some x

let maybe = new MaybeBuilder()
ley maybe2 = MaybeBuilder()

I'd like to ask, when is new not an optional to write?

Answer 1

您的两个示例使用完全不同的语言功能，即使它们使用相同的new关键字。

您永远不需要使用new来调用构造函数，就像在第二个示例中一样（与您必须使用它的C＃相比）。

但在F＃中，new关键字也用于对象表达式功能，如第一个示例所示。这是需要的，因为F＃中的花括号有不同的用途。编译器只知道您正在编写对象表达式，因为它看到{ new。

When `new` is not optional?

1 个答案: