F# 这是 Option.map 中的错误吗

Question

鉴于以下代码:

let mapOption (f : ('a -> 'b)) (x : 'a option) =
    match x with
    | Some x -> Some(f(x))
    | None -> None

let mapOptions (f : ('a -> 'b)) (xs : 'a option list) : 'b option list =
    xs
    |> List.map (fun (x : 'a option) -> mapOption f x)

let myList = [None; Some 1; Some 2; None]

let a = myList |> mapOptions (fun x -> x + 2)

let b = myList |> List.map(fun x-> x |> Option.map(fun y -> y + 2))

为什么 a 和 b 的结果等于:
[null; Some 3; Some 4; null]或 val it : int option list = [null; Some 3; Some 4; null]
不应该是:

[None; Some 3; Some 4; None]

Answer 1

None由 null 表示在 CLR 中。您可以通过试验 FSI 看到这一点:

> [Some 3; None];;
val it : int option list = [Some 3; null]

不过它仍然有效:

> [Some 3; None] |> List.choose id;;
val it : int list = [3]

所以 [null; Some 3; Some 4; null]与 [None; Some 3; Some 4; None] 相同:

> a = [None; Some 3; Some 4; None];;
val it : bool = true

哪里 a是来自 OP 的值。