F＃這是鑑於以下代碼Option.map

Question

了一個錯誤：

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)) 

爲什麼A和B等於結果：

[null; Some 3; Some 4; null]或val it : int option list = [null; Some 3; Some 4; null]

豈不是：

[None; Some 3; Some 4; None] 

Answer 1

None由表示CLR中的。你可以看到，通過與FSI試驗：

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

它仍然有效，但::

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

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

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

其中a是價值來自OP。