f# - 折叠选项列表

Question

给定一个列表[Some 1;一些 2; Some 3] 我想要一个输出 Some 6 。给定一个列表 [Some 1; None] 应该产生 None。

但我发现以干净的方式实现这一目标比我想象的要困难一些。

我能想到的最好的就是这个

let someNums = [Some 1; Some 2; Some 3]
someNums 
|> List.reduce (fun st v ->
  Option.bind (fun x ->
    Option.map (fun y -> x + y) st) v )

Answer 1

let lift op a b =
    match a, b with
    | Some av, Some bv  -> Some(op av bv)
    | _, _ -> None

let plus = lift (+)

[Some 1; Some 2; Some 3]
|> List.reduce plus
// val it : int option = Some 6


[Some 1; None]
|> List.reduce plus
// val it : int option = None

有折叠

[Some 1; None]
|> List.fold plus (Some 0)
// val it : int option = None

[Some 1; Some 2; Some 3]
|> List.fold plus (Some 0)
// val it : int option = Some 6

[Some 1; None; Some 2] 
|> List.fold plus (Some 0)
// val it : int option = None