在f中使用nullable<'t>#

与活动模式一样,选项在模式匹配中起着很好的作用,但似乎是通过使用活动模式(即 typeof 和 ?? )你的代码会吃掉更多的虱子。 基本问题是,如何处理可以为空的引用? 如果您的代码是长链计算,那么最好使用一元语法:

type Maybe<'a> = (unit -> 'a option)

let succeed x : Maybe<'a> = fun () -> Some(x)
let fail : Maybe<'a> = fun () -> None
let run (a: Maybe<'a>) = a()
let bind p rest = match run p with None -> fail | Some r -> (rest r)
let delay f = fun () -> run (f ())

type MaybeBuilder() =
  member this.Return(x) = succeed x
  member this.Let(p,rest) = rest p
  member this.Bind(p,rest) = bind p rest
  member this.Delay(f) = delay f

let maybe = new MaybeBuilder()

let add (a:'a) (b:'a) =
  maybe {
    match TryGetNumericAssociation<'a>() with
    | Some v -> return (v.Add(a,b))
    | _ -> return! fail
  }

let add3 (a:'a) (b:'a) (c:'a) =
  maybe {
    let! ab = add a b
    let! abc = add ab c
    return abc
  }

> let r1 = add 1 2;;
val r1 : (unit -> int option)
> r1();;
val it : int option = Some 3
> let r2 = add "1" "2";;
val r2 : (unit -> string option)
> r2();;
val it : string option = None
> let r3 = add3 "one" "two" "three";;
val r3 : (unit -> string option)
> r3();;
val it : string option = None