计算工作流的问题

Question

尝试遵循 f# 专家书中的示例，但工作流有问题...代码如下：

type Attempt<'a> = option<'a>
let succeed x    = Some (x)
let fail         = None 

let bind p rest  = 
    match p with 
    | None -> fail 
    | Some r -> rest r

let delay f = f()

type AttemptBuilder() = 

    member b.Return (x) = succeed x
    member b.Bind (p, rest) = bind p rest
    member b.Delay (f) = delay f
    member b.Let (p, rest):Attempt<'a> = rest p  //'
    member b.ReturnFrom x = x


// using it: 
let attempt = new AttemptBuilder()

let test foo = 
    attempt {
        if not foo then return! fail else return foo
    }

let check () = 
    attempt {

        let! n1 = test true
        let! n2 = test false
        let! n3 = test true
        let foo = n1,n2,n3
        return foo
    }
let foo = check ()

问题是，当所有值都为真时，我按预期得到一个 Some(true, true, true)，但如果传入的值之一为假，则 foo 为空（！）。有人ftw吗？

谢谢！

Answer 1

这只是因为None 在运行时实际上表示为null（参见Option<'T> page on MSDN 上的注释）。另外，请注意，您可以添加

member x.Zero() = fail

给你的构建器，然后你可以写测试为

let test x = attempt { if x then return foo }

这对我的眼睛来说有点清洁。