问题与计算工作流

Question

试图遵循专家F＃的书为例，其与工作流程的一个问题...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，当所有的值都是真实的，我按预期得到一个Some（true，true，true），但如果其中一个值传入为false，则foo为空（！）。任何人ftw？

谢谢！

Answer 1

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

member x.Zero() = fail 

你的建设者，然后你可以把测试作为

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

这是一个小清洁我的眼睛。