计算表达式不执行let

我使用的是f v 1.9.6.2,我定义了一个非常简单的计算表达式:

type MaybeBuilder() =
    member this.Let(x, f) =
        printfn "this.Let: %A" x
        this.Bind(Some x, f)
    member this.Bind(x, f) =
        printfn "this.Bind: %A" x
        match x with
        | Some(x) when x >= 0 && x <= 100 -> f(x)
        | _ -> None
    member this.Delay(f) = f()
    member this.Return(x) = Some x

let maybe = MaybeBuilder()

我在代码中撒了一些print语句,告诉我在计算表达式中调用了哪些方法。当我执行以下语句时:

maybe {
    let x = 12
    let! y = Some 11
    let! z = Some 30
    return x + y + z
}

我希望控制台打印出以下内容:

this.Let 12
this.Bind Some 12
this.Bind Some 11
this.Bind Some 30

但我的实际结果如下:

this.Bind: Some 11
this.Bind: Some 30

换句话说,f似乎没有执行 Let 成员。当我重写 让 要抛出异常,代码运行时没有异常。另外,当我评论 让 全体成员,我 不 获取一条错误消息,说明 The field, constructor or member 'Let' is not defined ,代码按预期执行。

(我试过使用Reflector来研究代码,但通常情况下,反编译的f会被破坏,超出可读性。)

看起来像 spec for computation expressions 改变了。是 let 绑定不再被视为语法糖,而是 让 计算工作流中不再需要成员?