为什么这个计算表达式生成器在我的 for 循环中需要“单位”？

这是一个后续问题这个问题 https://stackoverflow.com/questions/23122639/how-do-i-write-a-computation-expression-builder-that-accumulates-a-value-and-als.

我正在尝试创建一个计算表达式生成器，它通过自定义操作累积值，同时还支持标准 F# 语言构造。为了提供一个简单的示例来讨论，我使用了构建 F# 列表的计算表达式。感谢 kvb 和 Daniel 的建议，我取得了进一步的进展，但仍然遇到问题for loops.

建设者：

type Items<'a> = Items of 'a list

type ListBuilder() =
    member x.Yield(vars) = Items [], vars
    member x.Run(l,_) = l
    member x.Zero() = Items [], ()
    member x.Delay f = f()
    member x.ReturnFrom f = f

    member x.Combine((Items curLeft, _), (Items curRight, vars)) =
        (Items (curLeft @ curRight), vars)

    member x.Bind(m: Items<'a> * 'v, f: 'v -> Items<'a> * 'o) : Items<'a> * 'o =
        let (Items current, vals) = m
        x.Combine(m, f vals)

    member x.While(guard, body) =
        if not (guard()) then
            x.Zero()
        else
            x.Bind(body, fun () -> x.While(guard, body))

    member x.TryWith(body, handler) =
        try
            x.ReturnFrom(body())
        with e ->
            handler e

    member x.TryFinally(body, compensation) =
        try
            x.ReturnFrom(body())
        finally
            compensation()

    member x.Using(disposable:#System.IDisposable, body) =
        let body' = fun() -> body disposable
        x.TryFinally(body', fun () ->
            match disposable with
            | null -> ()
            | disp -> disp.Dispose())

    member x.For(xs:seq<'a>, body) =
        x.Using(xs.GetEnumerator(), fun enum ->
            x.While(enum.MoveNext, x.Delay(fun () -> body enum.Current)))

    [<CustomOperation("add", MaintainsVariableSpace=true)>]
    member x.Add((Items current, vars), [<ProjectionParameter>] f) =
        Items (current @ [f vars]), vars

    [<CustomOperation("addMany", MaintainsVariableSpace=true)>]
    member x.AddMany((Items current, vars), [<ProjectionParameter>] f) =
        Items (current @ f vars), vars


let listBuilder = ListBuilder()

let build (Items items) = items

这个版本允许我以前无法做的事情，例如：

let stuff = 
    listBuilder {
        let x = 5 * 47
        printfn "hey"
        add x
        addMany [x .. x + 10]
    } |> build

但是，我仍然收到此编译器错误：

let stuff2 =
    listBuilder {
        for x in 1 .. 50 do
            add x            
    } |> build

在这种情况下，IDE 会在 x 下划线for x in并告诉我，“这个表达式应该有类型unit，但这里有类型int。”

我不太清楚为什么它期望循环变量是单位类型。显然，我在某个地方得到了错误的方法签名，并且我怀疑我没有在应该经过的每个地方都经过我的累积状态，但是编译器错误实际上并不能帮助我缩小出错的范围。任何建议，将不胜感激。

直接原因是你的While函数限制类型body。但是，一般来说，您不能在同一计算表达式中同时使用自定义操作和控制流运算符，因此我认为即使您修复了签名，您也无法完全执行您想要的操作。