Question

这是this question的后续问题。

我试图创建一个计算表达式构建器，它通过自定义操作累积值，同时也支持标准的F＃语言构造。为了有一个简单的例子来讨论，我使用了一个构建F＃列表的计算表达式。感谢kvb和Daniel先生提出的建议，但仍然遇到for循环问题。

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

此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，我仍然在这个问题上遇到编译器错误：

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

在这种情况下，IDE正在for x in中强调x并告诉我，＆＃34;此表达式应该具有类型单位，但这里有类型int。＆＃34;

我不清楚为什么它期望循环变量属于单位类型。显然我在某个地方得到了错误的方法签名，我怀疑我不会在我应该的每个地方都经过我的累积状态，但编译错误实际上并没有帮助我缩小我出错的地方。任何建议将不胜感激。

Answer 1

直接原因是您的While函数限制了body的类型。但是，一般情况下，您不能同时使用自定义操作，也不能在同一计算表达式中控制流操作符，因此即使您修复了签名，我也不认为您能够完全按照自己的意愿行事。

为什么这个计算表达式构建器期望＆＃34; unit＆＃34;在我的for循环？

1 个答案: