如何修改我的Queue类以允许用户在F＃中创建未指定类型的空队列？ [英] How should I modify my Queue class to allow users to create empty queues of unspecified type in F#?

问题描述

我在F＃中创建了一个不可变的队列，如下所示：

I have created an immutable Queue in F# as follows:

type Queue<'a>(f : 'a list, r : 'a list) =    
    let check = function
        | [], r -> Queue(List.rev r, [])
        | f, r -> Queue(f, r)

    member this.hd =
        match f with
        | [] -> failwith "empty"
        | hd :: tl -> hd

    member this.tl =
        match f, r with
        | [], _ -> failwith "empty"
        | hd::f, r -> check(f, r)

    member this.add(x) = check(f, x::r)

    static member empty : Queue<'a> = Queue([], [])

我想创建一个空的 Queue ，但是我得到一个有价值的限制例外：

I want to create an instance of an empty Queue, however I get a value-restriction exception:

> let test = Queue.empty;;

  let test = Queue.empty;;
  ----^^^^

C： \Documents and Settings\juliet\Local Settings\Temp\stdin（5,5）：错误FS0030：
值限制。值'test'被推断为具有通用类型
val test：Queue<'_ a>
将test定义为一个简单的数据项，使其成为一个显式的
参数的函数或者，如果您不打算将其设为通用的，请添加类型注释。

C:\Documents and Settings\juliet\Local Settings\Temp\stdin(5,5): error FS0030: Value restriction. The value 'test' has been inferred to have generic type val test : Queue<'_a> Either define 'test' as a simple data term, make it a function with explicit arguments or, if you do not intend for it to be generic, add a type annotation.

基本上，我想要相同的功能在 Set 模块中看到，它允许我写：

Basically, I want the same kind of functionality seen in the Set module which allows me to write:

> let test = Set.empty;;

val test : Set<'a>

如何修改我的队列类允许用户创建空队列？

How can I modify my Queue class to allow users to create empty queues?

推荐答案

您需要使用GeneralizableValueAttribute，一个：

You need to use GeneralizableValueAttribute, a la:

type Queue<'a>(f : 'a list, r : 'a list) =  // '
    let check = function
        | [], r -> Queue(List.rev r, [])
        | f, r -> Queue(f, r)

    member this.hd =
        match f with
        | [] -> failwith "empty"
        | hd :: tl -> hd

    member this.tl =
        match f, r with
        | [], _ -> failwith "empty"
        | hd::f, r -> check(f, r)

    member this.add(x) = check(f, x::r)
module Queue =    
    [<GeneralizableValue>]
    let empty<'T> : Queue<'T> = Queue<'T>([], []) // '

let test = Queue.empty
let x = test.add(1)       // x is Queue<int>
let y = test.add("two")   // y is Queue<string>

您可以在语言规格。

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