字典<'K，'V>的类型扩展错误 [英] Type extension errors for Dictionary<'K, 'V>

问题描述

以下类型的扩展名

module Dict =

  open System.Collections.Generic

  type Dictionary<'K, 'V> with
    member this.Difference(that:Dictionary<'K, 'T>) =
      let dict = Dictionary()
      for KeyValue(k, v) in this do
        if not (that.ContainsKey(k)) then
          dict.Add(k, v)
      dict

给出错误：

签名和实现不兼容，因为类型参数TKey的声明需要格式为TKey：相等性的限制条件

The signature and implementation are not compatible because the declaration of the type parameter 'TKey' requires a constraint of the form 'TKey : equality

但是当我添加约束时会给出错误：

But when I add the constraint it gives the error:

此类型扩展名的声明类型参数与原始类型'Dictionary< ，>'

这是非常神秘的，因为以下类型的扩展名没有约束和作用。

This is especially mysterious because the following type extension doesn't have the constraint and works.

type Dictionary<'K, 'V> with
  member this.TryGet(key) =
    match this.TryGetValue(key) with
    | true, v -> Some v
    | _ -> None

现在我有奇怪的想法：只有在访问某些成员时，才需要约束？ / p>

Now I'm having weird thoughts: is the constraint required only when certain members are accessed?

推荐答案

module Dict =

  open System.Collections.Generic

  type Dictionary<'K, 'V> with
    member this.Difference(that:Dictionary<'K, 'T>) =
        let dict = Dictionary(this.Comparer)
        for KeyValue(k, v) in this do
            if not (that.ContainsKey(k)) then
                dict.Add(k, v)
        dict

编辑：

根据 F＃spec（14.11 CLI方法的附加限制）

一些特定的CLI方法和类型由F＃特别处理，因为它们在F＃编程中很常见，导致非常难以发现的错误。对于以下结构的每次使用，F＃编译器都会施加额外的临时约束：

Some specific CLI methods and types are treated specially by F#, because they are common in F# programming and cause extremely difficult-to-find bugs. For each use of the following constructs, the F# compiler imposes additional ad hoc constraints:

• x.Equals（yobj ）要求静态类型 x
$的类型 ty：等于 b $ b
• x.GetHashCode（）需要类型 ty：等于为静态类型 x


• 新词典< A，B>（）需要code> A：等于，因为任何不占用code的重载IEqualityComparer< T>

• x.Equals(yobj) requires type ty : equality for the static type of x
• x.GetHashCode() requires type ty : equality for the static type of x
• new Dictionary<A,B>() requires A : equality, for any overload that does not take an IEqualityComparer<T>

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