Question

我有一个实现IEnumerable<T>接口的类型，一切正常：

open System

type Bar() =

     interface Collections.IEnumerable with
         member x.GetEnumerator () = null

     interface Collections.Generic.IEnumerable<int> with
         member x.GetEnumerator () = null

但是如果类型通过基类型继承IEnumerable接口实现，则会出错：

open System

type Foo() =
     interface Collections.IEnumerable with
         member x.GetEnumerator () = null

type Bar() =
     inherit Foo()

     interface Collections.Generic.IEnumerable<int> with
         member x.GetEnumerator () = null

上面的代码会产生类型推断错误：

The member 'GetEnumerator<'a0 when 'a0 : null> : unit -> 'a0 when 'a0 : null' does not have the correct type to override any given virtual method
The member 'GetEnumerator<'a0 when 'a0 : null> : unit -> 'a0 when 'a0 : null' does not have the correct number of method type parameters. The required signature is 'GetEnumerator : unit -> Collections.Generic.IEnumerator<int>'.

我做错了什么或者这是一个F＃编译错误？

Microsoft (R) F# 2.0 Interactive build 4.0.30319.1

更新更多规范示例：

type IFoo     = abstract Bar : obj list
type IFoo<'a> = abstract Bar : 'a  list
                inherit IFoo
/* ok */
type Foo        = interface IFoo      with member x.Bar = []
                  interface IFoo<Foo> with member x.Bar = []
/* fail */
type FooBase    = interface IFoo      with member x.Bar = []
type FooDerived = interface IFoo<Foo> with member x.Bar = [] // <---
                  inherit FooBase
/*
 error FS0017: The member 'get_Bar : unit -> 'a list' does not
    have the correct type to override any given virtual method.
*/

Answer 1

编译器无法从“null”实现中推断出正确的类型。尝试

open System

type Foo() =
     interface Collections.IEnumerable with
         member x.GetEnumerator () = null

type Bar() =
     inherit Foo()

     interface Collections.Generic.IEnumerable<int> with
         member x.GetEnumerator () : Collections.Generic.IEnumerator<int> = null

更新：
原因是GetEnumerator类型实现的Bar方法的类型是不明确的，因为IEnumerable<'a>实现/继承了非泛型IEnumerable，它也指定了一个（非） -generic）GetEnumerator方法。那么，编译器应该如何推断出，如果他获得的所有方法都是null，那么您正在尝试实现哪种方法？因此，在这种情况下我们需要一个类型注释。

Answer 2

这不是一个错误，这只是一个类型推断失败，因为F＃可能在派生接口实现声明中实现继承的接口成员：

type IA = abstract A : int
type IB = inherit IA
type IC = inherit IB

type Baz =
     interface IC with
       member x.A = 1

因此，在我的示例中，我应该明确指定正确的返回类型，因为派生member x.GetEnumerator()类型中的Bar可能与IEnumerable.GetEnumerator()和IEnumerable<T>.GetEnumerator()都匹配。

这是一个F＃错误吗？

2 个答案: