f# - 这是 F# 错误吗？

Question

我有一个正在实现 IEnumerable<T> 的类型界面，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

但是如果类型继承 IEnumerable 就会出错通过基类型实现接口(interface):

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>'.

是我做错了什么还是 F# 编译器错误？

Microsoft (R) F# 2.0 Interactive build 4.0.30319.1

---

更新更规范的例子:

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

编译器无法从您的“空”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 的类型Bar 实现的方法类型不明确为 IEnumerable<'a>实现/继承非泛型 IEnumerable它还指定了一个(非通用)GetEnumerator方法。那么，编译器应该如何推断，如果他得到的都是空值，你到底在尝试实现哪个方法？因此在这种情况下我们需要一个类型注释。