[英]F# nested generic types not compatible with implemented type
Background: 背景:
Given the following two declarations in an F# program: 在F#程序中给出以下两个声明:
A
implements the interface Wrapped<int>
A
实现接口Wrapped<int>
B
implements the interface Wrapped<A>
B
实现Wrapped<A>
接口 We say that type A
is compatible with Wrapped<int>
and type B
is compatible with Wrapped<A>
- compatible, to my understanding, meaning that an A
can be passed into a function requiring a Wrapped<int>
. 我们说类型
A
与Wrapped<int>
兼容,类型B
与Wrapped<A>
兼容 - 据我所知,这意味着A
可以传递给需要Wrapped<int>
的函数。
Problem: 问题:
From my experience with programming, I would expect the following to also be true, given the above two statements: 根据我的编程经验,鉴于以上两个陈述,我希望以下内容也是如此:
B
should be compatible with the type Wrapped<Wrapped<int>>
B
应与Wrapped<Wrapped<int>>
类型兼容 since B
has A
as the type parameter where Wrapped<int>
should go, and A
and Wrapped<int>
are compatible. 因为
B
有A
作为Wrapped<int>
应该去的类型参数, A
和Wrapped<int>
是兼容的。
This is not the case. 不是这种情况。 The following implementation:
以下实施:
type Wrapper<'a> = abstract member Value : 'a
type A =
| A of int
interface Wrapper<int> with member this.Value = (let (A num) = this in num)
type B =
| B of A
interface Wrapper<A> with member this.Value = (let (B a) = this in a)
let f (x:Wrapper<Wrapper<int>>) =
x.Value.Value
let testValue = f (B (A 1))
has a compile error on B (A 1)
stating 在
B (A 1)
上有一个编译错误
The type
B
is not compatible with the typeWrapper<Wrapper<int>>
类型
B
与Wrapper<Wrapper<int>>
类型不兼容
Question: 题:
Since I was able to logically make the compatibility jump, am I doing something wrong while implementing this? 由于我能够逻辑地进行兼容性跳转,我在实现这个时做错了吗? Or does F# not have this "nested compatibility" feature, and if that's the case, is there a particular reason for not having it?
或者F#没有这种“嵌套兼容性”功能,如果是这种情况,是否有特殊原因没有它?
There is a workaround to this: 有一个解决方法:
type B =
| B of A
interface Wrapper<Wrapper<int>> with member this.Value = (let (B a) = this in a :> Wrapper<int>)
That will remove the compile error, though it feels a little bit wrong. 这将删除编译错误,虽然感觉有点不对劲。 I ask myself "What if I ever write a function to work on
Wrapper<A>
types? (if I ever add more Wrapper<A>
implementers) 我问自己“如果我编写一个函数来处理
Wrapper<A>
类型怎么办?(如果我添加更多Wrapper<A>
实现者)
The feature you're asking for is covariant types. 您要求的功能是协变类型。
Covariance permits a return type which is a subtype rather than that exactly defined by the generic type parameter (not that this is applicable only to interfaces, not concrete types). 协方差允许返回类型,它是一个子类型,而不是泛型类型参数精确定义的类型(不是这仅适用于接口,而不适用于具体类型)。 This allows you to downcast
IEnumerable<string> :?> IEnumerable<object>
as string :?> object
. 这允许您将
IEnumerable<string> :?> IEnumerable<object>
向下转换为string :?> object
。
Declaration is possible in the other .NET languages. 可以在其他.NET语言中声明。 Here's your example in C#:
这是你在C#中的例子:
interface Wrapper<out T> { }
class A : Wrapper<int> { }
class B : Wrapper<A> { }
var b = new B();
Action<Wrapper<Wrapper<int>>> unwrap = _ => { };
unwrap(b); //compiles
F# does not provide support for declaring covariant types, nor does it coerce types without explicit declaration. F#不提供对声明协变类型的支持,也没有在没有显式声明的情况下强制类型。 The reason for this is mostly that covariance leads to degraded type inferencing.
其原因主要是协方差导致类型推理降级。
Covariance in F# is possible with flexible types . 使用灵活类型可以实现F#中的协方差。 Here's an example in F# on the
seq
type which is defined as IEnumerable<out T>
. 这是在
seq
类型的F#中的一个例子,它定义为IEnumerable<out T>
。
let s = [1..10]
let r = s |> Seq.map(fun _ -> s)
let print1 (v: seq<seq<int>>) = printfn "%A" v
let print2 (v: seq<#seq<_>>) = printfn "%A" v
print1 r //does not compile
print2 r //compiles
There is a chance you could make this work if the generic parameters were marked covariant and used flexible types. 如果通用参数标记为协变并使用灵活类型,则有可能使这项工作成功。 You could have the interface declarations in C# and reference the assembly in F#.
您可以在C#中使用接口声明并在F#中引用程序集。
There is also mausch/VariantInterfaces which modifies the assembly based on a naming convention to add covariant / contravariant declarations, so if you had your type declarations in a separate assembly, you could run it in post-build. 还有mausch / VariantInterfaces根据命名约定修改程序集以添加协变/逆变声明,因此如果你在单独的程序集中有你的类型声明,你可以在后期构建中运行它。
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