Scala 类型的上限和下限 [英] Upper and Lower bound on scala type

问题描述

考虑以下层次结构:

class C1
class C2 extends C1
class C3 extends C2
class C4 extends C3

我想编写一个只接受 C2 和 C3 类型的函数.为此，我想到了以下几点:

I want to write a function that just accepts types C2 and C3. For that I thought of the following:

 def f [C >: C3 <: C2](c :C) = 0

我希望有以下行为

f(new C1)  //doesn't compile, ok
f(new C2)  //compiles, ok
f(new C3)  //compiles, ok
f(new C4)  // !!! Compiles, and it shouldn't 

问题是用 C4 调用它时，我不想允许，但编译器接受.我知道 C4 <: C2 是正确的，C4 可以看作是 C3.但是当指定绑定 [C >: C3 <: C2] 时，我希望编译器找到一个同时尊重两个边界的 C，而不是逐个.

The problem is when calling it with C4, which I don't want to allow, but the compiler accepts. I understand that C4 <: C2 is correct and that C4 can be seen as a C3. But when specifying the bound [C >: C3 <: C2], I would expect the compiler to find a C that respects both bounds at the same time, not one by one.

问题是:有什么方法可以实现我想要的，如果没有，编译器是否试图避免与此不一致?

Question is : Is there any way to achieve what I want, and if not, is the compiler trying to avoid some inconsistency with this?

编辑:从答案中我意识到我的假设是错误的.C4 总是满足 C >: C3，所以这两个界限确实得到尊重.我的用例的方法是 C3 <:<C.

Edit: from the answers I realized that my presumption is wrong. C4 always fulfills C >: C3, so both bounds are indeed respected. The way to go for my use case is C3 <:< C.

推荐答案

静态上，是的.施加此约束非常简单:

Statically, yes. It's pretty simple to impose this constraint:

def f[C <: C2](c: C)(implicit ev: C3 <:< C) = 0

f(new C4) 现在无法编译.

问题是，可能无法在编译时禁止以下行为:

The problem is, it's probably not possible to prohibit the following behaviour at compile time:

val c: C3 = new C4
f(c)

这里的变量c有静态类型C3，它通过编译器的任何类型的检查，但它在运行时实际上是一个C4.

Here variable c has static type C3, which passes any kind of typechecking by compiler, but it is actually a C4 at runtime.

在运行时，您当然可以使用反射或多态检查类型并抛出错误或返回 Failure(...) 或 None

At runtime you can of course check the type using reflection or polymorphism and throw errors or return Failure(...) or None

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