Scala 中方法类型参数化中的结构类型? [英] Structural type in method type parameterization in Scala?

问题描述

考虑以下 Scala 代码(例如，在 REPL 中)

Consider the following Scala code (e.g., in REPL)

object A{def foo:Unit = {}}
object B{def foo:Unit = {}}

def bar[T <: Any {def foo: Unit}](param: T*):Unit = param.foreach(x => x.foo)

bar(A, A)  // works fine
bar(B, B)  // works fine
bar(A, B)  // gives error

前两个工作正常.第三个报错:

The first two work fine. The third ones gives an error:

错误:推断的类型参数 [ScalaObject] 不符合方法栏的类型参数边界 [T <: Any{def foo: Unit}]

有什么方法可以做我想做的事吗?

Are there any ways to do what I want?

推荐答案

这通常称为结构类型，而不是鸭子类型.我编辑了你的标题.:)

This is usually called structural typing, not duck typing. I edited your title. :)

我认为您的问题是由定义类型参数 T 然后以不变方式使用它引起的.T 只能引用一种具体类型，但是你有不同类型的参数A 和B.

I think that your problem is caused by defining the type parameter T and then using it in an invariant way. T can only refer to one concrete type, but you have parameters of different types A and B.

这有效:

 def bar(param: {def foo: Unit}*) = param.foreach(x => x.foo)

使用类型别名也有效:

 type T = {def foo: Unit}
 def bar(param: T*) = param.foreach(x => x.foo)

这是可行的，因为编译器将简单地替换结构类型以代替其别名 T.替换后，这个例子和上面的完全一样.

This works because the compiler will simply substitute the structural type in place of its alias, T. After the substitution, this example is exactly the same as the one above.

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