是否可以使用任意类型参数定义返回类型？

Question

Assume, we have aa class with an type parameter

class A[T]

And we want to write a method, that returns objects of the type A with an arbritrary type parameter, like:

def f: A = { ... }

The compiler will complain about the missing type parameter for type A .

We can not solve this problem by writing A[Any] , since eg A[String] is no subtype of A[Any] . But we can reach this subtype relation with a covariant annotation of +T .

Is it possible write such a method f without using covariant annotation +T ?

Answer 1

You can use existential types:

def f: A[_] = { ... }

This is shorthand for:

def f: A[T forSome { type T }] = { ... }

You can even use upper (or lower) bounds:

def f: A[_ <: SomeType] = { ... }

You can assign any type T to this, however, you may not be able to do anything useful with the result