Scala-类型参数的超类型

Question

In Scala, how can I do something like this:

def cast [Type] (x: _ >: Type, errMsg: String): Type = {
    if (x.isInstanceOf[Type]) {
        x.asInstanceOf[Type]
    } else {
        throw new Exception(errMsg) 
    }
}

x: _ >: Type (a notation that does not exist in Scala) meaning "the type of x is any supertype of Type".

Answer 1

If x can be of any superType of Type , then certainly it can be Any . This is no constraint at all, you can just write x : Any

On another line, due to type erasure, you x.isInstanceOf[Type] will do no useful check. You cannot check on a type parameter. (You have to ensure that the type information will be available at runtime. You can get to something with Manifest ).

Answer 2

Try this:

def cast[T >: Type](x: T, errMsg: String): T = { ... }

However, you might want to reconsider the necessity of explicit typecasts in Scala.