如何使scala类型更高的类型起作用

Question

I'm trying to improve my understanding of higher-kinded type in Scala. While I went back to the basic, i got stuck on the following:

 def fooList[A <: Int](x: List[A]): List[A] = x.map{ e => e + 1 }

The Scala compiler does not accept it which, i do not understand. "A" must be a subclass of Int. hence whatever type under Int that shall be passed there should work. Why does it complain? Can someone advise me here ?

M

Answer 1

A really must be an Int in this case, because you can't create a sub-class of Int , but the compiler doesn't seem to want to prove that. That aside, since A <: Int , the + method of Int returns an Int .

So e + 1 is an Int and not an A (even if A must be an Int , anyway). Therefore, x.map(e => e + 1) returns a List[Int] and not a List[A] . In order to return List[A] , you need some class A with a + method that also returns A , which you don't.