Simple question about Scala generics

Question

What is the difference between X[Any] and X[_] ?

Let's consider, for example, two functions below:

def foo(x:X[_]){}

def foo(x:X[Any]){}

What is exactly the difference between these declarations above?

Answer 1

The first is an existential type, and the second is a normal type. The first syntax actually means this:

def foo(x:X[t] forSome { type t }){}

What this means is that x is of type X[t], where t can be any unspecified type t.

Intuitively, X[_] means the type parameter of X is irrelevant, whereas X[Any] says it must be Any.