Difference between F[_] and F[T] In Scala when used in type constructors

Question

This question is about _ as used in type constructor and not when used in defining existential types.

So the question is what is the difference when _ is us

Answer 1

To quote the specification:

The above scoping restrictions are generalized to the case of nested type parameter clauses, which declare higher-order type parameters. Higher-order type parameters (the type parameters of a type parameter t) are only visible in their immediately surrounding parameter clause (possibly including clauses at a deeper nesting level) and in the bounds of t. Therefore, their names must only be pairwise different from the names of other visible parameters. Since the names of higher-order type parameters are thus often irrelevant, they may be denoted with a _, which is nowhere visible.

Example

Here are some well-formed type parameter clauses:

[S, T]
[@specialized T, U]
[Ex <: Throwable]
[A <: Comparable[B], B <: A]
[A, B >: A, C >: A <: B]
[M[X], N[X]]
[M[_], N[_]] // equivalent to previous clause
[M[X <: Bound[X]], Bound[_]]
[M[+X] <: Iterable[X]]

So if you have no bounds, as in Functor [F[T]], there's no difference at all from Functor [F[_]].