confusing doc page on option-less pattern matching - strange implicit in unapply example

Problem

In this document page: https://github.com/lampepfl/dotty/blob/main/docs/_docs/reference/changed-features/pattern-matching.md

The initial definition of what an extractor is says that it is a an object having a special unapply looking like this:

def unapply[A](x: T)(implicit x: B): U
def unapplySeq[A](x: T)(implicit x: B): U

Why is there an (implicit x: B) ? And what is U?

The doc page never explains it and I can’t find any place in the examples further down where the mystic implicit of type B is used. And if it must be there, should this not be a using as this is Scala 3?

Proposal

If the implicit is there just to illustrate that there can be an implicit then I think it is better to remove it and just state in the text that a using-parameter is allowed.

[bjornregnell]

Aha, so the implicit param is there just to show that it can be there - but what is it good for? And what is the type B? And where is the type T? (I guess it should be A?) And I still think it should be a given if it must be there? And why x two times - that seems like a compile error (“x is already defined as parameter x”)

I think it would be better to start off with:

def unapply[A](x: A): U
def unapplySeq[A](x: A): U

As the above is the normal thing and later then say that a using-part is also possible if needed (but needed for what? And what is B? And we need another name than x for that second mystic using.

def unapply[A](x: A)(using y: B): U
def unapplySeq[A](x: A)(using y: B): U

“The different kinds of extractors are characterized by the result type U.”

Thanks! This explanation helps a lot to grok why U is there and now the following text makes much more sense. But maybe C instead of U to follow the A B C pattern of type names.

[som-snytt]

I’d suggest “the most general form is”, and note that the type param and the second parameter section are optional.

Also, “The different kinds of extractors are characterized by the result type U.”