Document using parameters on classes can provide given instances

[bishabosha]

We should probably document that using clauses with parameters annotated val/var generate a given instance in their body (this is obvious for Scala 2 users, but maybe not for someone who has only ever seen the new syntax, where using may have been understood as only a "consumer" of given instances)

Minimized example

class GivenIntBox(using val givenInt: Int)

@main def test =
  val b = GivenIntBox(using 23)

  import b.given

  println(summon[Int])

Output

23

[b-studios]

But this is also the case with contextual parameters on methods, and with contextual functions right? They also introduce a "given" in the corresponding scope. Retract...you are right .. it should be explained.

[amitport]

BUT, why does it generate a given instance? It seems completely surprising behavior to me. It would be much better to have an explicit declaration than to allow this, IMHO.

[som-snytt]

I donated some explanatory words which might be approximately correct and about in the right spot.

I remember there was a use case for export this.foo as bar, and extending that, export this.member as given T or something. But that is idle speculation.

I don't know that this is obvious for Scala 2 users. There are other gotchas around class parameters and aliasing, such as accidental shadowing, so maybe it's not completely surprising that some behaviors are completely surprising.

[amitport]

"it's not completely surprising that some behaviors are completely surprising" :) yes, one gets used to it with scala. Still, it's not the best quality of the language.