Add Future.discard extension method

[isaacl]

This method can be used to mark Future values that we're explicitly ignoring, to enable better unused value detection.

[isaacl]

@lenguyenthanh ^ for adding unused checks to lila

[ornicar]

scalalib already provides future.void and its type is Future[Unit] https://github.com/lichess-org/scalalib/blob/master/lila/src/main/scala/future.scala#L33

[isaacl]

Ah, I didn't notice this. But I'm not sure this void impl helps with unused object false positives, because the type is still non-unit and there's places in lichess where we 'fire and forget' Futures and scala thinks we're discarding data accidentally.

@lenguyenthanh would be able to comment more. I saw it mentioned in zulip but I can't find the thread anymore.

[ornicar]

extension [A](a: A) def discard: Unit = ()

would allow discarding any value and returning unit by adding .discard. As an alternative to ; ().

[lenguyenthanh]

this is the thread that @isaacl mentioned: https://hq.lichess.ovh/#narrow/stream/8-dev/topic/scala.20detection.20of.20dangling.20code

I think discard (or maybe ignored, effectful) is a better name for this function if We want to go this way. Which I believe is a way to avoid the problem instead of solving problem of dangling code. But I'm biased about purity.

[isaacl]

@lenguyenthanh Would discard or similar still be useful as an intermediate step to allow us to quickly enable unused detection? I'm not familiar enough with all the places we're ignore futures to tell how easy it would be to fix these. What do you think?

[lenguyenthanh]

I'm not familiar enough with all the places we're ignore futures to tell how easy it would be to fix these. What do you think?

It's quite ubiquitous in our code base, my gut feelings is about few hundreds. tbh, I'm not sure 🤷 . Maybe it's worth trying.

If We try, we can just add this extension function to lila and apply zero waste plugin and see how it go.

Example in lila-search: https://github.com/ghik/zerowaste (which is 100% pure I don't have to do any massaging)

[isaacl]

OK, I'll adjust this PR to add .discard. Although the paradigm of <expr producing Future>; () might also trick zerowaste, it wouldn't be as findable for later removal as explicitly calling a .discard method.