(very breaking) change in Future instances?

[yurique]

The following code works differently with cats 2.10.0 and cats 2.11.0+

def test(i: Int): Future[Unit] = 
  for {
    _ <- Future { println(s"start $i") } 
    _ <- futureSleep(100.millis) // just to avoid any timing issues, reproduces without it as well
    _ <- Future { println(s"end $i") } 
  } yield ()

List(1, 2, 3).traverse(test)

With 2.10.0 it prints (https://scastie.scala-lang.org/RYQcjXExR4m0NQQBHVo25w):

start 1
end 1
start 2
end 2
start 3
end 3

With 2.11.0+ (https://scastie.scala-lang.org/F21nFlI1Tl2bYm271o50ng):

start 1
start 2
start 3
end 1
end 2
end 3

[mpilquist]

Note the 2.11 behavior matches that of Future.traverse (https://scastie.scala-lang.org/aU1EoSBnQu2rbWiXtEbcsQ)

[yurique]

Right, it does 🤔.
But it doesn't match the traverse with IO: https://scastie.scala-lang.org/1JfGkbQOSTicrUyPaERQhw

Regardless, it still looks like a very breaking change.

[armanbilge]

it still looks like a very breaking change

looks like, but is not. See previous discussion:

• Changes in Future traverse behavior between 2.6 and 2.7 #4176
• Recommend Cats Effect IO as a replacement for every use case of Future #4230

[johnynek]

The problem is Future, and hence this function test: Int => Future[Unit] is not pure.

When you call test a side effect immediately starts. That is not true of the IO version. When you are working with Future you are working with an impure type and we don't limit the traverse implementation of data types to try to work that.

A PR was created where a lot of energy was spent to optimize traverse implementations: #4498 -- I think it is very likely the change you are seeing is due to that.

I guess what is happening is that since Future has a StackSafeMonad, it is traversing directly (which is what the standard library does), but that is triggering the side-effects in a different order.

[yurique]

looks like, but is not

It depends on how you look at it (or how much code you need to fix) 😆.

---

Thanks for chiming in, everyone! I'll close this one and will go hunting down my traverses :)