Desugaring of for with pattern match and if guard

Question
1

Please explain how does the following for desugar and why the last variant does not compile, while the one before it does:

trait A {
  def a: Boolean
}
trait B extends A {
  def b: Boolean
}

def f() = {
  val s = Seq[A]()

  val as = for (case x: A <- s if x.a) yield x // this compiles, the result is Seq[A]
  val bs = for (case x: B <- s) yield x // this compiles, the result is Seq[B]
  for (case x: B <- s) yield x.b // this compiles
  for (case x: B <- s if x.b) yield x // this gives an error: Found B => B, Required A => B
}

The behavior is the same with 3.3.7 and 3.7.4, with or without -preview.

2

It’s explained as a bug.

It should have the same behavior as:

for case (x: B) <- s if x.b yield x

where the extra parens say, “I am a pattern.” More precisely, it signals, “I am refutable.”

Without case, the parens matter:

  for x: B <- s if x.b yield x // B => Boolean
  for (x: B) <- s if x.b yield x // pattern's type B is more specialized...

But I don’t think parens should matter with case, which always takes a refutable pattern and always filters.

I don’t see a unit test for “parenless case", so I assume it’s a bug.

4 Likes
3

OK. Reported as For comprehension desugaring different when pattern not enclosed in parentheses · Issue #24736 · scala/scala3 · GitHub - and thanks for the workaround.

4

in the meantime you can write (for (case x: B ← s) yield x).filter(_.b)

5

That builds the target collection twice; I would advertise the judicious use of parentheses in my reply.

It uses withFilter instead of filter.

1 Like