Problem with understtanding Matchable trait in Scala 3

Question

When I try to do this in Scala 3.2.0:

  // CellTower from Google Maps API class unfortunately has no equals() 
  // implementation, but a toString(), so we use this for equality
  def isEqualTo(cellTowers: Seq[CellTower], others: Seq[CellTower]): Boolean =
    cellTowers.zip(others).fold(true)((c1, c2) => c1.toString == c2.toString)

I get:

Found:    Matchable
Required: Boolean
    cellTowers.zip(others).fold(true)((c1, c2) => c1.toString == c2.toString)

I read https://docs.scala-lang.org/scala3/reference/other-new-features/matchable.html, but I still don't understand the problem (and cannot create a solution)

Answer 1

It seems that you probably wanted to fold all the booleans with &&, but somehow forgot the actual folding operation:

def isEqualTo(cellTowers: Seq[CellTower], others: Seq[CellTower]): Boolean =
    cellTowers.zip(others).foldLeft(true){ case (b, (c1, c2)) => b && (c1.toString == c2.toString) }

This, however, would be much more easily expressed with forall:

def isEqualTo(cellTowers: Seq[CellTower], others: Seq[CellTower]): Boolean =
    cellTowers.zip(others).forall((c1, c2) => c1.toString == c2.toString)

(and it would also be faster, because it can return early in case it finds a false).

This still doesn't help you with sequences of different length though, so you probably should compare the size first.