Why doesn't "foldLeft" work with class members in Scala?-CodePudding

I am trying to remove classes with identical fields from a collection using foldLeft function in Scala.

For example, a Box class is defined as: class Box(color: String = "", size: Int = 0)

Let's create a simple collection Seq(Box, Box, Box) and try to filter it by the size field.

val s = Seq(Box("green", 1), Box("blue", 1), Box("red", 3))

s.foldLeft(Seq(s.head)){ (boxes, nextBox) =>
  if (boxes.last.size != nextBox.size) { // throws an exception
    boxes :  nextBox
  } else {
    boxes
  }
}

The above code throws an exception: value size cannot be accessed as a member of Box. The above code works fine with primitive types and for Seq(1, 1, 3) will produce 1, 3. It doesn't help if you explicitly tell the compiler (boxes: Seq[Box], nextBox: Box), it doesn't help even if you cast the value returned by last to Box.

Can you please explain why?

CodePudding user response：

The above code throws an exception: value size cannot be accessed as a member of Box.

Because Box doesn't have any public field called size; it has a constructor parameter called size which is different.
You can fix that doing class Box(color: String = "", val size: Int = 0) to make the constructor argument a field; or by using a case class that does that for you (and much more).

PS: It doesn't throw an exception, it fails with a compiler error; two very different things.

Also, what you are trying to do is just distinctBy; as I always say the Scaladoc is your friend.

final case class Box(color: String = "", size: Int = 0)

val result = boxes.distinctBy(_.size)

Since you seem very new to the language and with many basic errors / misconceptions I would advise you to pick some course or read a book about the language. Also, I would encourage you to join the Scala Discord server where you may ask questions and get more interactive help.

CodePudding user response：

Some code must be missing from your description, since you can't call Box(...) if Box is just a class. You could if it were a case class and then size would be accessible too, since a case class exposes all it's constructor arguments as members, whereas a standard class does not unless explicitly annotated with val:

scala> case class Box(color: String = "", size: Int = 0)
class Box

scala> val s = Seq(Box("green", 1), Box("green", 1), Box("green", 3))
val s: Seq[Box] = List(Box(green,1), Box(green,1), Box(green,3))

scala> s.foldLeft(Seq(s.head)){ (boxes, nextBox) =>
     |   if (boxes.last.size != nextBox.size) {
     |     boxes :  nextBox
     |   } else {
     |     boxes
     |   }
     | }
val res0: Seq[Box] = List(Box(green,1), Box(green,3))

Alternatively (notice the new of val in the class constructor and new to instantiate them):

scala> class Box(val color: String = "", val size: Int = 0)
class Box

scala> val s = Seq(new Box("green", 1), new Box("green", 1), new Box("green", 3))
val s: Seq[Box] = List(Box@159424e2, Box@29bcf51d, Box@1e54a6b1)

scala> s.foldLeft(Seq(s.head)){ (boxes, nextBox) =>
     |   if (boxes.last.size != nextBox.size) {
     |     boxes :  nextBox
     |   } else {
     |     boxes
     |   }
     | }
val res0: Seq[Box] = List(Box@159424e2, Box@1e54a6b1)