"Left Join" like production of two collections in for-yield-CodePudding

I have two collections - spells and charms.
charms might be empty

I need to do Magic of the combinations of spells and charms and if charms is empty I need Magic of spells alone

For non-empty charms this works:

def hocusPocus(spells: Seq[String], charms: Seq[String]): Seq[Magic] = {
  for {
    spell <- spells
    charm <- charms
  } yield {
    new Magic(spell, Some(charm))
  }
}

This snippet is simple and clean, but for empty charms it makes no Magic

Is there any possibility to do some Magic from spells only in such a clear way, if one is out of charms?

CodePudding user response：

This is one of those cases where it is easier to use flatMap map directly instead of using for (IMHO)
Also, it is a good use case to prefer a concrete collection like List over Seq

def hocusPocus(spells: List[String], charms: List[String]): List[Magic] =
  spells.flatMap { spell =>
    Magic(spell, None) :: charms.map(charm => Magic(spell, Some(charm)))
  }

Now, if you only need the Magic of spells alone if and only if charms is empty, you can do this:

def hocusPocus(spells: Seq[String], charms: Seq[String]): Seq[Magic] =
  if (charms.isEmpty)
    spells.map(spell => Magic(spell, None))
  else
    spells.flatMap { spell =>
      charms.map { charm =>
        Magic(spell, Some(charm))
      }
    }

CodePudding user response：

If it's possible to change type of Magic.charm to java.io.Serializable

you can write:

def hocusPocus(spells: Seq[String], charms: Seq[String]): Seq[Magic] = {
    for {
      spell <- spells
      charm <- if (charms.isEmpty) List(Option.empty) else charms
    } yield {
      new Magic(spell, charm)
    }
  }