Question

Given,

case class User(name: String, roles: List[String])
val users: List[User] = ...

I’d like to calculate a map —

val roleToUsers: Map[String, List[User]] = ???

I could do the concise:

(for (user <- users; role <- user.roles) yield (role, user)).groupBy(_._1).mapValues(_.map(_._2))

But the underscores make it a bit too cryptic for my liking. Is there a neater way to do this that doesn’t make it much more verbose?

Edit: List[Role] -> List[User]

Answer 1

Maybe this:

val lst = for {
  user <- users
  role <- user.roles
} yield (role, users collect {
  case user if user.roles contains role => user.name
})
val map = lst.toMap

Or, without for-comprehensions and with a minor optimization

users.flatMap(_.roles).distinct.map(role => 
  (role, users collect { 
    case user if user.roles contains role => user.name })).toMap