Easy idiomatic way to define Ordering for a simple case class

Question

I have a list of simple scala case class instances and I want to print them in predictable, lexicographical order using list.sorted, but receive "No implicit Ordering defined for ...".

Is there exist an implicit that provides lexicographical ordering for case classes?

Is there simple idiomatic way to mix-in lexicographical ordering into case class?

scala> case class A(tag:String, load:Int)
scala> val l = List(A("words",50),A("article",2),A("lines",7))

scala> l.sorted.foreach(println)
<console>:11: error: No implicit Ordering defined for A.
          l.sorted.foreach(println)
            ^

I am not happy with a 'hack':

scala> l.map(_.toString).sorted.foreach(println)
A(article,2)
A(lines,7)
A(words,50)

Answer 1

My personal favorite method is to make use of the provided implicit ordering for Tuples, as it is clear, concise, and correct:

case class A(tag: String, load: Int) extends Ordered[A] {
  // Required as of Scala 2.11 for reasons unknown - the companion to Ordered
  // should already be in implicit scope
  import scala.math.Ordered.orderingToOrdered

  def compare(that: A): Int = (this.tag, this.load) compare (that.tag, that.load)
}

This works because the companion to Ordered defines an implicit conversion from Ordering[T] to Ordered[T] which is in scope for any class implementing Ordered. The existence of implicit Orderings for Tuples enables a conversion from TupleN[...] to Ordered[TupleN[...]] provided an implicit Ordering[TN] exists for all elements T1, ..., TN of the tuple, which should always be the case because it makes no sense to sort on a data type with no Ordering.

The implicit ordering for Tuples is your go-to for any sorting scenario involving a composite sort key:

as.sortBy(a => (a.tag, a.load))

As this answer has proven popular I would like to expand on it, noting that a solution resembling the following could under some circumstances be considered enterprise-grade™:

case class Employee(id: Int, firstName: String, lastName: String)

object Employee {
  // Note that because `Ordering[A]` is not contravariant, the declaration
  // must be type-parametrized in the event that you want the implicit
  // ordering to apply to subclasses of `Employee`.
  implicit def orderingByName[A <: Employee]: Ordering[A] =
    Ordering.by(e => (e.lastName, e.firstName))

  val orderingById: Ordering[Employee] = Ordering.by(e => e.id)
}

Given es: SeqLike[Employee], es.sorted() will sort by name, and es.sorted(Employee.orderingById) will sort by id. This has a few benefits:

• The sorts are defined in a single location as visible code artifacts. This is useful if you have complex sorts on many fields.
• Most sorting functionality implemented in the scala library operates using instances of Ordering, so providing an ordering directly eliminates an implicit conversion in most cases.