Question

I’ve occasionally found myself wanting a very simple Ordering[A] for a class which does not have an ordering yet. Usually the projection of a tuple or a field of a class. The following class seems to do this generically

case object Ord { 
  case class DerivedOrdering[A,B](fn : B=>A)(implicit o : Ordering[A]) extends Ordering[B] { 
      def compare(a:B, b:B) = o.compare(fn(a), fn(b)) 
  }
}

I haven’t seen it in the scala library. Is it there somewhere?

Usage

Ord.DerivedOrdering[Int, (Int, List[Int])]( _._1)

Gives an ordering on (Int, List[Int]) ordering by the integer.

Answer 1

There is something you can do which is a method in the standard library

def by[T, S](f: T => S)(implicit ord: Ordering[S]): Ordering[T]

for example you can do the following:

implicit val tupleOrdering = Ordering.by[(Int, List[Int]), Int] {
  _._1
}

Reuben