Question

Say I have a list of names.

case class Name(val first: String, val last: String)

val names = Name("c", "B") :: Name("b", "a") :: Name("a", "B") :: Nil

If I now want to sort that list by last name (and if that is not enough, by first name), it is easily done.

names.sortBy(n => (n.last, n.first))
// List[Name] = List(Name(a,B), Name(c,B), Name(b,a))

But what, if I‘d like to sort this list based on some other collation for strings?

Unfortunately, the following does not work:

val o  = new Ordering[String]{ def compare(x: String, y: String) = collator.compare(x, y) }
names.sortBy(n => (n.last, n.first))(o)
// error: type mismatch;
// found   : java.lang.Object with Ordering[String]
// required: Ordering[(String, String)]
//   names.sortBy(n => (n.last, n.first))(o)

is there any way that allow me to change the ordering without having to write an explicit sortWith method with multiple if–else branches in order to deal with all cases?

Answer 1

One solution is to extend the otherwise implicitly used Tuple2 ordering. Unfortunately, this means writing out Tuple2 in the code.

names.sortBy(n => (n.second, n.first))(Ordering.Tuple2(o, o))