Design a generic trait in Scala

Question

I'm learning data structure recently. There is a case that I want to design a generic trait which type should support comparable. If I need to design a generic class, I can design like the following:

class SortedType [A: Ordering](val x: A)
val x = new SortedType(3)
val y = new SortedType("Hello, World!")

However, since in scala, the trait can't have parameters with context bounds, so I can't define a trait like this trait SortedType[A: Ordering]. How can I design the trait so that it's generic type support comparable? Thanks for your generous advice!

Andrey Tyukin · Accepted Answer · 2019-03-29 15:02:10Z

3

The constraint [A: Ordering] does not tell anything about the type A itself. Instead, it specifies that an (implicitl) instance of type Ordering[A] exists. The simplest way to guarantee the existence of an instance of type Ordering[A] is to simply provide a method def ord: Ordering[A].

So, you could make the ordering into a member of the trait, then accept an ordering as factory-method parameter:

trait SortedStuff[A] {
  def ord: Ordering[A]
  def x: A
}

object SortedStuff {
  def apply[A: Ordering](a: A) = new SortedStuff[A] {
    def ord = implicitly
    def x = a
  }
}

Note that this only makes sense if SortedStuff is some kind of module that is supposed to operate on a whole bunch of As. Attaching an Ordering to separate elements of A does not make any sense - ordering is a relation between elements, not a property of each single isolated element.

edited Mar 29, 2019 at 15:02

answered Mar 29, 2019 at 14:34

Andrey Tyukin

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