No implicit Ordering defined for ord

Question

I want to define a collection class and require its element being Ordered

Considering the code below:

class MyCollection[K: Ordered[K]] {
  def func(seq: Seq[K]): Unit = {
    seq.sorted
  }
}

The compiler will report error No implicit Ordering defined for ord: Ordering[K]

Am I doing anything wrong? Given that we already have the constraint K: Ordered[K]

Answer 1

You should use:

• either Ordered with

  • F-bound

    class MyCollection[K <: Ordered[K]] {
      def func(seq: Seq[K]): Unit =
        seq.sorted
    }
    

  • generalized constraint

    class MyCollection[K](implicit ev: K <:< Ordered[K]) {
      def func(seq: Seq[K]): Unit =
        seq.sorted
    }
    

  • view bound

    class MyCollection[K](implicit ev: K => Ordered[K]) {
      def func(seq: Seq[K]): Unit =
        seq.sorted
    }
    

    (from stronger to weaker assumption)

• or Ordering with context bound

  class MyCollection[K: Ordering] {
    def func(seq: Seq[K]): Unit =
      seq.sorted
  }
  

Ordered and Ordering are now defined so that the constraints K => Ordered[K] and K: Ordering are actually equivalent. Indeed, Ordering.ordered transforms one into another in one direction, Ordered.orderingToOrdered transforms in the other

def test[K](implicit ev: K => Ordered[K]) =
  implicitly[Ordering[K]] // compiles

def test[K: Ordering] =
  implicitly[K => Ordered[K]] // compiles

The context bound MyCollection[K: Ordering] is a syntax sugar for MyCollection[K](implicit ev: Ordering[K]). That's why [K: Ordered[K]] can't compile at all because of kind mismatch.

Ordering is a type class but Ordered is not. Ordered is an ordinary OOP trait (you're extending it in OOP style rather than define its implicit instances in FP style). That's why although [K: Ordered] compiles but it would be incorrect semantically (implicits will not be found).

Ordering and Ordered and comparing Options

Scala Ordering, Ordered, and View Bound

Get Ordering from Ordered in Scala