Question

I have this bit of code, which works:

val directions = rs.map(_.direction) // Direction extends Enumeration
directions == directions.sorted.reverse

I’d like to instead do something like this:

ratings.map(_.direction).isInBackwardsOrder


class RichSeq[T](seq: Seq[T]) {
    def isInBackwardsOrder = seq == seq.sorted.reverse
}

object RichSeq {
    implicit def seq2richSeq[T](seq: Seq[T]) = new RichSeq[T](seq)
}

I keep getting the following compilation error:

could not find implicit value for parameter ord: Ordering[T]
def isInBackwardsOrder = seq == seq.sorted.reverse

What I don’t understand is why it could find the implicit value for parameter ord, when it was in the original form, but cannot find it once I pull it into a utility class.

Thanks for the help,
Alex

Answer 1

In the original form, you had no generics. directions is a Seq[SomeWellKnownType], and at compile time, the compiler looks for an Ordering[SomeWellKnownType] in implicit scope, and finds one.

On the other hand, in RichSeq[T], the compiler must find an implicit Ordering[T] where T is a type parameter. No way to do that. You must ensure that the Ordering will be available when you create the RichSeq :

class RichSeq[T](seq: Seq[T])(implicit ev: Ordering[T]) {...

There is a shortcut for that, especially if you just need ev in implicit scope without refrencing it explicitly, as in here :

class RichSeq[T : Ordering](seq: Seq[T]) {...

Then you have the exact same problem in your implicit method, which is generic too, with the same solution :

implicit def seq2richSeq[T: Ordering](seq: Seq[T]) = new RichSeq[T](seq)

Then it should work. The seq2richSeq implicit conversion will kick in when an Ordering is available for the type of the elements in the Seq.