How to do ordering of a sealed trait?

Question

I have a distributed system defined by a sort of "state machine" ( "flow chart" )

each system writes there state in a shared "log"

I'm representing each state as part of a sealed trait and a given "status" for that state

I want to "merge/reduce" to a single status which represents the current progress.

(There are some relaxations in place as not all must succeed in order for the final status to complete successfully)

There are 2 sealed traits representing the flow:

sealed trait System
case object A extends System
case object B extends System
case object C extends System
...

sealed trait Status
case object Pending extends Status
case object InProgress extends Status
case object Success extends Status
case object Fail extends Status

Log:

A, Success
B, Fail
C, Pending
...
...

now there are a set of rules which I use to define a single status reduction

basically it gives a priority

A < B < C, ... < Z

and

Pending < InProgress < Success < Fail

so if there is a status of:

(A, Success) versus (C, Pending)

I want to reduce it to (C,Pending)

and if

(A,Success) versus (B, Fail)

I want to reduce it to (B, Fail)

I can model this as a simple integer comparison in my case (possibly with an outlier which I explicitly test for)

I'm not clear how to make the sealed traits comparible/orderable which would make my life way easier

something along these lines is adequate:

def reduce(states: Seq[(System,Status)]) : (System,Status) = {
    states.order... {left.system < right.system) && (a.status < b.status) ... possibly another ordering test ....}.tail // take the last one in the ordering
}

Answer 1

You can define a scala.math.Ordering[Status]:

object StatusOrdering extends Ordering[Status] {
  def compare(x: Status, y: Status): Int =
    (x, y) match {
      // assuming that the ordering is Pending < InProgress < Success < Fail...
      case (_, _) if (x eq y) => 0
      case (Pending, _) => -1
      case (_, Pending) => 1
      case (InProgress, _) => -1
      case (_, InProgress) => 1
      case (Success, _) => -1
      case (_, Success) => 1
      case _ => 0 // (Fail, Fail)
    }

In your reduce, you can then

import StatusOrdering.mkOrderingOps

and your Status objects will be enriched with < and friends.

It is also possible to have your trait extend Ordered[Status], which defines a canonical ordering in the trait:

sealed trait OrderedStatus extends Ordered[OrderedStatus] {
  def compare(that: OrderedStatus): Int =
    (this, that) match {
      case (x, y) if (x eq y) => 0
      case (Qux, _) => -1
      case (_, Qux) => 1
      case (Quux, _) => -1
      case (_, Quux) => 1
      case _ => 0
    }
}

case object Qux extends OrderedStatus
case object Quux extends OrderedStatus
case object Quuux extends OrderedStatus

Then you don't have to import mkOrderingOps, but I personally dislike the forward use of the extending case objects in the compare method (and the alternative of a boilerplate compare in each case object is even worse).