Summing a list of OptionT[Future, BigDecimal] in Scalaz

Question

I have a list of type List[Result[BigDecimal]] that I want to sum.

type Result[A] = OptionT[Future, A]

The rule would be that if there is any Future(None) then we get the result Future(None).

I have the function [1]:

def sum[A: Monoid](as: List[A]): A = {
  val M = implicitly[Monoid[A]]
  as.foldLeft(M.zero)(M.append)
}

However, I am missing the Monoid instance for Result[BigDecimal]. How do I define it using Scalaz?

learning Scalaz - sum function

Answer 1

I'm not sure why Scalaz doesn't provide this instance—it does provide a Monoid[Future[A]] where A has a monoid instance, and where there's an implicit execution context in scope. You can easily make your own, though, either by defining an isomorphism between Result and Future[Option[?]] and then using IsomorphismMonoid, or [this wouldn't actually have the desired semantics] by just defining one directly:

import scala.concurrent.ExecutionContext.Implicits.global
import scala.concurrent.Future
import scalaz._, Scalaz._

type Result[A] = OptionT[Future, A]

implicit def resultMonoid[A: Monoid]: Monoid[Result[A]] = new Monoid[Result[A]] {
  def zero: Result[A] = Monoid[A].zero.point[Result]
  def append(f1: Result[A], f2: => Result[A]): Result[A] = (f1 |@| f2)(_ |+| _)
}

And then (using Scalaz's own suml, but your sum would work as well):

scala> List(OptionT(Future(1.some)), OptionT(Future(2.some))).suml
res1: scalaz.OptionT[scala.concurrent.Future,Int] = OptionT(List())

scala> scala.concurrent.Await.result(res1.run, scala.concurrent.duration.Duration.Inf)
res2: Option[Int] = Some(3)

For what it's worth, Cats provides this instance, but (like the Future[Option[?]] instance in Scalaz) it has None as identity.