Writing Traverse Instance for Option

Question

Working on an exercise from Functional Programming in Scala, I implemented a Traverse instance for Option:

override def traverse[G[_],A,B](oa: Option[A])(f: A => G[B])(implicit G: Applicative[G]):
    G[Option[B]] =
  oa match {
    case None => G.unit(None)
    case _ => {
              val a: A = oa.get
              val x: G[B] = f(a)
              G.map(x)(Some(_))
            }
  }

Is this idiomatic? Besides condensing the case _'s 3 lines to 1, perhaps there's a more concise way to write this method?

oa map f produces a type of Option[G[B]]. Applying G.unit would give G[Option[G[B]]. — Kevin Meredith
– Kevin Meredith, Commented Jan 28, 2014 at 22:20

senia · Accepted Answer · 2014-01-28 22:22:23Z

2

Never use get method on option. In this case you could use case Some(a) instead.
You don't need curly braces in case branches

oa match {
  case None => G.unit(None)
  case Some(a) =>
    val x: G[B] = f(a)
    G.map(x)(Some(_))
}

I don't know if variable x makes any sense in this case. I'd just use this:

case Some(a) => G.map(f(a))(Some(_))

You could also use methods of Option like fold of map + getOrElse, but I guess pattern matching is the best solution in this case.

edited Jan 28, 2014 at 22:22

answered Jan 28, 2014 at 22:11

senia

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