More on those Monoids

I mentioned in a previous post about Scalaz' implementation of Monoids. Having run through the code with somebody better at Scala than I am, here are my notes.

Our code imports the implicit means of creating a monoid. The signature of the method is this:

  implicit def optionMonoid[A: Semigroup]: Monoid[Option[A]] = new Monoid[Option[A]] {

The [A: Semigroup] looks a little like a type parameter but it is totally different. It's a Context Bound that "are typically used with the so-called type class pattern, a pattern of code that emulates the functionality provided by Haskell type classes, though in a more verbose manner" (from this excellent explanation).

Context Bounds of the form [X: Y] say: I am expecting an implicit value of type Y[X]. In fact, this line is equivalent to Scalaz' implementation:

    implicit def optionMonoid[A](implicit x: Semigroup[A]): Monoid[Option[A]] = new Monoid[Option[A]] {

Now, recall that my code using the Scalaz library looked like this:

    implicit val stringSemigroup: Semigroup[String] = Semigroup.instance[String]((x, y) => x + y)

    val mySemigroup: Semigroup[Option[String]] = Semigroup[Option[String]]

The first line is me just defining my Semigroup - nothing interesting there.

The second is much more magic. It invokes this Scalaz code:

object Semigroup {
  @inline def apply[F](implicit F: Semigroup[F]): Semigroup[F] = F

which is somewhat confusing as F is used for more than one purpose. Basically, this line says: there should implicitly be a value of type Semigroup[F] where F is defined in the call site and I will return that value. (Note that the symbol 'F' is confusingly used to represent both the returned value and the type at the call site).

Well, the only implicit value that we defined is our own stringSemigroup and that's of type Semigroup[String] not Semigroup[Option[String]]. But then we also imported Scalaz' optionMonoid that says if there is a Semigroup[String] hanging around in the implicit ether, I can deliver a Monoid[Option[String]]. Since Monoid is a type of Semigroup, mySemigroup is legitimately given this Monoid[Option[String]].

Et voila. Not immediately obvious to a Java programmer but that's how Scala does it.

Further reading

1. Stackoverflow: Scalaz learning resources.