API Documentation: MonoidK

MonoidK is a universal monoid which operates on type constructors of one argument.

This type class is useful when its type parameter F[_] has a structure that can be combined for any particular type, and which also has an "empty" representation. Thus, MonoidK is like a Monoid for type constructors (i.e. parametrized types).

A MonoidK[F] can produce a Monoid[F[A]] for any type A.

Here's how to distinguish Monoid and MonoidK:

Let's compare the usage of Monoid[A] and MonoidK[F].

First some imports:

import cats.{Monoid, MonoidK}
import cats.syntax.all._

Just like Monoid[A], MonoidK[F] has an empty method, but it is parametrized on the type of the element contained in F:

// res0: List[String] = List()
// res1: List[String] = List()
// res2: List[Int] = List()

And instead of combine, it has combineK, which also takes one type parameter:

Monoid[List[String]].combine(List("hello", "world"), List("goodbye", "moon"))
// res3: List[String] = List("hello", "world", "goodbye", "moon")
MonoidK[List].combineK[String](List("hello", "world"), List("goodbye", "moon"))
// res4: List[String] = List("hello", "world", "goodbye", "moon")
MonoidK[List].combineK[Int](List(1, 2), List(3, 4))
// res5: List[Int] = List(1, 2, 3, 4)

Actually the type parameter can usually be inferred:

MonoidK[List].combineK(List("hello", "world"), List("goodbye", "moon"))
// res6: List[String] = List("hello", "world", "goodbye", "moon")
MonoidK[List].combineK(List(1, 2), List(3, 4))
// res7: List[Int] = List(1, 2, 3, 4)

MonoidK extends SemigroupK, so take a look at the SemigroupK documentation for more examples.