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Usage examples

This is interesting—how can we use Monoid in our day-to-day programs? The most compelling use case is to fold data structures. When you have Monoid[A], it is trivial to combine all of the elements of Vector[A] to obtain one A. For instance, we can get the sum of all of the elements of Vector[Int], as follows:

Vector(1,2,3).combineAll
// res8: Int = 6

This is equivalent to calling foldLeft on Vector:

Vector(1, 2, 3).foldLeft(0) { case (acc, i) => acc + i }

Indeed, foldLeft accepts two arguments, as follows:

• A start value, for which we can pass the monoid's empty value
• A function, for which we can pass the monoid's combine function

Cats also provide a foldMap function, which lets you transform the elements of a collection into  ...