scala212-cats-category-theory-composing-functors

Reference: https://bartoszmilewski.com/2015/01/20/functors/
Reference: https://typelevel.org/cats/typeclasses/functor.html

preface

Basic info about functors:

project description

We have optionList:Option[List[Int]] = Option(List(1, 2, 3)) and we want to square the elements.

pure Scala

def square(i: Int) = i * i
val optionList = Option(List(1, 2, 3))

optionList.map(_ . map(square)) should be (Some(List(1, 4, 9)))

Scala + Cats
- Option is a functor
- List is a functor
- Composition of two functors, when acting on objects, is just the composition of their respective object mappings.
```
def square(i: Int) = i * i
val optionList = Option(List(1, 2, 3))

Functor[Option].compose[List].map(optionList)(square) should be (Some(List(1, 4, 9)))
```