The magic of composition

During this talk, we are going to dive into Monocle and unravel the mystery surrounding optics.

Slides here.

Public

This talk is aimed for developers with basic notion of:

• Scala
• Functional programming

The examples are in written in Scala so a basic notion of the syntax is assumed.

If you are not acquainted with the words Functor, Appicative, Foldable, Traverse I would recommend visiting first the cats and typeclassopedia and get a basic notion of these concepts.

Motivation

case class Street(number: Int, name: String)
case class Address(city: String, street: Street)
case class Company(name: String, address: Address)
case class Employee(name: String, company: Company)

val employee = Employee("john", Company("awesome inc", Address("london", Street(23, "high street"))))

Old plain scala:

employee.copy(
  company = employee.company.copy(
    address = employee.company.address.copy(
      street = employee.company.address.street.copy(
        name = employee.company.address.street.name.capitalize // luckily capitalize exists
      )
    )
  )
)

With lenses:

(company composeLens
 address composeLens
 street composeLens streetName).modify(_.capitalize)(employee)

Libraries

Lenses provides more composable versions of the abstractions you already known how to use in Scala/Haskell.

• monocle (scala)
• lenses (haskell)

Both libraries provides rich APIs to work with lenses and a battery of utilities to make your life easier. Lenses is based on Van Laarhoven optics while Monocle uses an alternative encoding. The reason behind not using Van Laarhoven nor profunctor optics is because they are inefficient in scala and not well supported by the language (scala does not support N-rank types).

Hierarchy

Monocle lenses hierarchy

Lenses hierarchy in haskell

Composition

The result is their lowest upper bound in the hierarchy (or an error if that bound doesn't exist).

	Getter	Iso	Lens	Optional	Prism	Setter	Traversal
Getter	Getter	Getter	Getter	Fold	Fold	-	Fold
Iso	Getter	Iso	Lens	Optional	Prism	Setter	Traversal
Lens	Getter	Lens	Lens	Optional	Optional	Setter	Traversal
Optional	Fold	Optional	Optional	Optional	Optional	Setter	Traversal
Prism	Fold	Prism	Optional	Optional	Prism	Setter	Traversal
Setter	-	Setter	Setter	Setter	Setter	Setter	Setter
Traversal	Fold	Traversal	Traversal	Traversal	Traversal	Setter	Traversal

Credits:

• https://github.com/julien-truffaut/Monocle (J. Truffaut et al)
• https://hackage.haskell.org/package/lens (E. Kmett)