Opaque types

Magda Stożek

Get more clarity with

The perfect model

Precise
Readable
Prevents mistakes

Scala 2

Type aliases

Refined types

Value classes

type Age = Int

case class Age(value: Int) extends AnyVal

type Age = Int Refined Positive

Opaque type aliases

Demo:

why "opaque"
how to add API
- object methods
- instance methods
bounds

Newtypes

zero-cost wrapper
Scala 2
- scala-newtype
Scala 3
- opaque types?
- Monix's Newtypes
- ZIO Prelude
- Iron

Refined types

define allowed values
Scala 2
- refined
Scala 3
- refined?
- Iron

type Pages = Int
Pages(350)

if (value > 0) Some(value) else None

Ecosystem

Opaque types + Iron

Demo

Opaque types summary

Advantages:

simple and clear
no overhead
opaque

Disadvantages:

no API (*unless bounds)
no pattern matching
no toString/equals overriding

Read more

Opaque types documentation: https://docs.scala-lang.org/scala3/reference/other-new-features/opaques.html

Demo code: https://github.com/magdzikk/opaque-types

Discussion about newtypes in Scala: https://contributors.scala-lang.org/t/improve-opaque-types/4786

Iron docs: https://iltotore.github.io/iron/docs/index.html

Questions?

  opaque type EndOfPresentation = TimeToAskQuestions

http://bit.ly/magda-scalalove

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