zio / zio-prelude   1.0.0-RC27

Apache License 2.0 Website GitHub

A lightweight, distinctly Scala take on functional abstractions, with tight ZIO integration

Scala versions: 3.x 2.13 2.12
Scala.js versions: 1.x
Scala Native versions: 0.4

ZIO Prelude

ZIO Prelude is a lightweight, distinctly Scala takes on functional abstractions, with tight ZIO integration.

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ZIO Prelude is a small library that brings common, useful algebraic abstractions and data types to scala developers. It is an alternative to libraries like Scalaz and Cats based on radical ideas that embrace modularity and subtyping in Scala and offer new levels of power and ergonomics. It throws out the classic functor hierarchy in favor of a modular algebraic approach that is smaller, easier to understand and teach, and more expressive.

ZIO Prelude has three key areas of focus:

  1. Data structures, and type classes for traversing them. ZIO Prelude embraces the collections in the Scala standard library, and extends them with new instances and new useful additions.
  2. Patterns of composition for types. ZIO Prelude provides a small catalog of patterns for binary operators, which combine two values into another value of the same type. These patterns are named after the algebraic laws they satisfy: associativity, commutativity, and identity.
  3. Patterns of composition for type constructors. ZIO Prelude provides a catalog of patterns for binary operators on type constructors (things like Future, Option, ZIO Task). These patterns are named after the algebraic laws they satisfy (associativity, commutativity, and identity) and the structure they produce, whether a tuple or an either.

Design principles behind ZIO Prelude:

  1. Radical — So basically it ignores all dogma, and it is completely written with a new mindset.
  2. Orthogonality — The goal for ZIO Prelude is to have no overlap. Type classes should do one thing and fit it well. So there is not any duplication to describe type classes.
  3. Principled — All type classes in ZIO Prelude include a set of laws that instances must obey.
  4. Pragmatic — If we have data types that don't satisfy laws but that are still useful to use in most cases, we can go ahead and provide instances for them.
  5. Scala-First - It embraces subtyping and benefit from object-oriented features of Scala.

ZIO Prelude gives us:

  • Functional Data Types— Additional data types to supplement the ones in the Scala standard library such as Validation and NonEmptyList to enable more accurate domain modeling and handle common problems like data validation. For example:
    • NonEmptyList, NonEmptySet
    • ZSet, ZNonEmptySet
    • Validation, ZValidation
  • Functional Abstractions— Functional abstractions to describe different ways of combining data, making it easy for us to combine complex data types in a principled way.
  • New Types— that allow to increase type safety in domain modeling. Wrapping existing type adding no runtime overhead. These refined newtypes allow us to increase the type safety of our code base with zero overhead and minimal boilerplate.
  • ZPure— A description of a computation that supports logging, context, state, and errors, providing all the functionality traditionally offered by monad transformers with dramatically better performance and ergonomics.

The library has a small research-stage package (zio.prelude.fx) that provides abstraction over expressive effect types like ZIO and ZPure.

ZIO Prelude is a library focused on providing a core set of functional data types and abstractions that can help you solve a variety of day to day problems. The tools provided by ZIO Prelude fall into the following main categories:


In order to use this library, we need to add the following line in our build.sbt file:

libraryDependencies += "dev.zio" %% "zio-prelude" % "1.0.0-RC27"


In this example, we are going to create a simple voting application. We will use two features of ZIO Prelude:

  1. To become more type safety we are going to use New Types and introducing Topic and Votes data types.
  2. Providing instance of Associative type class for Votes data type which helps us to combine Votes values.
import zio.prelude._

object VotingExample extends scala.App {

  object Votes extends Subtype[Int] {
    implicit val associativeVotes: Associative[Votes] =
      new Associative[Votes] {
        override def combine(l: => Votes, r: => Votes): Votes =
          Votes(l + r)
  type Votes = Votes.Type

  object Topic extends Subtype[String]
  type Topic = Topic.Type

  final case class VoteState(map: Map[Topic, Votes]) { self =>
    def combine(that: VoteState): VoteState =
      VoteState(self.map combine that.map)

  val zioHttp    = Topic("zio-http")
  val uziHttp    = Topic("uzi-http")
  val zioTlsHttp = Topic("zio-tls-http")

  val leftVotes  = VoteState(Map(zioHttp -> Votes(4), uziHttp -> Votes(2)))
  val rightVotes = VoteState(Map(zioHttp -> Votes(2), zioTlsHttp -> Votes(2)))

  println(leftVotes combine rightVotes)
  // Output: VoteState(Map(zio-http -> 6, uzi-http -> 2, zio-tls-http -> 2))


Learn more on the ZIO Prelude homepage!


For the general guidelines, see ZIO contributor's guide.

Code of Conduct

See the Code of Conduct


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