Summary

ZIO Optics makes it easy to modify parts of larger data structures.

For example, say we have a web application where users can vote on which of various topics they are interested in. We maintain our state of how many votes each topic has received as a Ref[Map[String, Int]].

import zio._

lazy val voteRef: Ref[Map[String, Int]] =
  ???

If we want to increment the number of votes for one of the topics here is what it would look like:

def incrementVotes(topic: String): Task[Unit] =
  voteRef.modify { voteMap =>
    voteMap.get(topic) match {
      case Some(votes) =>
        (ZIO.unit, voteMap + (topic -> (votes + 1)))
      case None        =>
        val message = s"voteMap $voteMap did not contain topic $topic"
        (ZIO.fail(new NoSuchElementException(message)), voteMap)
    }
  }.flatten

This is alright, but there is a lot of code here for a relatively simple operation of incrementing one of the keys. We have to get the value from the Ref, then get the value from the Map, and finally set the new value in the Map.

We also have to explicitly handle the possibility that the value is not in the map. And this is all for a relatively simple data structure!

Here is what this would look like with ZIO Optics.

import zio.optics._

def incrementVotes(topic: String): Task[Unit] =
  voteRef.key(topic).update(_ + 1)

The key optic "zooms in" on part of a larger structure, in this case transforming the Ref[Map[String, Int]] into a Ref that accesses the value at the specified key. We can then simply call the update operator on Ref to increment the value.

The optic also handles the possibility of failure for us, failing with an OpticFailure that is a subtype of Throwable and contains a helpful error message if the key cannot be found.

ZIO Optics makes it easy to compose more complex optics from simpler ones, to define optics for your own data types, and to work with optics that use ZIO or STM effects.

Installation

Include ZIO Optics in your project by adding the following to your build.sbt file:

libraryDependencies += "dev.zio" %% "zio-optics" % zioOpticsVersion
Understanding Optics →