kenbot / goggles   1.0

MIT License GitHub

Pleasant, yet principled Scala optics DSL

Scala versions: 2.12 2.11

Goggles - pleasant, yet principled optics DSL

Build Status

Optics libraries are either too limited, or too hard to use. Goggles builds on Scala's powerful Monocle library, making immutability easy, fun, and boring, like it should be.

You already know how to use it.

import goggles._ 

case class Topping(cherries: Int)
case class Cake(toppings: List[Topping])
case class Bakery(cakes: List[Cake])

val myBakery = Bakery(List(Cake(List(Topping(0), Topping(3))), 

// List(0, 4)

set"$myBakery.cakes*.toppings[0].cherries" := 7
// Bakery(List(Cake(List(Topping(7), Topping(3))), 
//             Cake(List(Topping(7))), 
//             Cake(Nil)))

The DSL runs in the compiler, and is completely typesafe. It generates plain Monocle code.

Getting started

Goggles supports Scala 2.11 and 2.12. Add the following to your build.sbt file:

libraryDependencies ++= Seq("com.github.kenbot" %%  "goggles-dsl"     % "1.0",
                            "com.github.kenbot" %%  "goggles-macros"  % "1.0")

scalacOptions += "-Yrangepos" // Enables better error messages


1. Functional programming needs optics

In imperative programming, a game world might be updated like this: += 20

Even just holding a reference to the player, we can be confident that the change will be seen by everyone observing, without knowing anything about the external environment. However, mutability conveys a severe penalty in complexity of behaviour, and our human ability to reason about the code.

On the other hand, naively using immutable structures leads to unfortunate problems.

  game.copy(currentLevel = 
    game.currentLevel.copy(player = 
      game.currentLevel.player.copy(health = + 20

Ugly, yes, but it gets worse: recreating the object graph is a dire failure of modularity. We must now know exactly where the object sits in the world-structure, and how to recreate every detail up to the root. Modularity is supposed to be a flagship benefit of FP - how embarrassing!

Optics are a family of pure-functional techniques that model access and mutation with composable abstractions. They are the best answer that has emerged to this dilemma; without them FP is dismally unsuited to a range of mundane problems.

2. Power vs ease-of-use. Why choose?

The modifying-immutable-structures problem has been addressed in a variety of ways.

Dynamic environments such as Clojure, jq, and Javascript have features that allow easy manipulation of structures without mutation, but in a very constrained, domain-specific context.

Haskell's Control.Lens is wonderfully powerful and abstract, but has a reputation for being difficult to learn and use. Why can't we have our cake and eat it too?

3. Monocle + Goggles

Monocle is the leading optics library in Scala; it has a small, well-designed core, but its day-to-day user experience leaves a little to be desired. It has much of the power of Control.Lens, and also has much of the conceptual weight and learning curve.

This makes it an ideal core for building an optics DSL. Goggles aims to provide an intuitive, discoverable interface over Monocle for beginners, while helping experienced users get the job done with a minimum of fuss.


Navigate case class-like fields by name

import goggles._ 

case class City(name: String, population: Int)
case class State(name: String, capital: City)
val state = State("Victoria", City("Melbourne", 4087000))

// 4087000

set"$" += 1
// State("Victoria", City("Melbourne", 4087001))

The += is syntax sugar; it requires an implicit scala.Numeric in scope for the result type.

Interpolate any Monocle optic

import goggles._ 
import monocle.std.string.stringToInt

// Some('q')

set"${"113"}.$stringToInt.$intToChar" ~= (_.toUpper)
// "81" 

Compose Monocle optics

import goggles._
import monocle.std.string.stringToInt

val myLens = lens"$stringToInt.$intToChar"
// monocle.PPrism[String,String,Char,Char] = monocle.PPrism$$anon$1@2b6b0069

// Some('q')

Traverse over collections

import goggles._

case class Point(x: Double, y: Double)
val polygon = List(Point(0.0, 0.0), Point(0.0, 1.0), Point(1.0, 1.0), Point(1.0, 0.0))
// List(0.0, 0.0, 1.0, 1.0)

set"$polygon*.x" += 1.5
List(Point(1.5, 0.0), Point(1.5, 1.0), Point(2.5, 1.0), Point(2.5, 0.0))

Any type for which an implicit monocle.function.Each is in scope can use *

Select optional values

import goggles._

case class Estate(farm: Option[Farm])
case class Farm(prizeChicken: Option[Chicken])
case class Chicken(egg: Option[Egg])
case class Egg(weight: Double)
val estate = Estate(Some(Farm(Some(Chicken(Some(Egg(2.3)))))))

// Some(2.3)

set"$" *= 2
// Estate(Some(Farm(Some(Chicken(Some(Egg(4.6)))))))

Any type for which an implicit monocle.function.Possible is in scope can use ?

Select indexed values

import goggles._

sealed trait Square
case object - extends Square
case object X extends Square
case object O extends Square

val ticTac: Vector[Vector[Square]] = 
    Vector(X, -, -),
    Vector(O, X, -),
    Vector(-, O, O))

val i = 0
// Some(X) 

set"$ticTac[2][0]" := O
//  Vector(
//    Vector(X, -, -),
//    Vector(O, X, -),
//    Vector(O, O, O))

Any type for which an implicit monocle.function.Index is in scope can use [i] with an index.

Great compilation error messages

Helpful compiler errors are a first class part of Goggles' design, hopefully encouraging exploration, clarifying optics concepts and allowing the functionality to be discoverable.

scala> get"$myBasket.items*"
<console>:18: error: Int doesn't have a 'foo' method

 Sections  │ Types                         │ Optics
 $myBasket │ ShoppingBasket                │
 .items    │ ShoppingBasket  ⇒  List[Item] │ Lens
 *         │ List[Item]      ⇒  Item       │ Traversal
 .qty      │ Item            ⇒  Int        │ Lens, returning Traversal
 foo       │ Int             ⇒  ???        │


Comparison to other approaches

Goggles itself

set"$myBakery.cakes*.toppings[0].cherries" := 7

Goggles is not an optics library itself; it is only a new user interface built on a subset of Monocle, and interoperates seamlessly with the rest. It uses whitebox macros, meaning that the contents of the macro decide the static return type.

Goggles takes the view that macros that base their behaviour on the structure of code rather than its value are not referentially-transparent, and not consistent with the best traditions of FP. Repurposing Scala's syntax to do things that aren't Scala is surprising to users and imposes an unnecessary cognitive burden.

Extensions to StringContext form the main mechanism, because:

  • It isn't Scala, and the String clearly demarcates regular Scala from the designated DSL area.
  • This gives us enormous flexibility to choose the syntax we want.

There are some disadvantages:

  • There is no IDE support out of the box: it just looks like a string to IDEs. (Could this be remedied with plugins?)
  • Because interpolated optics get evaluated before the rest of the macros, the type inference is poor for arguments.

QuickLens is designed to be a lightweight alternative to Monocle; it is solely focused on manipulating case classes. It uses a fluent API with blackbox macros, which deconstruct the given code tree to discover path information. It supports several features like "each" traversal and indexing, but lacks an overarching, cohesive optics model outside of the DSL. In addition, the fluent API supports manipulating several points in the path at once, and Prism-style navigation of sum types.


Monocle itself contains some internal syntactic helpers, including a simple DSL for convenient case class manipulation. Currently it uses a blackbox macro to deconstruct a code tree, which generates a monocle.Lens.


Shapeless offers Lens and Prisms, which allow automatic navigation of case classes and sum types. It uses Dynamic to allow Scala-like syntax, and uses a thicket of typeclasses to prove validity. It supports indexing and products, but not traversals. As with many of Shapeless' concepts, understanding the mechanism used requires a high level of proficiency, and the compile errors are quite unhelpful.