import sbt._ resolvers += Resolver.sonatypeRepo("releases") resolvers += Resolver.sonatypeRepo("snapshots") val scalaVersion = "2.11.7" // or "2.10.6" val libraryVersion = "1.2.0-M1" // or "1.3.0-SNAPSHOT" libraryDependencies ++= Seq( "com.github.julien-truffaut" %% "monocle-core" % libraryVersion, "com.github.julien-truffaut" %% "monocle-generic" % libraryVersion, "com.github.julien-truffaut" %% "monocle-macro" % libraryVersion, "com.github.julien-truffaut" %% "monocle-state" % libraryVersion, "com.github.julien-truffaut" %% "monocle-law" % libraryVersion % "test" ) // for @Lenses macro support addCompilerPlugin("org.scalamacros" %% "paradise" % "2.0.1" cross CrossVersion.full)
Table of contents
- Lens Creation
- Generic Optics and Instance Location Policy
- Optics Hierarchy
- Maintainers and Contributors
Monocle is a
Lens library, or more generally an Optics library where Optics gather the concepts
Iso. Monocle is strongly inspired by Haskell Lens.
What does it mean?
Optics are a set of purely functional abstractions to manipulate (get, set, modify) immutable objects. Optics compose between each other and particularly shine with nested objects.
Why do I need this?
Scala already provides getters and setters for case classes but modifying nested object is verbose which makes code difficult to understand and reason about. Let's have a look at some examples:
case class Street(name: String, ...) // ... means it contains other fields case class Address(street: Street, ...) case class Company(address: Address, ...) case class Employee(company: Company, ...)
Let's say we have an employee and we need to set the first character of his company street name address in upper case. Here is how we could write it in vanilla Scala:
val employee: Employee = ... 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 } ) ) )
As you can see copy is not convenient to update nested objects as we need to repeat at each level the full path to reach it. Let's see what could we do with Monocle:
val _name : Lens[Street , String] = ... // we'll see later how to build Lens val _street : Lens[Address , Street] = ... val _address: Lens[Company , Address] = ... val _company: Lens[Employee, Company] = ... (_company composeLens _address composeLens _street composeLens _name).modify(_.capitalize)(employee) // you can achieve the same result with less characters using symbolic syntax (_company ^|-> _address ^|-> _street ^|-> _name).modify(_.capitalize)(employee)
ComposeLens takes two
Lens, one from A to B and another from B to C and creates a third
Lens from A to C.
Therefore, after composing _company, _address, _street and _name, we obtain a
String (the street name).
In the above example, we used capitalize to upper case the first letter of a
It works but it would be clearer if we could use
Lens to zoom into the first character of a
However, we cannot write such a
Lens because a
Lens defines how to focus from an object
S into a mandatory
A and in our case, the first character of a
String is optional as a
String might be empty. For this
we need a sort of partial
Lens, in Monocle it is called
import monocle.function.headOption._ // to use headOption (a generic optic) import monocle.std.string._ // to get String instance for HeadOption ((_company composeLens _address composeLens _street composeLens _name composeOptional headOption).modify(toUpper)(employee)
Similarly to composeLens, composeOptional takes two
Optional, one from A to B and another from B to C and
creates a third
Optional from A to C. All
Lens can be seen as
Optional where the optional element to zoom to is always
present, hence composing an
Optional and a
Lens always produces an
Optional (see class diagram for full inheritance
relation between Optics).
For more examples, see the
There are 3 ways to create
Lens, each with their pro and cons:
The manual method where we construct a
val _company = Lens[Employee, Company](_.company)( c => e => e.copy(company = c)) // or with some type inference val _company = Lens((_: Employee).company)( c => e => e.copy(company = c))
The semi-automatic method using the
val _company = GenLens[Employee](_.company) val _name = GenLens[Employee](_.name) // or val genLens = GenLens[Employee] val (_company, _name) = (genLens(_.company) , genLens(_.name))
Finally, the fully automatic method using the
Lensfor every accessor of a case class in its companion object (even if there is no companion object defined). This solution is the most boiler plate free but it has several disadvantages:
- users need to add the macro paradise plugin to their project.
- poor IDE supports, at the moment only IntelliJ recognises the generated
requires access to the case classes since you need to annotate them.
@Lenses case class Employee(company: Company, name: String, ...) // generates Employee.company: Lens[Employee, Company] // and Employee.name : Lens[Employee, String] // you can add a prefix to Lenses constructor @Lenses("_") case class Employee(company: Company, name: String, ...) // generates Employee._company: Lens[Employee, Company]
@Lenses are both limited to case classes
Generic Optics and Instance Location Policy
A generic optic is an optic that is applicable to different types. For example,
headOption is an
S to its optional first element of type
A. In order to use
headOption (or any generic optics), you
- import the generic optic in your scope via
- have the required instance of the type class
monocle.HeadOptionin your scope, e.g. if you want to use
List[Int], you need an instance of
HeadOption[List[Int], Int]. This instance can be either provided by you or by Monocle.
Monocle defines generic optic instances in the following packages:
monocle.stdfor standard Scala library and Scalaz classes, e.g.
List, Vector, Map, IList, OneAnd
monocle.genericfor Shapeless classes, e.g.
An example shows how to use Monocle imports.
- Core defines the main library concepts: optics, typeclass, syntax. Core only depends on scalaz for type classes.
- Law defines properties for optics using discipline and scalacheck.
- Macro defines a set of macros to generate optics automatically.
- Generic is an experiment to provide highly generalised Optics using shapeless.
Maintainers and Contributors
The current maintainers (people who can merge pull requests) are:
and the contributors (people who committed to Monocle).