Battle tested Scala library for boilerplate-free data transformations.
In the daily life of a strongly-typed language's programmer sometimes it happens we need to transform an object of one type to another object which contains a number of the same or similar fields in their definitions.
case class MakeCoffee(id: Int, kind: String, addict: String)
case class CoffeeMade(id: Int, kind: String, forAddict: String, at: ZonedDateTime)
Usual approach is to just rewrite fields one by one
val command = MakeCoffee(id = Random.nextInt,
kind = "Espresso",
addict = "Piotr")
val event = CoffeeMade(id = command.id,
kind = command.kind,
forAddict = command.addict,
at = ZonedDateTime.now)
While the example stays lean, in real-life code we usually end up with tons of such boilerplate, especially when:
- we maintain typed schema and want to migrate between multiple schema versions
- we apply practices like DDD (Domain-Driven-Design) where suggested approach is to separate model schemas of different bounded contexts
- we use code-generation tools like Protocol Buffers that generate primitive
types like
Int
orString
, while you'd prefer to use value objects in you domain-level code to improve type-safety and readability
Chimney provides a compact DSL with which you can define transformation rules and transform your objects with as little boilerplate as possible.
import io.scalaland.chimney.dsl._
val event = command.into[CoffeeMade]
.withFieldComputed(_.at, _ => ZonedDateTime.now)
.withFieldRenamed(_.addict, _.forAddict)
.transform
Underneath it uses Scala macros to give you:
- type-safety at compile-time
- fast generated code, almost equivalent to handwritten version
- excellent error messages
- minimal overhead on compilation time
Getting started
To include Chimney to your SBT project, add the following line to your build.sbt
:
// if you use Scala on JVM-only
libraryDependencies += "io.scalaland" %% "chimney" % chimneyVersion
// if you cross-compile to Scala.js and/or Scala Native
libraryDependencies += "io.scalaland" %%% "chimney" % chimneyVersion
where the latest versions available on Maven for each platform are
Library is currently supported for Scala 2.12.x and 2.13.x on JVM, SJS 1.x, SN 0.4. Other versions should be considered EOL.
Due to some compiler bugs, it's recommended to use at least Scala 2.12.1.
Trying out with Scala CLI/Ammonite
If you are using Scala CLI you can try out Chimney by adding it with using
clause:
//> using scala "2.13.10"
//> using lib "io.scalaland::chimney:0.6.2"
import io.scalaland.chimney.dsl._
case class Foo(x: String, y: Int, z: Boolean = true)
case class Bar(x: String, y: Int)
object Main extends App {
println(Foo("abc", 10).transformInto[Bar])
println(Bar("abc", 10).into[Foo].enableDefaultValues.transform)
}
or run the Ammonite REPL:
scala-cli repl --ammonite --scala "2.13.10" --dependency "io.scalaland::chimney:0.6.2"
Loading...
Welcome to the Ammonite Repl 2.5.5-17-df243e14 (Scala 2.13.10 Java 17.0.1)
@ case class Foo(x: String, y: Int, z: Boolean = true)
defined class Foo
@ case class Bar(x: String, y: Int)
defined class Bar
@ import io.scalaland.chimney.dsl._
import io.scalaland.chimney.dsl._
@ Foo("abc", 10).transformInto[Bar]
res3: Bar = Bar(x = "abc", y = 10)
@ Bar("abc", 10).into[Foo].enableDefaultValues.transform
res4: Foo = Foo(x = "abc", y = 10, z = true)
If you don't have Scala CLI installed you can use this quick script that downloads coursier and uses it to fetch Ammonite REPL with the latest version of Chimney. It drops you immediately into a REPL session.
curl -s https://raw.githubusercontent.com/scalalandio/chimney/master/try-chimney.sh | bash
Documentation
Chimney documentation is available at https://scalalandio.github.io/chimney
Building documentation locally
In order to build documentation locally, you need to install Sphinx documentation generator first.
Then in project's root directory run command:
sbt makeSite
HTML Documentation should be generated at target/sphinx/html/index.html
.
Alternatively use Docker:
docker run --rm -v ./docs:/docs sphinxdoc/sphinx:3.2.1 bash -c "pip install sphinx-rtd-theme && make html"
Thanks
Thanks to JProfiler (Java profiler) for helping us develop the library and allowing us to use it during development.
Thanks to SwissBorg for sponsoring the development time for this project.