Welcome to Chimney’s documentation!¶
Chimney is a 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") // MakeCoffee(24, "Espresso", "Piotr") val event = CoffeeMade(id = command.id, kind = command.kind, forAddict = command.addict, at = ZonedDateTime.now) // CoffeeMade(24, "Espresso", "Piotr", "2020-02-03T20:26:59.659647+07:00[Asia/Bangkok]")
While the example stays short, 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
String, 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 // CoffeeMade(24, "Espresso", "Piotr", "2020-02-03T20:26:59.659647+07:00[Asia/Bangkok]")
Read Getting started with transformers to learn more about Chimney’s transformers.
Beside transformers, Chimney supports case class patching as well. It’s a bit different type of transformation which happens when you hold an object of some type and want to modify only subset of its fields with values taken from other (patch) object.
case class User(id: Int, email: String, address: String, phone: Long) case class UserUpdateForm(email: String, phone: Long) val user = User(10, "firstname.lastname@example.org", "Broadway", 123456789L) val updateForm = UserUpdateForm("email@example.com", 123123123L) user.patchUsing(updateForm) // User(10, "firstname.lastname@example.org", "Broadway", 123123123L)
Read Getting started with patchers to learn more about Chimney’s patchers.
Chimney uses Scala macros to give you:
type-safety at compile-time
fast generated code, almost equivalent to hand-written version
excellent error messages
minimal overhead on compilation time
a way to express custom transformation logic
- Getting started with transformers
- Customizing transformers
- Standard transformers
- Default values support
- Plugging in own transformations
- Java beans
- Unsafe options transformation
- Lifted transformers
- Cats integration