Adversaria is a very tiny library which provides a few tools for working with static annotations in Scala, by making them available through typeclass interfaces.
The nature of annotations in Scala is that they are very rarely the best solution for any task. The can, however, be convenient in certain circumstances, and this small domain is where Adversaria aims to help.
Currently there are three use cases:
- getting all the annotations applied to a particular type
- finding the parameter of a case class to which a particular annotation has been applied
- getting all the annotations applied to a particular case class field
We expect the list of supported use cases to grow as additional suggestions are received.
An early release of Adversaria is available on Maven Central, and can be included in an sbt project by including,
"com.propensive" %% "adversaria" % "0.2.0"
If we were to define the following annotations,
import scala.annotation.StaticAnnotation final case class id() extends StaticAnnotation final case class count(n: Int) extends StaticAnnotation
we could apply them to some case classes, such as,
@count(10) case class Company(name: String) case class Person(name: String, @id email: String)
Perhaps we would like to find out the annotations on
Company. We can get this information by requesting an implicit
TypeMetadata[Company] value, and accessing its
annotations field, like so,
val info = implicitly[TypeMetadata[Company]] println(info.annotations) > List(count(10))
TypeMetadata implicit should resolve for any type, regardless of whether it has any annotations or not.
Another supported use case is to find the field of a case class which has been annotated with a particular annotation, if and only if that annotation exists. We use the
FindMetadata typeclass for this. It takes two type parameters: the type of the annotation, and the type to check, respectively,
val idField = implicitly[FindMetadata[id, Person]] println(idField.get(Person("John Smith", "email@example.com))) > firstname.lastname@example.org
However, attempting to resolve such an implicit on a case class which has no field annotated with that annotation will fail at compile-time:
val idField = implicitly[FindMetadata[id, Company]] ! adversaria: could not find a parameter annotated with type @id
This library is very experimental, and provided for evaluation purposes only. It should not be considered production-ready, and the API is liable to change without warning.
Adversaria is provided under the Apache 2.0 license.