Providing typeclass interfaces to user-defined Scala annotations

Adversaria is a tiny library which provides a few tools to make it easier to work with static annotations in Scala, by making them available through typeclass interfaces.

• access all annotations on a type through a typeclass
• resolve a typeclass instance only if a type has an annotated field
• makes annotations more useful and accessible in Scala
• no macro code is required to use annotations

Annotations in Scala are rarely the best solution for any task, but can nevertheless be convenient as a "feature of last resort" when no other solution provides the right ergonomics. This small domain is where Adversaria helps.

Currently three use cases are supported:

• getting all the annotations applied to a particular type
• finding the particular parameter of a case class to which a certain annotation has been applied
• getting every annotation applied to a particular case class field

This list of supported use cases is likely to grow.

If we define the following annotations in the standard way (each starting with a lower-case letter, as is the convention)

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: Text)
case class Person(name: Text, @id email: Text)

We would like to write code that can access annotations such as @count and @id through a simple typeclass interface.

Elsewhere, we may have a polymorphic method, say inspect, which inspects an instance of a type:

def inspect[T](value: T): Unit

If we would like to get the annotations on T that are subtypes of count, we can get these with the typeclass, Annotations:

def inspect[T](value: T)(using anns: Annotations[count, T]): Unit =
  anns.collect:
    case `count`(n: Int) => println(t"count = $n")

If inspect is called with a type, T, that does not have any @count annotations, then no contextual Annotations[count, T] instance will be constructed, and the code will not compile. So inspect[Company](company) would compile, while inspect[Person](person) would not.

Three methods also provide access to annotations on fields:

• Annotations.field[T](fn) will return a list of annotations on the case class field indicated by the lambda, fn. This lambda must be a simple field accessor, such as _.email, otherwise the method will not compile.
• Annotations.fields[T, A] will return a list of CaseField instances providing access to the name, annotation and value (if given an instance of T to dereference) for each annotation on any field with an annotation of type A in T's definition.
• Annotations.firstField[T, A] will return the first such field, if it exists.

Adversaria is classified as fledgling. For reference, Soundness projects are categorized into one of the following five stability levels:

• embryonic: for experimental or demonstrative purposes only, without any guarantees of longevity
• fledgling: of proven utility, seeking contributions, but liable to significant redesigns
• maturescent: major design decisions broady settled, seeking probatory adoption and refinement
• dependable: production-ready, subject to controlled ongoing maintenance and enhancement; tagged as version 1.0.0 or later
• adamantine: proven, reliable and production-ready, with no further breaking changes ever anticipated

Projects at any stability level, even embryonic projects, can still be used, as long as caution is taken to avoid a mismatch between the project's stability level and the required stability and maintainability of your own project.

Adversaria is designed to be small. Its entire source code currently consists of 142 lines of code.

Adversaria will ultimately be built by Fury, when it is published. In the meantime, two possibilities are offered, however they are acknowledged to be fragile, inadequately tested, and unsuitable for anything more than experimentation. They are provided only for the necessity of providing some answer to the question, "how can I try Adversaria?".

1. Copy the sources into your own project

   Read the fury file in the repository root to understand Adversaria's build structure, dependencies and source location; the file format should be short and quite intuitive. Copy the sources into a source directory in your own project, then repeat (recursively) for each of the dependencies.

   The sources are compiled against the latest nightly release of Scala 3. There should be no problem to compile the project together with all of its dependencies in a single compilation.

2. Build with Wrath

   Wrath is a bootstrapping script for building Adversaria and other projects in the absence of a fully-featured build tool. It is designed to read the fury file in the project directory, and produce a collection of JAR files which can be added to a classpath, by compiling the project and all of its dependencies, including the Scala compiler itself.

   Download the latest version of wrath, make it executable, and add it to your path, for example by copying it to /usr/local/bin/.

   Clone this repository inside an empty directory, so that the build can safely make clones of repositories it depends on as peers of adversaria. Run wrath -F in the repository root. This will download and compile the latest version of Scala, as well as all of Adversaria's dependencies.

   If the build was successful, the compiled JAR files can be found in the .wrath/dist directory.

Contributors to Adversaria are welcome and encouraged. New contributors may like to look for issues marked beginner.

We suggest that all contributors read the Contributing Guide to make the process of contributing to Adversaria easier.

Please do not contact project maintainers privately with questions unless there is a good reason to keep them private. While it can be tempting to repsond to such questions, private answers cannot be shared with a wider audience, and it can result in duplication of effort.

Adversaria was designed and developed by Jon Pretty, and commercial support and training on all aspects of Scala 3 is available from Propensive OÜ.

Adversaria are miscellaneous collections of notes or annotations, after which the library is named.

In general, Soundness project names are always chosen with some rationale, however it is usually frivolous. Each name is chosen for more for its uniqueness and intrigue than its concision or catchiness, and there is no bias towards names with positive or "nice" meanings—since many of the libraries perform some quite unpleasant tasks.

Names should be English words, though many are obscure or archaic, and it should be noted how willingly English adopts foreign words. Names are generally of Greek or Latin origin, and have often arrived in English via a romance language.

The logo is an arobase or "at-sign", being the Scala (and Java) symbol which introduces an annotation.

Adversaria is copyright © 2024 Jon Pretty & Propensive OÜ, and is made available under the Apache 2.0 License.