Hyperbole

Assistance with Scala 3 metaprogramming

Hyperbole is a nascent set of tools to make it easier to write macros in Scala. To begin, it includes an introspect method which can provide useful reflection detail about how the source of an expression translates into an AST.

Features

• See a full introspection of an expression's AST
• Renders the AST structure in an easy-to-read tree format
• View source code alongside the AST
• Also view the "expanded" source code alongside, showing "invisible" syntax
• Produces output at compiletime or runtime, invoked inside or outside of a macro

Availability Plan

Hyperbole has not yet been published. The medium-term plan is to build Hyperbole with Fury and to publish it as a source build on Vent. This will enable ordinary users to write and build software which depends on Hyperbole.

Subsequently, Hyperbole will also be made available as a binary in the Maven Central repository. This will enable users of other build tools to use it.

For the overeager, curious and impatient, see building.

Getting Started

Hyperbole runs at compiletime to inspect an expression, but can present its output in different ways:

• as a message reported at compiletime
• as a terminal compile error
• as a value which can be used in some way at runtime

It can be invoked on an expression in ordinary code, or in an inline context where a Quotes instance is available, on a lifted Expr or a Tree value.

In all cases, the macro is invoked with an introspect method, either passing the expression to be inspected, or a lifted Expr expression value in a quoted context, disambiguated by overloading. In a quoted context, an additional optional parameter, terminate, may be specified as true to indicate that the output should be reported as a compiler error rather than an informational message at compiletime; by default it is false.

A contextual Introspection value determines what result should be yielded from a call to introspect. This given value can determine not only the result type, but whether the expression is evaluated or its value retained. Three implementations offered:

• introspection.println—records the introspection details to stdout with println, and returns the expression value
• introspection.text—constructs a Text value containing the introspection details, ignoring the expression without evaluating it

The default is to log using Eucalyptus. Note that this contextual value is not necessary for the introspect method that take an Expr value.

For example, in a macro method body,

import hyperbole.*
def macroImpl(expr: Expr[T])(using Quotes): Expr[Unit] =
  import quotes.reflect.*
  introspect(expr, terminate = true)

or, for example, in a Probably test,

import hyperbole.*, introspection.log
import probably.*
import eucalyptus.*, logging.stdout

test(t"Two joined lists are not empty"):
  val xs = List(1)
  val ys = List(2)
  introspect(xs ++ ys)
.assert(!_.isEmpty)

Status

Hyperbole is classified as embryonic. For reference, Scala One 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.

Hyperbole is designed to be small. Its entire source code currently consists of 79 lines of code.

Building

Hyperbole 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 Hyperbole?".

1. Copy the sources into your own project

   Read the fury file in the repository root to understand Hyperbole'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 Hyperbole 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 hyperbole. Run wrath -F in the repository root. This will download and compile the latest version of Scala, as well as all of Hyperbole's dependencies.

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

Contributing

Contributors to Hyperbole 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 Hyperbole 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.

Author

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

Name

Hyperbole is a tool for working with macros (short for macroinstructions) whose name implies a large—or hyperbolic—size.

In general, Scala One 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.

Logo

The logo shows a set of shapes formed from hyperbolic curves, reminiscent of a butterfly.

License

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