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

Getting Started

Hyperbole runs at compiletime to expose the TASTy trees of expressions. It can be invoked either from a macro, or within user code. In both cases, it produces a single string showing a tabular representation of the TASTy tree, for displaying in the console.

Inside a Macro

Within a macro, where there is a Quotes instance available, any Expr value can be introspected by calling its introspect extension method.

This will construct an instance of Teletype, a kind of string including ANSI control codes. An ordinary Text value can be obtained using Teletype#plain, which can be printed during compilation (as information or as an error), or even used in the expansion of the macro.

In Ordinary Code

The TASTy tree for any expression can also be obtained in ordinary code, using the global introspect method.

For example, we can introspect the expression 1 + x:

val x = 5
introspect(1 + x)

which will expand to the string,

TASTy             Param   Source   Code
â•
─────────────────────────────────────────────────────╴
â–ª Inlined                 1 + x    1.+(rs$line$2.x)
└─▪ Apply                 1 + x    1.+(rs$line$2.x)
  ├─▪ Select      +       1 +      1.+
  │ └─▪ Literal   1       1        1
  └─▪ Ident       x           x    rs$line$2.x

We could then compare the AST to a variant such as this:

val y: 5 = 5
introspect(y + 1)

which produces the simpler tree,

TASTy         Param   Source   Code
â•
─────────────────────────────────────╴
â–ª Inlined             y + 1    6
└─▪ Literal   6       y + 1    6

thanks to constant folding taking advantage of the singleton type of y.

Status

Hyperbole is classified as embryonic. 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.

Hyperbole is designed to be small. Its entire source code currently consists of 112 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, 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.

Logo

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

License

Hyperbole is copyright © 2025 Jon Pretty & Propensive OÜ, and is made available under the Apache 2.0 License.