Typonym

Collections raised to the type-level in Scala

When writing macros or generic type-level code, we often need a way to statically represent collections of values, without them to existing at runtime and requiring allocation on the heap. Scala's Tuple type provides a generic way of representing a heterogeneous sequence of values at the type-level, but the inline or macro code that's necessary to work with them—in particular, to decompose them into values—can be intricate, fragile and repetitive. Typonym makes it easy to convert between value-level and type-level representations of three collection types, Lists, Sets and Maps, as well as straigtforward conversions of singleton literal types.

Features

Provides type-level encodings of Lists, Sets and Maps
Convert between runtime values and static types
Automatically convert singleton literal types
Types can be trivially composed for more complex structures
Types may be reified from a static type in "normal" code
Types can also be converted from a Type or TypeRepr value from within a macro implementation

Availability

Getting Started

All terms and types are defined in the typonym package:

import typonym.*

Application

Scala's rich type system may be used as a means of carrying static information from one point in the code to another, so that it may be used for static analysis and to influence typechecking. Much as a runtime value may be passed as a parameter to a method, and influence that method's runtime behavior, a compiletime type may be passed as a type parameter, and affect how that method is typechecked.

The runtime behavior of a method is, assuredly, implemented with ordinary calling and branching logic, operating on runtime values and runs, of course, at runtime. And we write almost all code this way because it's easier.

In contrast, typechecking behavior (ignoring, for a moment, its relationship with the runtime behavior it constrains) may be implemented using type-level constructs that operate on types, and are not so much run as applied at compiletime. This is a harder way of implementing logic: type-level operations are less powerful and less expressive than those available at runtime. So it is not ideal.

However, macros offer the ability to define type-level behavior using the power, expressivity and simplicity of value-level code, which will be run at compiletime.

Such code often benefits from collections, such as lists, sets and maps, which presents a need to represent such types in the type system, and to convert between types (both static types and value-level reflections of those types in macros) and standard Scala collections of values.

Type-level Representations

Singleton types of Ints, Strings, Doubles and Booleans have a straightforward isomorphism from their value-level to their type-level representations.

A Scala List will be encoded as a Tuple parameter to the TypeList type constuctor. For example, the list, List("one", "two", "three") would be represented as, TypeList[("one", "two", "three")] (by trivially composing it with the type-level representation of Strings).

Reification

Typonym offers two overloaded variants of the reify method for transforming types into values, to be invoked with a static type; or with an instance of Type, from within a macro implementation.

For example, to convert the List example above to a value, we could call,

val list: List[String] = reify[TypeList[("one", "two", "three")]]

or in a more general context,

inline def printElements[ItemsType <: TypeList[?]]: Unit =
  reify[ItemsType].foreach(println)

Similarly, within a macro,

def printLongest[ItemsType <: TypeList[?]: Type](using Quotes): Expr[Unit] =
  val items: List[String] = reify(Type.of[ItemsType])
  println(items.maxBy(_.length))
  '{()}

Status

Typonym is classified as embryotic. 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.

Typonym is designed to be small. Its entire source code currently consists of 63 lines of code.

Building

Typonym 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 Typonym?".

Copy the sources into your own project

Read the fury file in the repository root to understand Typonym'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.
Build with Wrath

Wrath is a bootstrapping script for building Typonym 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 typonym. Run wrath -F in the repository root. This will download and compile the latest version of Scala, as well as all of Typonym's dependencies.

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

Contributing

Contributors to Typonym 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 Typonym 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

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

Name

A typonym is a name derived from a type. Similarly, Typonym derives a value from a type (and types from values).

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 the square brackets which usually delimit a type in Scala.

propensive/typonym