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https://github.com/propensive/typonym

Collections raised to the type-level in Scala
https://github.com/propensive/typonym

collections dependent-types scala type-level type-level-programming

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Collections raised to the type-level in Scala

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README 

        
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# 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, `List`s, `Set`s and `Map`s, as well as straigtforward

conversions of singleton literal types.



## Features



- Provides type-level encodings of `List`s, `Set`s and `Map`s

- 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 Plan



Typonym has not yet been published. The medium-term plan is to build Typonym

with [Fury](https://github.com/propensive/fury) and to publish it as a source build on

[Vent](https://github.com/propensive/vent). This will enable ordinary users to write and build

software which depends on Typonym.



Subsequently, Typonym 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](#building).



## Getting Started



All terms and types are defined in the `typonym` package:

```scala

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 `Int`s, `String`s, `Double`s and `Boolean`s 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 `String`s).



### 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,

```scala

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

```

or in a more general context,

```scala

inline def printElements[ItemsType <: TypeList[?]]: Unit =

  reify[ItemsType].foreach(println)

```



Similarly, within a macro, 

```scala



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, 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.



Typonym is designed to be _small_. Its entire source code currently consists

of 62 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?".



1. *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.



2. *Build with [Wrath](https://github.com/propensive/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`](https://github.com/propensive/wrath/releases/latest), 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](https://github.com/propensive/typonym/labels/beginner).



We suggest that all contributors read the [Contributing

Guide](/contributing.md) 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](https://propensive.com/).



## Name



A typonym is a name derived from a type. Similarly, _Typonym_ derives a _value_

from a type (and types from values).



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 the square brackets which usually delimit a type in Scala.



## License



Typonym is copyright © 2024 Jon Pretty & Propensive OÜ, and

is made available under the [Apache 2.0 License](/license.md).