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https://github.com/propensive/kaleidoscope

Statically-checked inline matching on regular expressions in Scala
https://github.com/propensive/kaleidoscope

capture-groups pattern-matching regex regular-expression scala

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Statically-checked inline matching on regular expressions in Scala

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# Kaleidoscope



__Statically-typed inline pattern matching on regular expressions__



__Kaleidoscope__ is a small library to make pattern matching against strings more

pleasant. Regular expressions can be written directly in patterns, and

capturing groups bound directly to variables, typed according to the group's

repetition. Here is an example:

```scala

case class Email(user: Text, domain: Text)



email match

  case r"$user([^@]+)@$domain(.*)" => Email(name, domain)

```



Strings are widely used to carry complex data, when it's wiser to use

structured objects. Kaleidoscope makes it easier to move away from strings.



## Features



- pattern match strings against regular expressions

- regular expressions can be written inline in patterns, anywhere a string could match

- direct extraction of capturing groups in patterns

- typed extraction (into `List`s or [Vacuous](https://github.com/propensive/vacuous/) `Optional`s) of variable-length capturing groups

- static checking of regular expression syntax

- simpler "glob" syntax is also provided



## Availability



Kaleidoscope is available as a binary for Scala 3.4.0 and later, from [Maven

Central](https://central.sonatype.com). To include it in an `sbt` build, use

the coordinates:

```scala

libraryDependencies += "dev.soundness" % "kaleidoscope-core" % "0.1.0"

```



## Getting Started



Kaleidoscope is included in the `kaleidoscope` package, and exported to the

`soundness` package.



To use Kaleidoscope alone, you can include the import,

```scala

import kaleidoscope.*

```

or to use it with other [Soundness](https://github.com/propensive/soundness/) libraries, include:

```scala

import soundness.*

```



Note that Kaleidoscope uses the `Text` type from

[Anticipation](https://github.com/propensive/anticipation) and the `Optional`

type from [Vacuous](https://github.com/propensive/vacuous/). These offer some

advantages over `String` and `Option`, and they can be easily converted:

`Text#s` converts a `Text` to a `String` and `Optional#option` converts an

`Optional` value to its equivalent `Option`. The necessary imports are shown in

the examples.



You can then use a Kaleidoscope regular expression—a string prefixed with

the letter `r`—anywhere you can pattern match against a string in Scala. For example,

```scala

import anticipation.Text



def describe(path: Text): Unit =

  path match

    case r"/images/.*" => println("image")

    case r"/styles/.*" => println("stylesheet")

    case _             => println("something else")

```

or,

```scala

import vacuous.{Optional, Unset}



def validate(email: Text): Optional[Text] = email match

  case r"^[a-z0-9._%+-]+@[a-z0-9.-]+\.[a-z]{2,6}$$" => email

  case _                                            => Unset

```



Such patterns will either match or not, however should they match, it is

possible to extract parts of the matched string using capturing groups. The

pattern syntax is exactly as described in the [Java Standard

Library](https://docs.oracle.com/javase/7/docs/api/java/util/regex/Pattern.html),

with the exception that a capturing group (enclosed within `(` and `)`) may be

bound to an identifier by placing it, like an interpolated string substitution,

immediately prior to the capturing group, as `$identifier` or `${identifier}`.



Here is an example of using a pattern match against filenames:

```scala

enum FileType:

  case Image(text: Text)

  case Stylesheet(text: Text)



def identify(path: Text): FileType = path match

  case r"/images/${img}(.*)"  => FileType.Image(img)

  case r"/styles/$styles(.*)" => FileType.Stylesheet(styles)

```



Alternatively, as with patterns in general, this can be extracted directly in a

`val` definition.



Here is an example of matching an email address:

```scala

val r"^[a-z0-9._%+-]+@$domain([a-z0-9.-]+\.$tld([a-z]{2,6}))$$" =

  "[email protected]": @unchecked

```



The `@unchecked` annotation ascribed to the result is standard Scala, and

acknowledges to the compiler that the match is _partial_ and may fail at

runtime.



If you try this example in the Scala REPL, it would bind the following values:

```

> domain: Text = t"example.com"

> tld: Text = t"com"

```



In addition, the syntax of the regular expression will be checked at

compile-time, and any issues will be reported then.



### Repeated and optional capture groups



A normal, _unitary_ capturing group, like `domain` and `tld` above, will

extract into `Text` values. But if a capturing group has a repetition suffix,

such as `*` or `+`, then the extracted type will be a `List[Text]`. This also

applies to repetition ranges, such as `{3}`, `{2,}` or `{1,9}`.



Note that `{1}` will still extract a `Text` value. The type is determined

statically from the pattern, and not dynamically from the runtime scrutinee.



A capture group may be marked as optional, meaning it can appear either zero or

one times. This will extract a value with the type `Optional[Text]`; that is,

if it present it will be a `Text` value, and if not, it will be `Unset`.



For example, see how `init` is extracted as a `List[Text]`, below:

```scala

import gossamer.{drop, Rtl}



def parseList(): List[Text] = "parsley, sage, rosemary, and thyme" match

  case r"$only([a-z]+)"                      => List(only)

  case r"$first([a-z]+) and $second([a-z]+)" => List(first, second)

  case r"$init([a-z]+, )*and $last([a-z]+)"  => init.map(_.drop(2, Rtl)) :+ last

```



### Escaping



Note that inside an extractor pattern string, whether it is single- (`r"..."`)

or triple-quoted (`r"""..."""`), special characters, notably `\`, do not need

to be escaped, with the exception of `$` which should be written as `$$`.



It is still necessary, however, to follow the regular expression escaping

rules, for example, an extractor matching a single opening parenthesis would be

written as `r"\("` or `r"""\("""`.



## Globs



Globs offer a simplified and limited form of regular expression. You can use

these in exactly the same way as a standard regular expresion, using the

`g"..."` interpolator instead.



## Status



Kaleidoscope is classified as __maturescent__. 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.



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

of 531 lines of code.



## Building



Kaleidoscope 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 Kaleidoscope?".



1. *Copy the sources into your own project*

   

   Read the `fury` file in the repository root to understand Kaleidoscope'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 Kaleidoscope 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 `kaleidoscope`.

   Run `wrath -F` in the repository root. This will download and compile the

   latest version of Scala, as well as all of Kaleidoscope's dependencies.



   If the build was successful, the compiled JAR files can be found in the

   `.wrath/dist` directory.



## Contributing



Contributors to Kaleidoscope are welcome and encouraged. New contributors may like

to look for issues marked

[beginner](https://github.com/propensive/kaleidoscope/labels/beginner).



We suggest that all contributors read the [Contributing

Guide](/contributing.md) to make the process of contributing to Kaleidoscope

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



Kaleidoscope 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



Kaleidoscope is named after the optical instrument which shows pretty patterns to its user, while the library also works closely with patterns.



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 is a loose allusion to a hexagonal pattern, which could appear in a kaleidoscope.



## License



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

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