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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:

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 Lists or Vacuous Optionals) 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. To include it in an sbt build, use the coordinates:

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,

import kaleidoscope.*

or to use it with other Soundness libraries, include:

import soundness.*

Note that Kaleidoscope uses the Text type from Anticipation and the Optional type from 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,

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,

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, 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 before to the capturing group, as $identifier or ${identifier}.

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

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, these can be extracted directly in a val definition (though this is common usage).

Here is an example of matching an email address:

val r"^[a-z0-9._%+-]+@$domain([a-z0-9.-]+\.$tld([a-z]{2,6}))$$" =
  "test@example.com": @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} (exactly three times), {2,} (at least twice) or {1,9} (at least once, and at most nine times). Note that {1} will still extract a Text value, not a List[Text].

The type of each captured group 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:

import gossamer.{skip, 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(_.skip(2, Rtl)) :+ last

Escaping

Escaping happens at two levels between source code and regular expression. First when source code is interpreted as a string. And again when that string is interpreted as a regular expression pattern.

This is particularly apparent when pattern matching a single backslash (\) in Java: we must write java.util.regex.Pattern.compile("\\\\"). The backslash in the regular expression needs to be escaped with another backslash; then each of those backslashes must be escaped in order to embed it in a string.

The situation is improved in Kaleidoscope patterns, written as single- (r"...") or triple-quoted (r"""...""") interpolated strings: special characters, notably \, do not need to be escaped. This means that only the regular expression escaping rules need to be considered.

An exception is $ (which is used to indicate a substitution) and 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"""\(""".

Regex values

Regular expressions may be defined as values outside of pattern matches, using the same syntax. For example,

val IpAddress = r"([0-9]+\.){3}[0-9]+"

which may then be used in a pattern,

input match
  case IpAddress(addr) => addr

but can only represent an entire match, and cannot extract capturing groups.

Such values are instances of Regex and provide access to the source pattern as a Text value, Regex#pattern, as well as the position and nature of any groups within the pattern.

Regexs are used in other Soundness libraries wherever a regular expression is required. More importantly, Text is never used for a value that represents a regular expression. So it's possible to know from a method's signature whether its parameter is interpreted as a regular expression or as a direct string.

Indeed, in Gossamer, the method sub (for making substitutions in a textual value) is overloaded to take either a Regex or a Text parameter, and to behave accordingly.

Globs

Globs offer a simplified but limited form of regular expression. You can use these in exactly the same way as a standard regular expresion, using the g"..." interpolator instead.

For example,

path match
  case g"/usr/local/bin/$name"     => name
  case g"/home/*/.local/bin/$name" => name

The appearance of a * in a glob will match any sequence of characters, except for / and \. A ? will match exactly one character. An extractor, such as $name above, is equivalent to *, but binds the value to an identifier.

As an expression (rather than a pattern), and interpolated string, g"", will also produce a Regex value, and can be used anywhere a Regex is valid. In fact, globs are implemented as a simpler front-end to regular expressions. So it would be possible to write, path.sub(g"/home/*/.local", t"/usr/local") or path.sub(r"/home/[^/]*/.local", t"/usr/local") to achieve the same goal.

Equality

It is even possible, sometimes, to equate regular expressions and globs. For example, g".local/*" == r"\.local/[^/\\]*" returns true because they are represented by identical underlying patterns. However, the inequality of two Regex instances does not necessarily indicate any difference in the behavior of two Regexs: it may be impossible to find any input where one matches and the other does not, while they are implemented differently. (As a trivial example, consider r"[xy]" and r"[yx]": non-equal Regexs, with identical behavior.)

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