Rudiments

Rudimentary utilities for writing everyday Scala

Rudiments provides a small collection of tiny but useful utilities for everyday programming in Scala, and could be considered an enhanced "predef".

Features

• implementation of an easier-to-use Y-combinator method, fix with a recursion helper method, recur
• typesafe and mutation-safe reimplementations of several String operations
• String extractors for primitive types
• unit, only, twin and triple convenience methods to make some code patterns slightly more concise
• a typesafe str"" string interpolator where all substitutions must be Strings

Availability Plan

Rudiments has not yet been published. The medium-term plan is to build Rudiments with Fury and to publish it as a source build on Vent. This will enable ordinary users to write and build software which depends on Rudiments.

Subsequently, Rudiments 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.

Getting Started

Utilities in Rudiments are mostly provided through extension methods, and importing the rudiments package will bring all utility methods into scope.

Y-Combinator

An implementation of a Y-Combinator, called fix, is provided, implemented using a Scala 3 context function, which enables slightly more favorable syntax than was possible in Scala 2. This method makes it easier to write code in a point-free style.

The fix method takes a type parameter (which must be explicitly specified for type inference) and a lambda as its first parameter, to which an additional parameter should be supplied as its initial value. Crucially, in the body of fix's lambda, a call to recur should be used to signal recursion.

This is best illustrated with an example. Here is an implementation of a factorial function.

def factorial(n: Int): Int =
  fix[Int] { i => if i <= 0 then 1 else i*recur(i - 1) } (n)

This avoids the explicit definition of a private or nested helper function, which would normally be necessary for a definition such as this.

Primitive String Extractors

Extractors for all the primitive types are provided for matching on Strings. These are defines as extensions to the (virtual) companion objects of the primitive types, so they have the same names as the types they extract.

Here is an example of them in use:

def parse(number: String): Boolean | Int | Double =
  number match
    case Boolean(b) => b
    case Int(i)     => i
    case Double(d)  => d
    case _          => 0

Typesafe String operations

The extension method String#cut has identical semantics to String#split, but returns an immutable IArray instead of an Array. Likewise, the methods String#bytes and String#chars mirror String#getBytes and String#toCharArray, returning an IArray[Byte] and IArray[Char] respectively.

The bytes method currently uses the UTF-8 in all cases, though this may change if there is sufficient demand.

Four variants of the extension method join are provided on Traversable[String] instances, which provide the same functionality as mkString (but only operating on Strings) with one additional two-parameter version that's specialized for natural language lists where each element is separated by a comma except the last, which is preceded by a word such as and or or.

For example,

List("one", "two", "three").join(", ", " or maybe ")

will produce the string one, two or maybe three.

Lightweight system property access

Many JVM system properties are available in the map, System.getProperties and are typically given identifiers written in a dot-notation style, such as user.dir. Rudiments provides syntactic sugar for accessing these dynamically through the Sys object, for example,

val pwd: Option[String] = Sys.user.dir()

unit

Often a side-effecting expression returns a value which is not used at a particular call site, and can be discarded. However, the expression's return type can result in type-inference choosing an undesired return type, when Unit would be preferable, or a compile-time warning about discarded values may be produced.

The unit extension method silently discards the return value of any expression, and instead produces a Unit, ().

only

The only extension method applies a partial function to a value and lifts the result into an option.

For example,

val result: Option[Int] = string.only { case Int(i) => i*i }

str"" Interpolator

The s"" interpolator takes parameters of Any type as substitutions, calling String#toString on them as necessary. This may be considered too permissive, so str"" is provided as a typesafe alternative that requires every substitution to be a String.

twin and triple

These two extension methods produce a two-tuple and a three-tuple (respectively) of repetitions of the value it is applied to. This can be useful in a subsequent map operation.

Status

Rudiments 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.

Rudiments is designed to be small. Its entire source code currently consists of 716 lines of code.

Building

Rudiments 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 Rudiments?".

1. Copy the sources into your own project

   Read the fury file in the repository root to understand Rudiments'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 Rudiments 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 rudiments. Run wrath -F in the repository root. This will download and compile the latest version of Scala, as well as all of Rudiments's dependencies.

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

Contributing

Contributors to Rudiments 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 Rudiments 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

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

Name

The word rudiment is defined as, "the principle which lies at the bottom of any development; an unfinished beginning", which is apt for a library whose purpose is to provide such common functionality that it might lie at the start of many other libraries.

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 burning sun, the basis for life in our solar system; it represents the foundational nature of Rudiments.

License

Rudiments is copyright © 2024 Jon Pretty & Propensive OÜ, and is made available under the Apache 2.0 License.