👻 Monads

⚠️ The owner organisation on https://www.npmjs.com/ has been changed to @hqoss. Please use @hqoss/monads going forward. Please note that all new versions will use camelCase in order to follow JavaScript conventions.

Type safe Option, Result, and Either types; inspired by Rust.

Table of contents

• ⏳ Install

• 📝 Usage

  • Option<T>
  • Result<T, E>
  • Either<L, R>
  • API Docs

• Core design principles

• Node version support

  • Why ES2018

• Testing

• TODO

⏳ Install

npm install @hqoss/monads

⚠️ NOTE: The project is configured to target ES2018 and the library uses commonjs module resolution. Read more in the Node version support section.

📝 Usage

Option<T>

See full documentation

import { Option, Some, None } from "@hqoss/monads"

function divide(numerator: number, denominator: number): Option<number> {
  if (denominator === 0) {
    return None
  } else {
    return Some(numerator / denominator)
  }
};

// The return value of the function is an option
const result = divide(2.0, 3.0)

// Pattern match to retrieve the value
const message = result.match({
  some: res => `Result: ${res}`,
  none: "Cannot divide by 0",
})

console.log(message) // "Result: 0.6666666666666666"

Result<T, E>

See full documentation

import { Result, Ok, Err } from "@hqoss/monads"

function getIndex(values: string[], value: string): Result<number, string> {
  const index = values.indexOf(value)

  switch (index) {
    case -1:
      return Err("Value not found")
  default:
    return Ok(index)
  }
}

console.log(getIndex(["a", "b", "c"], "b")) // Ok(1)
console.log(getIndex(["a", "b", "c"], "z")) // Err("Value not found")

Either<L, R>

See full documentation

import { Either } from "@hqoss/monads"

function getLabel(uncertainDate: Either<Date, string>) {
  return uncertainDate.match({
    left: date => date.toLocaleDateString(),
    right: text => `<abbr title="${text}">an uncertain date</abbr>`,
  })
}

API Docs

See full API Documentation here.

Core design principles

• Code quality; This package may end up being used in mission-critical software, so it's important that the code is performant, secure, and battle-tested.

• Developer experience; Developers must be able to use this package with no significant barriers to entry. It has to be easy-to-find, well-documented, and pleasant to use.

• Modularity & Configurability; It's important that users can compose and easily change the ways in which they consume and work with this package.

Node version support

The project is configured to target ES2018. In practice, this means consumers should run on Node 12 or higher, unless additional compilation/transpilation steps are in place to ensure compatibility with the target runtime.

Please see https://node.green/#ES2018 for reference.

Why ES2018

Firstly, according to the official Node release schedule, Node 12.x entered LTS on 2019-10-21 and is scheduled to enter Maintenance on 2020-10-20. With the End-of-Life scheduled for April 2022, we are confident that most users will now be running 12.x or higher.

Secondly, the 7.3 release of V8 (ships with Node 12.x or higher) includes "zero-cost async stack traces".

From the release notes:

We are turning on the --async-stack-traces flag by default. Zero-cost async stack traces make it easier to diagnose problems in production with heavily asynchronous code, as the error.stack property that is usually sent to log files/services now provides more insight into what caused the problem.

Testing

Ava and Jest were considered. Jest was chosen as it is very easy to configure and includes most of the features we need out-of-the-box.

Further investigation will be launched in foreseeable future to consider moving to Ava.

We prefer using Nock over mocking.

TODO

A quick and dirty tech debt tracker before we move to Issues.

• Write a Contributing guide
• Complete testing section, add best practices
• Describe scripts and usage, add best practices
• Add typespec and generate docs
• Describe security best practices, e.g. npm doctor, npm audit, npm outdated, ignore-scripts in .npmrc, etc.
• Add "Why should I use this" section