Real World Halogen [Draft]

Functional languages like PureScript, ReasonML, and Elm offer powerful features to manage complexity and help you reliably design, build, and refactor apps of any size. Yet, despite functional programming’s long history, there are few resources that teach how to build a non-trivial applications with these languages.

This guide demonstrates how to build a real world single-page application in PureScript and its most popular framework, Halogen. It comes with over 2,000 lines of commented code so you can see exactly how the ideas presented here translate to idiomatic PureScript.

I’m a senior software engineer at Awake Security and previously worked at CitizenNet. Both companies leverage PureScript to build single-page applications. I’m convinced it’s the best language for the web available today, and by the end of this guide you’ll be well-equipped to use it to build reliable applications of your own.

A word of caution

I wrote this rough draft before the implementation. Consider it a preview draft, not the official, completed version: most of the content is still valid and I recommend reading chapters 3 and 4 in particular. But any content using snippets from the implementation are not up-to-date. Please refer to the thoroughly-commented source code instead.


This is not a gentle introduction to PureScript or Halogen. It is intended for advanced beginners or intermediate PureScript developers who can build small Halogen apps but don’t yet feel comfortable building real world applications with the language & framework. If you feel lost when you begin reading, I recommend checking out learning resources including:

This guide is written for Halogen v4, but v5 is coming out soon. Make sure to browse the Halogen repository at the v4 tag, not the master branch.