Introduction

Transit is a PureScript library for building type-safe state machines. It provides a type-level DSL for specifying state transitions. You define your state machine once using this specification, and the compiler ensures your implementation matches it — eliminating bugs from invalid transitions, missing cases, or documentation drift.

This tutorial will guide you through the basics of Transit by showing its main features based on examples. We’ll start with a simple door state machine and gradually add more complexity.

About This Documentation

All code examples in this documentation are extracted from actual, type-checked PureScript source files. Also, whenever you find an assertion or a full unit test, we ensure that it ran and passed. In this sense, this text is not just documentation, but also a test suite. At the bottom of every code example you can find a link to the actual source file, so you can get a better picture of the context and see information about the imports used.

Similarities to Haskell’s Servant library

If you’re familiar with Servant¹ from Haskell, Transit follows a similar philosophy: just as Servant uses a REST API type-level specification to ensure type-safe routing functions and generate OpenAPI documentation, Transit uses a state machine graph type-level specification to ensure type-safe update functions and generate state diagrams.