Documentation

Eventium is a typed, composable event-sourcing and CQRS library for Haskell. Instead of storing the current state of something, you store the sequence of events that happened to it, and derive state by replaying them. That constraint forces clarity — and Eventium gives you small, well-typed pieces to build on:

• A projection is a fold: a seed plus an event handler.
• A command handler is a pure function that validates intent and returns events.
• A process manager is a pure function that reacts to events with commands.
• A read model is a queryable view kept up to date from the event log.
• An event store is a record of functions — swap in-memory, SQLite, or PostgreSQL at the boundary.

Everything is a plain value, not a typeclass, so it composes by function application rather than instance resolution.

Start here

1. Getting Started — install Eventium and build a runnable counter.
2. Event Sourcing & CQRS — the mental model the rest of the docs build on.

Core concepts

• Projections — reconstruct state from events.
• Command Handlers — validate commands, emit events, handle concurrency.
• Process Managers — coordinate workflows across aggregates, with compensation.
• Read Models — build queryable views, in memory or persisted.
• Design & Internals — how the event store works and why it’s built this way.

Learn by example

The examples section walks through three complete apps, from a 100-line counter to a full CQRS banking system.

Reference

Once published, the API documentation will live on Hackage. The source is on GitHub.