Eparch

EPARCH OF THE CITY (ἔπαρχος τῆς πόλεως), successor of the late Roman URBAN PREFECT, the governor of Constantinople. ¹

Eparch is a Gleam library that wraps certain Erlang/OTP behaviours with a type-safe API, making your byzantine systems shine with a great type system.

Supported OTP Behaviours

Module	Wraps	Purpose
eparch/state_machine	gen_statem	Type-safe finite state machines
eparch/event_manager	gen_event	Broadcast event managers with dynamic handlers

Full API reference: https://hexdocs.pm/eparch.

Key Differences from gen_statem

Erlang's gen_statem	eparch/state_machine
Separate handle_call, handle_cast, handle_info	Single handler dispatching on a unified Event type
Raw action tuples	Type-safe Action values
state_enter always on	Opt-in via with_state_enter()
Multiple return tuple formats	Single Step type (NextState, KeepState, Stop)

Key Differences from gen_event

Erlang's gen_event	eparch/event_manager
Separate handler callback module per handler	Single Handler builder (new_handler/2, on_terminate/2)
Handler identified by {Module, Id} tuple	Opaque HandlerRef returned by add_handler
handle_call for per-handler queries	Embed process.Subject(reply) in your event type instead
add_sup_handler	add_supervised_handler

Installation

gleam add eparch

Usage

See the Quick Start guide for full walkthroughs, or run some of the example projects that live in the examples/ directory.

Development

The project uses devenv and Nix for a hermetic development environment:

nix develop

Or, if you are already using direnv:

direnv allow .

Footnotes

1. The Oxford Dictionary of Byzantium, Vol II. ↩