harel

What it is

David Harel’s statecharts extend finite state machines with nested states, parallel regions and history, which makes complex behaviour (a microwave, a traffic light, a voice dialogue) much easier to describe. SCXML is the W3C standard for writing statecharts as XML.

harel parses SCXML documents into typed Rust structures (<state>, <parallel>, <final>, transitions, data model, <invoke>), checks that they are well formed, and writes them back out as XML.

Using it

use harel::{parse_scxml, validate, to_xml};

let xml = std::fs::read_to_string("examples/microwave.scxml")?;
let chart = parse_scxml(&xml)?;     // typed statechart

validate(&chart)?;                  // structural checks (targets exist, ids are unique, ...)
println!("{} top-level states, initial = {:?}", chart.states.len(), chart.initial);

let round_trip = to_xml(&chart);    // back to SCXML

parse_scxml_with_options adjusts how strictly the input is read. The repository ships example charts: a microwave (also a parallel variant), a traffic light, a calculator and a blackjack game.

Ideas for using it

• Model checking input. Translate a statechart into a transition system and check safety properties with a model checker, for example “the door never opens while heating”.
• Lint statecharts in CI. Validate every .scxml file in a repository on each commit.
• Generate code or diagrams. Walk the typed structure to emit a Rust state machine or a Graphviz diagram.

Status

Prototype: parsing, validation and writing work; there is no interpreter that runs a chart.