• The build hook no longer fails a Flutter build. A hook is run once for every kind of asset the embedder is collecting, and flutter run runs one pass that asks for none at all; the hook read the code-asset configuration before checking whether code assets had been asked for, which throws, and took the whole build down with it before the app started. It now answers that pass with nothing. dart test and dart run never made the call, so this was only ever reachable from Flutter.
• example/flutter points the same generators, shrinking and stateful testing at a user interface, in three shapes: a configuration sweep whose oracle is Flutter's own error reporting, a model the screen is held against, and a monkey that reads the semantics tree and insists only that the app go on working. It is a package of its own rather than part of the published archive — Flutter cannot be a dependency of a pure Dart package — and it is where hegel_flutter is being worked out.

• oneOf takes optional weights, pairing one to one with its options: an option is drawn in proportion to its share, and a counterexample still shrinks toward the first option — weight shapes the distribution, never the shrink. Each weight is at least one and their total has to fit a draw; anything else is refused where it was written. The first consumer's document generators are what pulled this out of the deferred list.
• TestCase.collect(value, label: ...) tallies observations per label across the valid cases of a run, and the distribution prints at the end from Verbosity.verbose up — the answer to "did the generators actually produce the shapes this property is supposed to exercise?". Observations made by a case assume rejected, or inside a stateful rule that declined, are discarded with their case.

First release.

Writing properties #

• property() registers an ordinary package:test test, so groups, -N, tags, skips and the IDE's run button all work on it. runProperty() is the same runner for harnesses that are not package:test.
• Draw with tc.draw(generator, name: ...), narrow a case with tc.assume, explain one with tc.note, and steer generation with tc.target.
• Failures are reported as the error your own expect raised, with its own stack, under the counterexample the engine shrank to. A run that finds several distinct bugs reports all of them.
• Counterexamples are kept in an example database and replayed ahead of anything new, so a property keeps failing until the bug is fixed; reproduce: replays one from a blob, which is how a CI failure comes back to a machine with a debugger.
• HEGEL_TEST_CASES and HEGEL_DATABASE override a run without editing it.

Generators #

• Numbers, text, and time: integers, bigIntegers, doubles, booleans, durations, text, characters, fromRegex, emails, urls, domains, bytes, dates, times, dateTimes, uuids, ipAddresses.
• Collections: lists with optional uniqueness, sets, maps, all with length bounds.
• Composition: map, where, flatMap on every generator, plus just, sampledFrom, oneOf, optional, tuple2–tuple4, composite, and deferred for recursive shapes.
• package:hegel/generators.dart is the catalog on its own, for prefixing.

Stateful testing #

• StateMachine, Rule, Invariant and runStateful: the engine picks the rules, and shrinks the sequence to the shortest script that still breaks an invariant.
• Rule.precondition keeps a rule off the table until it can run; Pool<T> holds what one step made for a later step to act on, and the engine chooses and shrinks over which.
• maxConcurrency: above one runs the rules several at a time, interleaving at every await in a rule body, with each worker's steps tagged in the report. Rule.group says what may overlap with what.

The engine #

• Distributed through a build hook: the pinned release is downloaded, verified against a checked-in SHA-256, and published as a code asset, with a local-engine override via pub user-defines. Nothing to install.
• Linux x64 and arm64, macOS arm64, Windows x64 and arm64 — the targets hegel-rust publishes.
• Use dart build cli rather than dart compile exe to bundle an application that depends on this package; dart run and dart test need nothing.

Notes #

• Pure-Dart properties run under flutter test today. Widget-level property testing is planned as a separate hegel_flutter package, because flutter_test pins test_api exactly.
• Requires Dart 3.13 or later, for build hooks and code assets.