regent

Optimistic, message-driven state engine: a journal of sealed facts folded into keyed stores and units, traversed by one ordered queue of regents. Pure Dart.

Regent in 60 seconds

In standard state-management terms:

  • Msg — an event/action: an immutable fact you dispatch.
  • Store / Unit — a reducer with its state: a Store folds messages into a keyed collection, a Unit into a single value.
  • Guard — middleware as a pure function: it can drop, rewrite, or fan out a message before the rows below it see it.
  • Mint — a guard deriving a new message from one it judged (run as its own round, re-judged from the top; never journaled — it re-derives on replay).
  • Ledger / journal — the runtime plus its event log: the journal keeps every dispatched message; reads and streams come from at(position).
  • Regent — any row in the ordered queue a message walks: a store, a unit, or a guard.
import 'package:regent/regent.dart';

sealed class CartMsg extends Msg { const CartMsg(); }
final class ItemAdded extends CartMsg {
  const ItemAdded(this.productId);
  final String productId;
}

final class Cart extends Unit<List<String>, CartMsg> {
  const Cart() : super(const []);
  @override
  List<String> reduce(List<String> items, CartMsg msg) => switch (msg) {
    ItemAdded(:final productId) => [...items, productId],
  };
}

const cart = Cart();

void main() {
  final ledger = Ledger.root(cart);
  ledger.dispatch(const ItemAdded('sku-42'));
  print(ledger.at(cart).state); // [sku-42]
}

The rest of this document uses these terms with their full meanings.

The queue of regents

Dispatch a Msg; it enters the journal (the complete, ungated record) and walks the QUEUE — an ordered list of REGENTS:

  • A store row is a pure READER standing at its place: it folds what passes (Store.reduce over a keyed collection, Unit.reduce over one value) and can never touch the message. What it sees is whatever survived the guards above its row.
  • A guard row is a pure JUDGE of the flow: it folds nothing and holds no state. Its verdict is a set of LAUNCHES targeting the only two indices that preserve the theorem no row ever sees a message that skipped a guard above it: .forward(msg) continues THIS round below (pass, drop via {}, rewrite, fan out); .mint(msg) DERIVES a new fact as its own round from index 0, after this round completes — re-judged by every guard, never journaled (it re-derives on replay), required to commute with its siblings. A Veto is the boolean specialization (pass or drop).

One order, two opposite relationships to it: moving a store changes what IT sees; moving a guard changes what EVERYONE below it sees. The record always keeps the original fact — guards shape the admitted feed, never the record.

Two doors

The app is a const VALUE — a Regency of rows in traversal order plus the merge edges (each projection carries its own endpoints). Reader rows get NAMES — const globals the consumer owns (const canonicalization makes the global and any equal construction ONE instance, so the name IS the row):

const catalogCovered = CatalogCovered();
const localCatalog = LocalCatalog();
const catalog = Catalog();

const app = Regency({
  catalogCovered,
  CachedCatalogGate(),  // set order is the queue — placement is protection
  localCatalog,
  catalog,
}, merges: {LocalCatalogSupports()});

final ledger = Ledger.root(app); // splices rows, wires merges

Regencies nest (a segment splices at its position) and a plain regent is a one-row graph: Ledger.root(const NavUnit()). A FEATURE whose rows are provably self-contained travels as one named graft — its merge edges ride along and resolve before the root's own:

final class WishlistFeature extends Regency {
  const WishlistFeature()
      : super(const {WishlistCap(), wishlist},
              merges: const {LocalWishlistSupports()});
}

Grouping is splice-in-place and reads stay FLAT — read(wishlist) never knows the grouping exists, so a graft can never change what a row means, only where the set is written down. The ledger then has exactly TWO doors:

  • dispatch(msg) — state a fact.
  • at(position) — stand at a position, typed by the spec instance: at(catalog) is the store's live memory, at(viewer) the unit's, at(const CachedCatalogGate()) the guard's story (GuardEvents: judged input + verdict — dropped, forwarded, minted), at(.entry) the complete pre-judgment RECORD, at(.exit) the admitted feed (at(.exit).msgs<OrderPlaced>() — effects tap here, so nothing fires on a dropped message).

On every handle, PLURAL members are streams (msgs<T>(), states, statesBefore, events — all derived from the one atomic events, so nothing races the fold) and SINGULAR members are values now (state for a unit, entities/[id]/ids for a store — merge-resolved; folded is the unmerged fold truth on both, what guards judge through and what replay snapshots).

Guards read the world only through read(catalog) — the ledger's own folded state by REGENT IDENTITY (the const global or an equal construction name the same row). With canon's generator the graph is annotated (@canon const app = Regency(...)) and each row CLASS gains a read extension, so the same globals answer everywhere: read(catalog) in a judge, catalog.of(context) in a build, catalog.entities now — the generator never invents a name.

Optimism

Optimism is ROWS, never memory machinery — a store's memory holds nothing but its fold. The write dock: a side store holds the pending prediction as honest state (base has no arm for it), a merge edge applies it at read, a guard settles it against echoes by STATE COMPARISON, and a deadline EFFECT dispatches a timeout fact the guard judges like any other. Pending, settled, in-flight, covered — every status a UI could render is a row, so everything replays and confirm/revert/amend orders are statable as laws.

Beyond the fold

  • Events — each store emits one post-fold event per delivered family message (msg, before, after, changed keys): effects observe cause and consequence atomically, so they can never race the fold. Sugar: transitions(), entering(state), on<M>().
  • In-flight as a row — a request fact folds its key in, the answering facts (success or failure) fold it out; presence = loading, read with the same surface as any state. A guard reading it drops duplicate asks; a scope-entry FACT judged by a gate replaces every fetch-on-entry bridge. No machinery, no sidecar.
  • Merges — read-time edges, never copied state: a unit's state answers a keyed surface at its own Identifiable id (merge), or a whole store lends its rows to another's reads through a projection (mergeStore) — the shadow-store pattern: a disk cache folds into its own store and supports the main store's reads until the authority covers.

The one clock, and why there is no effects runner

Regent has exactly ONE clock: the dispatch stream. Nothing else is allowed to mark time — not a Duration parameter in the state tier, not a wall-clock read in a fold. So the first thing every arrival from an effects-as-values framework asks for — a runner with restartable / latest / debounce modes — regent deliberately does not ship. Not for lack of ambition: the modes have no work left to do here.

The reason is that the wire is already a queue of facts, not a Future per call. A network ask isn't a promise you cancel; it's a fact you send. You cannot unsend it — you can only ignore its answer, and ignoring is a judgment, which is pure by nature. So every mode collapses into citizens:

runner mode regent
restartable / latest a CURRENT-intent row + a staleness veto
droppable the in-flight row + its gate
sequential the outbox — an ordered queue already

Search-as-you-type, whole (the wire correlation is the trick — the answer echoes its question, so staleness is judgeable in a pure fold):

final class Search extends Unit<String?, SearchMsg> {   // the CURRENT intent
  const Search() : super(null);
  @override
  String? reduce(String? q, SearchMsg msg) => switch (msg) {
    SearchQueryMsg(:final query) => query,              // newest intent wins
    SearchResultsMsg() => q,
  };
}

/// An answer to a question that is no longer current is dropped for every
/// row below — the whole "cancel the stale request" story, as a judgment.
final class StaleSearchGate extends Veto<SearchResultsMsg> {
  const StaleSearchGate();
  @override
  bool block(SearchResultsMsg msg, ReadStore read) => msg.query != read(search);
}

The effect that remains is a TRANSLATOR — fact in, I/O, fact out. No state, no branches, and its worst possible sin (a stale or duplicate answer) is eaten by the veto above: the edge may be racy, the ledger cannot be.

ledger.at(.exit).msgs<SearchQueryMsg>().listen((msg) async {
  try {
    dispatch(SearchResultsMsg(
        query: msg.query, products: await api.searchProducts(msg.query)));
  } on ApiException {
    dispatch(SearchFailedMsg(query: msg.query));  // failure is a fact too
  }
});

And when a consumer genuinely wants to wait? The timer is a translator like any other edge: wall-time in, ONE fact out — after(d, msg) — and what the elapsed time MEANS is judged in the queue against the present (an epoch or a state check on the due-fact; a settle tick for a superseded epoch is a stale fact like any other). A timer nobody cancels is harmless when its tick is judged. Cancelling is then a cost optimization, never a correctness requirement — and replay() never arms a timer at all, so a law test proves the whole debounce by dispatching the due-facts, with zero real waiting.

What irreducibly stays at the edge: the sites that HOLD a cancellable resource — upload bytes, platform streams, the socket itself. There the decision is still a fold (the saga says cancelled; the connection unit says offline) and the edge merely releases the resource. That is a small, stable, hand-written set — which is why effects stay one honest file.

The edge spends; the queue decides.

Message conventions

The structure prevents most failure modes; message taxonomy discipline prevents the rest. These are the rules the engine can't enforce for you:

  • Messages are facts, not calls. Name an inbound message for the fact it states (ProductLoaded, UsernameTaken), an outbound one for the intent it declares. A message never names its handler.
  • Semantic outcomes, never generic errors. UsernameTaken, not Error("username taken") — an expected outcome is a message the reducer and UI handle like any other fact.
  • One sealed family per entity concern. The family (ProductMsg, CartMsg) is exactly what one store reduces — sealed, so the reduce is exhaustively matched and a new variant is a compile error until every store answers it. NO row reduces the root Msg — a row whose facts cross families (a shadow, a dock, an in-flight unit) declares a sealed GROUP its facts implements (a family base may join a group wholesale), so even a shadow's delegation arm is typed: final UserMsg m => const Users().reduce(rows, m).
  • Guards are pure. A guard reads the world only through read — never dispatches, never touches the world. Placement is semantics: declare guards above the rows they protect.
  • The locality axiom. Every regent invocation is a pure function of (current state, message) — never of why the cursor arrived, what round it is, or what minted what. STORES TRANSFORM STATE AND NOTHING ELSE; GUARDS ENQUEUE CURSORS (at 0 or x+1) AND NOTHING ELSE. History reaches the future only through state, so replay totality is a theorem, provenance is invisible (if causation matters, it goes ON the fact), and every regent is table-testable with (state, msg) pairs — judgments are values.
  • Mints derive, never sequence. A legitimate mint is a fact the fold already implies, restatable as a law about state ("whenever X folds, Y exists"). Sequencing over time belongs to effects; a mint chain past the depth budget throws — a design diagnosis, not a runtime hazard.

Store keys are gradually typed

A store's key type may be the raw codec type or the id's generated extension type — both are always valid, and they are runtime-identical (extension types erase):

final class Products extends Store<String, Product, ProductMsg> { … }     // day one
final class Products extends Store<ProductId, Product, ProductMsg> { … }  // hardened

Write String before the first generation exists (nothing else compiles yet); tighten to ProductId whenever you like — or never. The typed key buys exactly one thing: nominal protection on the store's key axis (products[someUserId] stops compiling). Everything else — verbs, entity fields, derived reads — is typed independently and works the same either way.

Libraries

regent