Marten and event sourcing (Decide / Evolve)

No package: this is a recipe. The library side is compiled and tested in examples/Nullean.Entities.Integration.Examples/Marten; the session calls are Marten's.

Marten's aggregate conventions (a static Create(event) and instance Apply(event) methods) are plain methods that return the next value, which is exactly what the library wants. Write the decision as a function from state and command to events, fold with the aggregate's own Apply, and the aggregate's own code is what gets tested:

var spec = EntitySpec.EventSourced<Invoice, InvoiceCommand, object>(
    initial, InvoiceDecider.Decide, (invoice, e) => invoice.Evolve(e),
    [new Pay(50m), new Pay(100m), new Void()], InvoiceConstraints.All);

spec.Spec.Exhaustive();
spec.Exhaustive(NoPaymentAfterVoid);

1. the state rules hold after every command sequence
2. so does a temporal rule over the emitted events

In the command handler, run the same check before appending:

var state = await session.Events.AggregateStreamAsync<Invoice>(id);
var events = InvoiceDecider.Decide(state, command).ToList();
var next = events.Aggregate(state, (s, e) => s.Evolve(e));
if (InvoiceConstraints.All.Check(state, next) is { Count: > 0 } violations) return Refuse(violations);
session.Events.Append(id, events);

The exhaustive search found, in Integration/Marten/Invoice.cs, a decide without the overpayment guard and one that pays a voided invoice, each with the shortest command sequence.

To check a recorded stream instead, Trace.Check(events, rules) takes the events as Timestamped<TEvent> (Marten stores a timestamp with each event) and returns every violation of a temporal rule with its time. examples/Apps has an event-sourced cart built on the same pattern, in memory.