Pointer-heavy codebases

Most real services are built around a big mutable aggregate behind a relational store: a *Order with twenty fields, loaded by ID, mutated in place, saved in a transaction. Crucible’s kernel wants a value context. The reconciliation is a single rule.

Don’t make the aggregate the context C. Use a small value projection of the state-relevant fields as C. The machine decides transitions and emits effects over that projection; the host applies those effects to its real aggregate at the boundary, inside its existing transaction, using its existing mutation and persistence code.

// Projection: only the fields the machine reasons about.
type OrderView struct {
    Total    int
    PaidCent int
    RushFlag bool
}

func (o *Order) project() OrderView {
    return OrderView{Total: o.Total(), PaidCent: o.Paid(), RushFlag: o.IsRush()}
}

Why value semantics is load-bearing

A value C is not a stylistic preference. It is what makes the kernel’s guarantees hold. Under a value context every step is a pure function of (state, context, event), which is exactly what lets you snapshot an instance, replay it deterministically, persist it durably, and verify it statically (the entire analysis & verification toolbox assumes it). C = *T is an available escape hatch, but choosing it forfeits all of that: a guard or action could mutate shared state under you, and snapshot/replay/verify stop meaning anything. Project a value; keep the guarantees.

The boundary recipe

On the way in, when you hydrate an aggregate that claims to be in some state, use Assay to check it is legally there before you resume driving it:

order := repo.Load(ctx, id)              // hydrated externally
view := order.project()

if err := machine.Assay(order.State(), view); err != nil {
    return fmt.Errorf("order %s not legal in %s: %w", id, order.State(), err)
}

Then the round trip is always the same shape:

inst := machine.Cast(view, state.WithInitialState(order.State()))
res := inst.Fire(ctx, event)             // pure decision over the projection

for _, eff := range res.Effects {        // []Effect: the data the host applies
    apply(order, eff)                    // YOUR mutation, on YOUR aggregate
}
order.SetState(res.NewState)
repo.Save(ctx, order)                    // YOUR persistence, YOUR transaction

flowchart LR
    L[load aggregate] --> P[project to value C]
    P --> A[Assay legal?]
    A --> F[Fire]
    F --> E[apply effects to aggregate]
    E --> S[persist in your txn]

Before: the machine reaches into the aggregate, mutates it, and persistence is tangled into transition logic. After: the machine touches nothing but a value snapshot, hands you effects, and your data layer stays the sole owner of mutation and IO.

This recipe applies effects synchronously at the call site today. The event-driven IO seams let the host stop hand-wiring dispatch: crucible/sink fans effects out to many destinations fire-and-forget (and the state-to-sink bridge makes “fan every transition out” a one-liner), while a broker for pub/sub transport is on the roadmap. The projection-and-apply boundary above is exactly the seam those plug into.