Guards

A guard is a condition that must hold for a transition to fire. Crucible offers two complementary styles, and both must pass when present.

Named guards reference a function registered by name; the params curry one generic guard into many specific ones. Chain multiple When calls and they are AND-ed together:

Transition(Authorizing).On(Authorized).
    When("minSubtotal", map[string]any{"floor": 1500}).
    When("inServiceArea").
    GoTo(Active).Assign("recordHold")

Expression guards build a boolean tree from declarative nodes, with no Go callback required, so they serialize cleanly. Combine state.And, state.Or, and state.Not with field predicates and state.StateIn:

generous := state.Field[Stage]("subtotal").Ge(state.Int[Stage](5000))

Transition(Authorizing).On(Authorized).
    WhenExpr(state.Or(
        generous,
        state.And(
            state.Field[Stage]("subtotal").Ge(state.Int[Stage](2000)),
            state.Field[Stage]("priority").In(
                state.Str[Stage]("fast"), state.Str[Stage]("express"),
            ),
        ),
    )).
    GoTo(Active).Assign("recordHold")

Field predicates support .Eq, .Ne, .Lt, .Le, .Gt, .Ge, and .In, with operands built from state.Str, state.Int, state.Float, and state.Bool. state.StateIn(s) lets a guard depend on the active configuration. When a transition carries both When and WhenExpr, every named guard and the whole expression must evaluate true.

stateDiagram-v2
    Authorizing --> Active : Authorized [generous OR fast-lane]
    Authorizing --> Rejected : Declined

Guards are pure and side-effect free: they read context, they never mutate it. Keep the decision in the guard and the consequence in an assign or effect.