Behavior Layer Reference¶

State machines and workflows in USL.

Behavior Definition¶

behavior OrderLifecycle for Order {
  initial state Pending

  state Pending {
    on confirm -> Confirmed
  }

  state Confirmed {
    // ...
  }
}

Transitions¶

Basic Transition¶

on event -> TargetState

With Guards¶

on confirm -> Confirmed
  requires this.hasPayment && this.hasAddress

With Effects¶

on confirm -> Confirmed
  effects {
    this.status = Status.Confirmed
    this.confirmedAt = now()
  }

With Events¶

on confirm -> Confirmed
  effects {
    emit OrderConfirmed(this.id)
  }

Nested States¶

state Active {
  state Working {
    on pause -> Paused
  }

  state Paused {
    on resume -> Working
  }

  on deactivate -> Inactive
}

Parallel States¶

parallel state Processing {
  region Payment {
    state PendingPayment
    state Paid
  }

  region Shipping {
    state PendingShipment
    state Shipped
  }
}

Verification¶

USL verifies: - All transitions preserve invariants - Initial state is valid - No unreachable states - Deterministic transitions

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