Model robust contract state with sealed fields and state machines

Model robust contract state with sealed fields and state machines

Intermediate development begins with stronger state modeling: immutable configuration, explicit states, and controlled transitions.

Learning objectives

• Use sealed fields for immutable configuration.
• Represent multi-step workflows with explicit states.
• Reject illegal transitions in the contract.
• Test the state machine independently from the UI.

Core lesson

The Compact security guide distinguishes ordinary ledger fields from sealed fields. A sealed field is useful for configuration that should not change after initialization.

A state machine turns an informal workflow into a finite set of states and allowed transitions. This matters in auctions, approvals, games, escrow, credential issuance, and other multi-step DApps.

The contract, not the front-end, must enforce transitions. A malicious client can call exported circuits directly even if the normal UI hides an action.

State tests should cover every legal path and attempt illegal calls in each state. This is one of the best ways to find missing authorization or lifecycle checks.

Recommended workflow

1. List every application state.
2. Define allowed transitions.
3. Mark configuration that should be sealed.
4. Encode transition assertions.
5. Write positive and negative transition tests.

Engineering and security note

State machines should reject unexpected transitions even when the UI never exposes those buttons.

Hands-on goal

Build a three-state approval workflow: DRAFT -> SUBMITTED -> APPROVED, with explicit rejection of invalid transitions.

Open-source references

• Midnight Docs - Smart contract security
• Midnight Docs - Test and debug
• Midnight Docs - Compact

Compatibility note: The version numbers in this package are a research snapshot from 2026-09-09. Before running commands, compare them with the current Midnight compatibility matrix and release notes.