Sealed Types & Error Handling

Use Sealed Types & Error Handling to express stable APIs and exhaustive domain models. Prefer composition, sealed hierarchies, constrained generics, and focused extensions while minimizing hidden mutation and unchecked casts.

Build a Complete Mental Model

Use Sealed Types & Error Handling to express stable APIs and exhaustive domain models. Prefer composition, sealed hierarchies, constrained generics, and focused extensions while minimizing hidden mutation and unchecked casts.

Apply It in Modern Dart

Run and test an original Sealed Types & Error Handling example with null, empty, boundary, invalid, async-error, and cancellation cases. Use dart format, dart analyze, and dart test, and verify Future or Stream subscriptions and isolate boundaries where relevant.

After This Lesson

• Explain nullability, mutability, and async contracts
• Write an example that passes dart analyze
• Test a failure or async case

sealed class LoadState {}
class Loading extends LoadState {}
class Ready extends LoadState { Ready(this.items); final List<String> items; }
class Failed extends LoadState { Failed(this.message); final String message; }

String label(LoadState state) => switch (state) {
  Loading() => 'Loading',
  Ready(items: final items) => 'Ready: ${items.length}',
  Failed(message: final message) => 'Error: $message',
};

void main() => print(label(Ready(['A', 'B'])));

Ready: 2

Dart class modifiers — Dart