Sealed Classes

Sealed Classes

About

• Sealed classes define a fixed hierarchy of subtypes.
• Enable exhaustive pattern matching.
• Compile-time safety for state modeling.
• Alternative to traditional inheritance.

Main Topics

1. Basic Sealed Class

   • Definition: sealed modifier with subtypes.
   • Example:

     sealed class Result {}
     class Success extends Result {}
     class Failure extends Result {}

2. Pattern Matching

   • Definition: Exhaustive switch expressions.
   • Example:

     String handleResult(Result r) => switch(r) {
       Success() => 'Success',
       Failure() => 'Failure'
     };

3. State Modeling

   • Definition: Representing finite states.
   • Example:

     sealed class AuthState {}
     class LoggedIn extends AuthState {}
     class LoggedOut extends AuthState {}

4. Algebraic Types

   • Definition: Product and sum types.
   • Example:

     sealed class Shape {}
     class Circle extends Shape {}
     class Rectangle extends Shape {}

How to Use

• Declare: Use sealed modifier
• Extend: Define all allowed subtypes
• Match: Use with switch expressions
• Exhaust: Compiler checks completeness

How It Works

• Hierarchy: Fixed at compile-time
• Patterns: Compiler knows all cases
• Safety: Prevents missing cases
• Performance: Optimized by compiler

Example Session:

void main() {
  Result res = Success();
  print(handleResult(res)); // Compiler knows all cases
}

Conclusion

Sealed classes provide compiler-verified state modeling through restricted hierarchies, enabling exhaustive pattern matching and robust domain modeling in Dart applications.