dart-3-updates

Dart 3 Updates Skill

Apply Dart 3 language features — branches, patterns, pattern types, and records — correctly and idiomatically.

When to Use

Use this skill when:

• Writing or refactoring switch statements or if-else chains.
• Creating new data-holding classes and deciding between sealed classes, records, or plain classes.
• Destructuring values from maps, lists, records, or objects.
• Modernizing pre-Dart-3 code to use patterns, exhaustiveness checks, or switch expressions.

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1. Branches

if / if-case

// Standard if
if (score >= 90) {
  grade = 'A';
} else if (score >= 80) {
  grade = 'B';
} else {
  grade = 'C';
}

// if-case: match and destructure against a single pattern
if (pair case [int x, int y]) {
  print('$x, $y');
}

• if conditions must evaluate to a bool.
• In if-case, variables declared in the pattern are scoped to the matching branch.
• If the pattern does not match, control flows to the else branch (if present).

switch statements

switch (command) {
  case 'quit':
    quit();
  case 'start' || 'begin': // logical-or pattern
    startGame();
  default:
    print('Unknown command');
}

• Each matched case body executes and jumps to the end — break is not required.
• Non-empty cases can end with continue, throw, or return.
• Use default or _ to handle unmatched values.
• Empty cases fall through; use break to prevent fallthrough in an empty case.
• Use continue with a label for non-sequential fallthrough.
• Use logical-or patterns (case a || b) to share a body between cases.

switch expressions

final color = switch (shape) {
  Circle() => 'red',
  Square() => 'blue',
  _ => 'unknown',
};

• Omit case; use => for bodies; separate cases with commas.
• Default must use _ (not default).
• Produces a value.

Exhaustiveness

• Dart checks exhaustiveness in switch statements and expressions at compile time.
• Use default/_, enums, or sealed types to satisfy exhaustiveness.