Dart 3 Updates Agent Skill

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.

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.

sealed class Shape {}
class Circle extends Shape {}
class Square extends Shape {}

// Dart knows all subtypes — no default needed:
String describe(Shape s) => switch (s) {
  Circle() => 'circle',
  Square() => 'square',
};

Guard clauses

switch (point) {
  case (int x, int y) when x == y:
    print('Diagonal: $x');
  case (int x, int y):
    print('$x, $y');
}

• Add when condition after a pattern to further constrain matching.
• Usable in if-case, switch statements, and switch expressions.
• If the guard is false, execution proceeds to the next case.

2. Patterns

Patterns represent the shape of a value for matching and destructuring.

Uses

// Variable declaration
var (a, [b, c]) = ('str', [1, 2]);

// Variable assignment (swap)
(b, a) = (a, b);

// for-in loop destructuring
for (final MapEntry(:key, :value) in map.entries) { ... }

// switch / if-case (see Branches section)

• Wildcard _ ignores parts of a matched value.
• Rest elements (...) in list patterns ignore remaining elements.
• Case patterns are refutable: if no match, execution continues to the next case.
• Destructured values in a case become local variables scoped to that case body.

Object patterns

var Foo(:one, :two) = myFoo;

JSON / nested data validation

if (data case {'user': [String name, int age]}) {
  print('$name, $age');
}

3. Pattern Types

| Pattern | Syntax | Description |

|---|---|---|

| Logical-or | p1 \|\| p2 | Matches if any branch matches. All branches must bind the same variables. |

| Logical-and | p1 && p2 | Matches if both match. Variable names must not overlap. |

| Relational | == c, < c, >= c | Compares value to a constant. Combine with && for ranges. |

| Cast | subpattern as Type | Asserts type, then matches inner pattern. Throws if type mismatch. |

| Null-check | subpattern? | Matches non-null; binds non-nullable type. |

| Null-assert | subpattern! | Matches non-null or throws. Use in declarations to eliminate nulls. |

| Constant | 42, 'str', const Foo() | Matches if value equals the constant. |

| Variable | var name, final Type name | Binds matched value to a new variable. Typed form only matches the declared type. |

| Wildcard | _, Type _ | Matches any value without binding. |

| Parenthesized | (subpattern) | Controls precedence. |

| List | [p1, p2] | Matches lists by position. Length must match unless a rest element is used. |

| Rest element | ..., ...rest | Matches arbitrary-length tails or collects remaining elements. |

| Map | {'key': subpattern} | Matches maps by key. Missing keys throw StateError. |

| Record | (p1, p2), (x: p1, y: p2) | Matches records by shape; field names can be omitted if inferred. |

| Object | ClassName(field: p) | Matches by type and destructures via getters. Extra fields ignored. |

• Use parentheses to group lower-precedence patterns.
• All pattern types can be nested and combined.

4. Records

// Create
var record = ('first', a: 2, b: true, 'last');

// Type annotation
({int a, bool b}) namedRecord;

// Access
print(record.$1);   // positional: 'first'
print(record.a);    // named: 2

• Records are anonymous, immutable, fixed-size aggregates.
• Each field can have a different type (heterogeneous).
• Fields are accessed via built-in getters ($1, $2, .name); no setters.
• Two records are equal if they have the same shape and equal field values.
• hashCode and == are automatically defined.

Multiple return values

(String name, int age) userInfo(Map<String, dynamic> json) {
  return (json['name'] as String, json['age'] as int);
}

var (name, age) = userInfo(json);
// Named fields:
final (:name, :age) = userInfo(json);

Records vs. data classes

Use a record when:

• Returning multiple values from a single function (small, one-time use).
• Grouping a few values locally with no reuse across the codebase.
• You need structural equality with no additional behavior.

Use a class when:

• The type is reused across multiple files or features.
• You need methods, encapsulation, inheritance, or copyWith.
• The type is part of a public API or long-lived data model.
• Changing the shape must be caught by the type system across the codebase.

Other best practices

• Use typedef for record types to improve readability and maintainability.
• Changing a record type alias does not guarantee type safety across the codebase — only classes provide full abstraction.

5. Migration Workflow

When modernizing pre-Dart-3 code, follow these steps:

Step 1 — Replace if-else chains with switch expressions

// Before (pre-Dart 3)
String label;
if (status == Status.loading) {
  label = 'Loading...';
} else if (status == Status.success) {
  label = 'Done';
} else {
  label = 'Error';
}

// After (Dart 3)
final label = switch (status) {
  Status.loading => 'Loading...',
  Status.success => 'Done',
  Status.error => 'Error',
};

Step 2 — Convert abstract class hierarchies to sealed classes

// Before
abstract class Result {}
class Success extends Result { final String data; Success(this.data); }
class Failure extends Result { final String error; Failure(this.error); }

// After — enables exhaustive switch
sealed class Result {}
final class Success extends Result { const Success(this.data); final String data; }
final class Failure extends Result { const Failure(this.error); final String error; }

Step 3 — Use destructuring for multiple return values

Replace wrapper classes used solely for returning multiple values with records.

Step 4 — Validate

Run dart analyze to confirm exhaustiveness and type safety after each change.