Records and patterns

Learn records and patterns design.

Records

Records, introduced in Dart 3, are a lightweight data grouping feature. (int, String) holds an integer and a string together. With final pair = (42, 'Hello'); you store two values in a single variable.

Fields are accessed with .$1 and .$2: pair.$1 returns 42. Named fields are also possible: ({int x, int y}). Records are struct-like but inheritance-free and value-based.

Patterns

Patterns, introduced in Dart 3, are a syntax that deconstructs structures like records, lists, and maps. Pattern matching happens in a switch expression: switch (pair) { case (int x, String y): print('$x $y'); }.

Patterns extract values and provide matching. With destructuring, variable binding and guards (when), powerful flow control is possible. Patterns make code more declarative.

Exercise A switch expression with a record pattern

Define a String describePoint(int x, int y) function whose body uses a switch expression: return switch ((x, y)) { (0, 0) => 'Origin', (int a, int b) => '$a,$b' };. In main add print(describePoint(0, 0)); and print(describePoint(3, 4));.

Show example solution

String describePoint(int x, int y) {
  return switch ((x, y)) {
    (0, 0) => 'Origin',
    (int a, int b) => '$a,$b',
  };
}

void main() {
  print(describePoint(0, 0));
  print(describePoint(3, 4));
}

Test yourself

Record field access

final pair = (42, 'Hello');
print(pair.____);

Show answer

final pair = (42, 'Hello');
print(pair.$1);

Match the pattern parts

(int, String)

Record type
case (int x, String y)

Pattern match
$1

First field access
switch

Flow expression
When are records useful?
- To store a single value
- To group multiple values in a lightweight way
- For inheritance instead of a class
- To store async operation results
Show answer

To group multiple values in a lightweight way — records group multiple values in a lightweight, inheritance-free way.

Next lesson: Summary - Language design

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