Records and patterns
- Dart course
- Expert
- Lesson 5/47
- 5 min
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 $y'); }.
Patterns extract values and provide matching. With destructuring, variable binding and guards (when), powerful flow control is possible. Patterns make code more declarative.
Practice A switch expression with a record pattern
Define a String describePoint(int x, int y) function whose body uses a switch expression: return switch ((x, y)) { (0, 0) => 'Origin', (int a, int b) => '$a,$b' };. In main add print(describePoint(0, 0)); and print(describePoint(3, 4));.
Starter code
void main() {
// define describePoint function (switch + record pattern)
// print calls for two points
}Sample solution
String describePoint(int x, int y) {
return switch ((x, y)) {
(0, 0) => 'Origin',
(int a, int b) => '$a,$b',
};
}
void main() {
print(describePoint(0, 0));
print(describePoint(3, 4));
}Expected output
Origin 3,4
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Record field access
final pair = (42, 'Hello');
print(pair.____);final pair = (42, 'Hello');
print(pair.$1);Explanation: pair.$1 returns the first field (42).
2 Match the pattern parts
| Term | Meaning |
|---|---|
(int, String) |
Record type |
case (int x, String y) |
Pattern match |
$1 |
First field access |
switch |
Flow expression |
3 When are records useful?
- To store a single value
- To group multiple values in a lightweight way Correct answer
- For inheritance instead of a class
- To store async operation results
Explanation: records group multiple values in a lightweight, inheritance-free way.