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Dart Record Destructuring

Record destructuring unpacks a record into separate variables in one declaration. The pattern names each part, so the values are ready to use at once.

Unpacking a returned record

describe returns a record with a number and a word. The declaration pattern unpacks both parts into value and word.

(int, String) describe(int n) {
  if (n < 0) {
    return (n, 'negative');
  }
  return (n, 'ok');
}

void main() {
  var (value, word) = describe(5);
  print('$value $word');
}

Swapping two values

a starts at 1 and b starts at 2. The pattern assignment rebuilds the pair in the other order, so the print shows the swapped values.

void main() {
  var a = 1;
  var b = 2;
  (a, b) = (b, a);
  print('a $a b $b');
}

Named fields with the shorthand

named holds two named fields. The shorthand pattern binds x and y variables with the same names as the fields.

void main() {
  var named = (x: 3, y: 4);
  var (:x, :y) = named;
  print('x $x y $y');
}

Destructuring in a for-in loop

pairs holds two records. Each loop step unpacks the current record into n and word before the print runs.

void main() {
  var pairs = [(1, 'one'), (2, 'two')];
  for (var (n, word) in pairs) {
    print('$n $word');
  }
}

Example

void main() {
  var (first, :label) = (9, label: 'nine');
  print('$first $label');
  var (amount, :unit) = (250, unit: 'metres');
  print('$amount $unit');
}

Key points

  • Destructuring unpacks a record into named variables in one line.
  • A positional pattern binds the fields in order, from first to last.
  • A named pattern uses the shorthand (:x, :y) to bind variables that share the field names.
  • A pattern assignment such as (a, b) = (b, a) swaps two values with no extra variable.
  • Destructuring also works in a for-in loop over a list of records.

In this example, each declaration mixes one positional field with one named field. The first pattern binds 9 to first and nine to label, so the first print shows 9 nine. The second pattern binds 250 to amount and metres to unit, so the second print shows 250 metres.

Note

The pattern must match the shape of the record. A pattern with the wrong fields does not compile.

Try it Yourself

Edit the code, then press Run.

Running is simulated · DartPad

Exercise

What does var (a, b) = (10, 20); do?