sync* generators in Dart

Lazy Iterables: nothing runs until something pulls.

// sync* builds a lazy Iterable; nothing runs until something pulls.
Iterable<int> fibonacci() sync* {
  var a = 0, b = 1;
  while (true) {
    yield a;
    (a, b) = (b, a + b);
  }
}
Iterable<String> numbered(Iterable<String> items) sync* {
  var n = 1;
  for (final item in items) {
    yield '$n. $item';
    n++;
  }
}

void main() {
  print(fibonacci().take(8).toList());
  print(numbered(['scan', 'count', 'log']).join('  '));
}

How it works

1. sync* plus yield builds an Iterable on demand.
2. An infinite while (true) is safe — take(8) pulls just eight.
3. (a, b) = (b, a + b) swaps through a record, no temp variable.

Keywords and builtins used here

• String
• final
• for
• in
• int
• sync
• var
• void
• while
• yield

The run, in numbers

Lines

20

Characters to type

402

Tokens

135

Three-star pace

65 tpm

At the three-star pace of 65 tokens a minute, this run takes about 125 seconds.

Type this snippet

Step 3 of 3 in Async, step 24 of 27 in Language basics.

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