then<R2> method
Returns a parser that sequences the receiver and other, returning their
results as a typed 2-element Record.
Unlike seq, which returns an untyped List<dynamic>, then preserves
the static types of all matched parsers. Chained invocations of then
flatten automatically into larger typed records up to 9 elements.
For example:
// Produces a (String, int) record:
final pair = letter().then(digit().map(int.parse));
// Flattens into a 3-element record (String, int, String):
final triplet = pair.then(letter());
Implementation
@useResult
SequenceParser2<R1, R2> then<R2>(Parser<R2> other) =>
SequenceParser2<R1, R2>(this, other);