dart: lazy fibonacci generator

main.dart

void main() {
  final fib = fibStream();
  print(fib.take(10));
}

Iterable<int> fibStream() sync* {
  final firstIt = fromList([0]).followedBy(fibStream());
  final secondIt = fromList([0,1]).followedBy(fibStream());;
  yield* zipWith(sumInt, firstIt, secondIt);
}

Iterable<T> zipWith<T,S>(T Function(S a, S b) f, Iterable<S> a, Iterable<S> b) sync* {
  if (a.isNotEmpty && b.isNotEmpty) {
    final firstVal = f.call(a.first, b.first);
    final firstIt = fromList<T>([firstVal]);
    yield* firstIt.followedBy(zipWith<T,S>(f, tail(a), tail(b)));
  }
}

Iterable<int> limitlessIt (int start) sync* {
  final curIt = Iterable<int>.generate(1, (_) => start);
  yield* curIt.followedBy(limitlessIt(start+1));
}


Iterable<T> fromList<T>(List<T> list) {
  return Iterable<T>.generate(list.length, (i) => list[i]);
}

Iterable<T> tail<T>(Iterable<T> it) {
  return it.skip(1);
}

int sumInt(int a, int b) => a + b;