recursive functions within the class - dart

Question

The below code worked fine with me while it is a stand a lone function
chunkBySize(List list, int size) => list.isEmpty
? list
: ([list.take(size)]..addAll(chunkBySize(list.skip(size), size)));
And I was able to call it smoothly as:
void main() {
var list = new List();
list.addAll([1,2,3]);
print(chunkBySize(list, 2));
}
but While trying to use it in a structure it failed with me, so I forced to write in the below way:
import 'dart:collection';
class functionalList<E> extends ListBase<E> {
final List<E> l = []; // or List l = new List();
functionalList();
.
.
.
chunkBySize(int size) => _chunkBySize(l, size);
_chunkBySize(List list, int size) => list.isEmpty
? list
: ([list.take(size)]..addAll(_chunkBySize(list.skip(size), size)));
}
and was able to call it by:
void main() {
var list = new functionalList();
list.addAll([1,2,3]);
print(list.chunkBySize(2));
}
Is there a way to simplify it within the class boy, i.e. replacing the below by single statement:
chunkBySize(int size) => _chunkBySize(l, size);
_chunkBySize(List list, int size) => list.isEmpty
? list
: ([list.take(size)]..addAll(_chunkBySize(list.skip(size), size)));

Answer 1

You can declare a local helper function inside the chunkBySize method.
class FunctionalList<E> extends ListBase<E> {
...
List<Iterable<E>> chunkBySize(int size) {
List<Iterable<E>> helper(Iterable<E> list, int size) => list.isEmpty
? const []
: ([list.take(size)]..addAll(_chunkBySize(list.skip(size), size);
return helper(this, size);
}
}
I don't find this particularly more readable than having an external helper function, but in this case it actually gives you better typing.
I'd probably write the function as:
List<Iterable<E>> chunkBySize(int size) {
List<Iterable<E>> result = [];
for (int i = 0; i < this.length; i += size) {
result.add(this.getRange(i, min(i + size, this.length)));
}
return result;
}
Is both easier to read and more efficient.
The recursive function that repeatedly callas addAll will have time complexity that is quadratic in the length of the original list because it keeps copying the elements from one list to a another.