Add const BuiltList<T>.empty()

[cbenhagen]

This would allow us to use it as a default for a named parameter.

Related to https://github.com/google/built_collection.dart/issues/164 which is blocked by missing language features.

[davidmorgan]

Unfortunately this is not as straightforward as it sounds.

BuiltList computes hashCode lazily, so it can't be const. We would have to provide a different implementation class just for the empty const, which likely has other implications.

I realize the lack of const is annoying, I just haven't figured out a good way around it yet :/

One approach for named parameters would be to do

doSomething({Iterable<T> values = const []}) {
  values = BuiltList.of(values);
}

[cbenhagen]

Thank you for the workaround. Unfortunately this would not work for freezed classes. What other implications would you expect with a different implementation for the empty special case? Just wondering, I am sure you have put a lot of thought in it already.

[davidmorgan]

If a compiler notices that there is only a single implementation of a class in the whole program it can optimize--turning dynamic calls into static calls. I suspect that would apply here.

built_value supports defaulting collections from built_collection to empty collections, maybe freezed could/should do that same?

[derolf]

As a workaround, I do:

import 'package:built_collection/built_collection.dart';

// ignore: avoid_implementing_value_types
class EmptyBuiltList<E> implements BuiltList<E> {
  const EmptyBuiltList();

  BuiltList<E> get _list => BuiltList<E>();

  @override
  BuiltList<E> operator +(BuiltList<E> other) => _list + other;

  @override
  E operator [](int index) => _list[index];

  @override
  bool any(bool Function(E p1) test) => _list.any(test);

  @override
  List<E> asList() => _list.asList();

  @override
  Map<int, E> asMap() => _list.asMap();

  @override
  Iterable<T> cast<T>() => _list.cast<T>();

  @override
  bool contains(Object? element) => _list.contains(element);

  @override
  E elementAt(int index) => _list.elementAt(index);

  @override
  bool every(bool Function(E p1) test) => _list.every(test);

  @override
  Iterable<T> expand<T>(Iterable<T> Function(E p1) f) => _list.expand(f);

  @override
  E get first => _list.first;

  @override
  E firstWhere(bool Function(E p1) test, {E Function()? orElse}) => _list.firstWhere(test, orElse: orElse);

  @override
  T fold<T>(T initialValue, T Function(T p1, E p2) combine) => _list.fold(initialValue, combine);

  @override
  Iterable<E> followedBy(Iterable<E> other) => _list.followedBy(other);

  @override
  void forEach(void Function(E p1) f) => _list.forEach(f);

  @override
  Iterable<E> getRange(int start, int end) => _list.getRange(start, end);

  @override
  int indexOf(E element, [int start = 0]) => _list.indexOf(element, start);

  @override
  int indexWhere(bool Function(E p1) test, [int start = 0]) => _list.indexWhere(test, start);

  @override
  bool get isEmpty => _list.isEmpty;

  @override
  bool get isNotEmpty => _list.isNotEmpty;

  @override
  Iterator<E> get iterator => _list.iterator;

  @override
  String join([String separator = '']) => _list.join(separator);

  @override
  E get last => _list.last;

  @override
  int lastIndexOf(E element, [int? start]) => _list.lastIndexOf(element, start);

  @override
  int lastIndexWhere(bool Function(E p1) test, [int? start]) => _list.lastIndexWhere(test, start);

  @override
  E lastWhere(bool Function(E p1) test, {E Function()? orElse}) => _list.lastWhere(test, orElse: orElse);

  @override
  int get length => _list.length;

  @override
  Iterable<T> map<T>(T Function(E p1) f) => _list.map(f);

  @override
  BuiltList<E> rebuild(Function(ListBuilder<E> p1) updates) => _list.rebuild(updates);

  @override
  E reduce(E Function(E p1, E p2) combine) => _list.reduce(combine);

  @override
  Iterable<E> get reversed => _list.reversed;

  @override
  E get single => _list.single;

  @override
  E singleWhere(bool Function(E p1) test, {E Function()? orElse}) => _list.singleWhere(test, orElse: orElse);

  @override
  Iterable<E> skip(int n) => _list.skip(n);

  @override
  Iterable<E> skipWhile(bool Function(E p1) test) => _list.skipWhile(test);

  @override
  BuiltList<E> sublist(int start, [int? end]) => _list.sublist(start, end);

  @override
  Iterable<E> take(int n) => _list.take(n);

  @override
  Iterable<E> takeWhile(bool Function(E p1) test) => _list.takeWhile(test);

  @override
  ListBuilder<E> toBuilder() => _list.toBuilder();

  @override
  BuiltList<E> toBuiltList() => _list.toBuiltList();

  @override
  BuiltSet<E> toBuiltSet() => _list.toBuiltSet();

  @override
  List<E> toList({bool growable = true}) => _list.toList(growable: growable);

  @override
  Set<E> toSet() => _list.toSet();

  @override
  Iterable<E> where(bool Function(E p1) test) => _list.where(test);

  @override
  Iterable<T> whereType<T>() => _list.whereType<T>();
}

That way, I can pass it to freezed as default value:

@freezed
abstract class Foo with _$Foo {
  factory Foo({
    @JsonKey() @Default(EmptyBuiltList<int>()) BuiltList<int> bars,
  }) = _Foo;

  factory Foo.fromJson(Map<String, dynamic> json) => _$FooFromJson(json);
}

The annoying piece is that json_serializers don't allow non-literal default values in @JsonKey...

[derolf]

Unfortunately this is not as straightforward as it sounds.

BuiltList computes hashCode lazily, so it can't be const. We would have to provide a different implementation class just for the empty const, which likely has other implications.

You could store the _hashcode in an expando.

[davidmorgan]

Closing in favour of #164