Support "const" constructors for Build collections

[RobertSzymacha]

Having const constructors would make it possible to use it with named, optional parameters (with default values)

[davidmorgan]

Thanks.

With the current model of const, this is hard--there's no way to do any processing, so we can't calculate and store the hash code, for example. (Usually the hashcode is lazily calculated). We'd have to have an alternative implementation just for const.

Hopefully relaxing the rules around const (or around defaults) comes up as a language change at some point.

[elliottmb]

@davidmorgan Given the expansion of const in the last two releases of dart, is this potentially back on the table?

[davidmorgan]

I'm afraid not; the same problems remain, we'd have to provide multiple implementations.

[elliottmb]

What about at least the addition of an empty constructor that is const? I have a number of classes that currently null coalesce an optional field. They can't be const due to it being an empty BuiltList. I don't want to pass that burden onto the user by making it a required one they must pass an empty instance to.

[davidmorgan]

I think we could have a separate named const constructor, e.g. BuiltList.empty. But that's pretty verbose.

[elliottmb]

While verbose, it is not unheard of in the dart community. Stream.empty() for streams, Observable.empty() in rxdart, and a number of others.

Just tossing it out there, as I selfishly would benefit from this smaller case. Ideally having const constructors for all cases would be nice, but given the non-final _hashCode I see the dilema.

[davidmorgan]

Yes, I see that it's an awkward corner.

Personally I prefer not to use optional named parameters precisely because defaults don't work well. built_value is designed to fill that corner of the language--builders can correctly handle whether something is passed or not.

I'm still hopefully that the language will improve things here.

[kentcb]

Arrived here looking for exactly the same thing: const BuiltList<whatever>.empty(), or perhaps BuiltList<whatever>.empty as a static const field.

[knaeckeKami]

I currently have a file with

final emptyBuiltList = BuiltList.of(const <Null>[]);

final emptyBuiltSet = BuiltSet.of(const <Null>[]);

I need empty lists/sets in a couple of places (e.g. initialization, resetting the state).

I was annoyed by having to specify by constructing a new empty collection each then, and by specifying the type parameter for the empty list.

By using the Null type (which is a bottom type) , I can avoid creating unnecessary new objects, and specifying the type parameters for empty collections and probably gain a tiny bit of performance, too.

BuiltList<int> emptyInts = emptyBuiltList; compiles and runs fine.

Maybe it would be nice if this was included in built_collection, but I don't mind having a file with a couple of one-liners either.

[davidmorgan]

Thanks!

I think you'll run into trouble if you call toBuilder on those, though? Which might actually make them dangerous if you pass them to another method that does that.

[knaeckeKami]

Wow, you're right. Didn't run into this yet.
Don't use this code.

[davidmorgan]

I realized only recently that toBuilder should be toBuilder<T> where you specify which type you want to be able to add :/

[cbenhagen]

Added a separate issue for the special case of an empty const. I think there are many use cases where this might be helpful.