Built Union: sum-types for Dart

built_union.dart introduces Union values, also known as Tagged unions (C), Enums (Rust), Sum types (Haskell).

A Union value is type that has a few different variants.
At any given time, the Union value has a value of a certain single variant.

You want to use Built Union if you need to represent a type that means:
“Either this or that, but not both at the same time”.

Fully compatible with BuiltValue.

Created as part of the Offst project.

How to use?

Add to your pubspec.yaml file:

dependencies:
    meta: ^1.1.0
    built_union: ^0.1.0
    # built_value: ^7.0.6
dev_dependencies:
    built_union_generator: ^0.1.0
    # built_value_generator: ^7.0.8 
    build_runner: ^1.0.0

Note that versions might differ from this example, so make sure to use the
correct versions by checking at pub.dev.

You will most likely need built_value and built_value_generator for the
creation of BuiltValues and serialization. If you don’t, you can omit those
dependencies.

Examples

Quick links:

• Definitions example
• Usage example

Consider the following SimpleUnion definition:

@BuiltUnion()
class SimpleUnion extends _$SimpleUnion {
  static Serializer<SimpleUnion> get serializer => _$simpleUnionSerializer;
  SimpleUnion.empty() : super.empty();
  SimpleUnion.integer(int integer) : super.integer(integer);
  SimpleUnion.tuple(int tupleInt, String tupleString)
      : super.tuple(tupleInt, tupleString);
  SimpleUnion.string(String string) : super.string(string);
  SimpleUnion.builtList(BuiltList<int> builtList) : super.builtList(builtList);
}

The definition above means that SimpleUnion always must be in one of a few states:

• empty
• integer
• tuple
• string
• builtList

Some examples for instantiating SimpleUnion:

final simpleUnionEmpty = SimpleUnion.empty();
final simpleUnionInteger = SimpleUnion.integer(3);
final simpleUnionTuple = SimpleUnion.tuple(4, 'four');
final simpleUnionString = SimpleUnion.string('String');
final simpleUnionBuiltList = SimpleUnion.builtList(BuiltList([1,2,3,4]));

Serialization

Built Union supports json serialization.
Example:

final simpleUnions = [
  SimpleUnion.empty(),
  SimpleUnion.integer(3),
  SimpleUnion.tuple(4, 'four'),
  SimpleUnion.string('string'),
  SimpleUnion.builtList(BuiltList([1,2,3,4])),
];
for (final simpleUnion in simpleUnions) {
  final serialized = serializersWithPlugin.serialize(simpleUnion, specifiedType: FullType(SimpleUnion));
  JsonEncoder encoder = new JsonEncoder.withIndent('  ');
  print(encoder.convert(serialized));
  final simpleUnion2 = serializersWithPlugin.deserialize(serialized, specifiedType: FullType(SimpleUnion));
  expect(simpleUnion, simpleUnion2);
}

Resulting json:

"empty"
{
  "integer": 3
}
{
  "tuple": [
    4,
    "four"
  ]
}
{
  "string": "string"
}
{
  "builtList": [
    1,
    2,
    3,
    4
  ]
}

Unimplemented features

• Generics

Hacking guide

• Run tool/presubmit to format, build, analyze and test all packages.
• To work locally, run tool/local_deps enable. This will make sure that
  dependencies point to your local copy, and not to a remote copy. To restore
  remote dependencies, run tool/local_deps disable.

Thanks

Special thanks to David Morgan for the
guidance during the development of this package.

See the original issue
that led to the creation of this package.