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Type-safe, boilerplate-free union types for Dart



Add the following to you pubspec.yaml:

  union: ^1.0.0


A union is a class that holds a value which can be of different types, and allow manipulating that value in a type-safe way.

Unions comes in different variants, where the class name "Union" is suffixed by a number (such as Union2 or Union3). That number represents the number of different types the value stored can take.

For example, if a value can be both a String and a int, that makes two different types. We can therefore use Union2 like so:

Union2<String, int> myUnion;


Union2<int, String> myUnion;

Note that we cannot assign Union2<int, String> to Union2<String, int> (or the opposite).

Assigning a value to a union

We've seen previously that a Union2<String, int> stores a value that is either a String or an int.

But such union cannot contain a type that is neither String nor int.

As such, to ensure the type safety, instead of a single constructor that take the current value of any given type, unions have multiple constructor: One constructor for each type.

For example, Union2<String, int> expose two constructors:

  • new Union2.first(String value)
  • new Union2.second(int value)

Whereas Union4<String, int, double, List<int>> have four constructors:

  • new Union4.first(String value)
  • new Union4.second(int value)
  • new Union4.third(double value)
  • new Union4.forth(List<int> value)

We can therefore do:

Union2<String, int> myUnion;

myUnion = Union2.first('hello world');
myUnion = Union2.second(42);

Reading the value

Unions expose the current value publicly through value.

But since the value can be of different types, a small trick is necessary to preserve type safety:

On a Union2<SomeClass, AnotherClass>, the type of value is neither of type SomeClass nor of type AnotherClass. Instead it has the type of their nearest common interface.

Union2<String, int> example;
// String and int have no shared, so value is of type Object
Object value = example.value;
int impossible = example.value; // COMPILE ERROR, not type safe

Union2<int, double> example2;
// int and double have a common interface: num. Therefore is of type num
num value2 = example2.value; // type safe, there's no cast or compile error.
int stillImpossible = example2.value; // COMPILE ERROR, value is a num not a int

Union2<String, String> example3;
// The type is always a String, so value is a String too
String value3 = example3.value; // type safe, there's no cast or compile error.

NOTE: Such powerful type inference is made possible only thanks to type extension.

Applying operations on the value

To prevent mistakes, it is not recommended to use the is operator to determine what the value currently is.


Union2<String, int> union;
if (union.value is String) {
  print('String: ${union.value}');
} else if (union.value is int) {
  print('int: ${union.value}');

This is a bad practice because it is easy to forget to handle one of the potential types that the value can take.

Instead, use one of the availble methods on unions, such as:

  • map, which transforms the current value in another value.
  • forEach, which allows performing some logic based on the value type.
  • join, which fuse all the different types in a single type.


Union2<String, int> union;
  (String value) => print('String: $value'),
  (int value) => print('int: $value'),

This is better because the code will not compile if we forgot to handle one of the potential types that value can take.

Making a reusable union

Sometimes, we have a specific combination of types that we want to want to reuse all the time.

In such case, having to specify the generic types of a Union can be a bit verbose. Similarly, the constructors name (first, second, ...) may not be ideal.

In that situation, what we can do is subclassing the desired Union, as followed:

class AsyncState<T> extends Union3<T, Loading, Exception> {
    const AsyncState.value(T value): super.first(value);
    const AsyncState.loading(): super.second(const Loading());
    const AsyncState.exception(Exception error): super.third(error);

This allows us to write:

final state = AsyncState<int>.value(42);
final loading = const AsyncState<int>.loading();

instead of:

final state = Union3<int, Loading, Exception>.first(42);
final loading = const Union3<int, Loading, Exception>.second(const Loading());

Then, we may want to write an extension on the original Union, to easily convert raw unions to our custom union:

extension ToAsyncState<T> on Union3<T, Loading, Exception> {
  AsyncState<T> toAsyncState() {
    return join(
      (v) => AsyncState.value(v),
      (_) => const AsyncState.loading(),
      (v) => AsyncState.exception(v),

This allows us to write:

var state = AsyncState<int>.value(42);
state = state
      (v) => v * 2,
      (v) => v,
      (v) => v,