Nested class declaration

[feduke-nukem]

Proposal: Opportunity to declare classes inside other classes like in Kotlin:

sealed class SomeState<T>{
    class Loading<T> : SomeState<T>()

    data class Success<T>(val value:T) : SomeState<T>()

    data class Error<T>(val error: Throwable) : SomeState<T>()
}

fun handle(state: SomeState<Int>){
    when(state){
        is SomeState.Error -> TODO()
        is SomeState.Loading -> TODO()
        is SomeState.Success -> TODO()
    }
}

Dart:

sealed class SomeState<T> {
  class Loading<T> extends SomeState<T> {}

  class Success<T> extends SomeState<T> {
    final T value;

    Success(this.value);
  }

  class Error<T> extends SomeState<T> {
    final Object error;

    Error(this.error);
  }
}

void handle(SomeState<int> state) {
  switch (state) {
    case SomeState.Loading():
    case SomeState.Success():
    case SomeState.Error():
  }
}

Use case: to increase readability especially for sealed classes

Without such syntax we should always add "noisy" prefix/suffix to sealed subclasses' names especially when super class has long name:

sealed class SomeVeryVeryVeryLongNamedState<T> {}

class SomeVeryVeryVeryLongNamedStateLoading<T>
    extends SomeVeryVeryVeryLongNamedState<T> {}

class SomeVeryVeryVeryLongNamedStateSuccess<T>
    extends SomeVeryVeryVeryLongNamedState<T> {
  final T value;

  SomeVeryVeryVeryLongNamedStateSuccess(this.value);
}

class SomeVeryVeryVeryLongNamedStateError<T>
    extends SomeVeryVeryVeryLongNamedState<T> {
  final Object error;

  SomeVeryVeryVeryLongNamedStateError(this.error);
}

void handle(SomeVeryVeryVeryLongNamedState<int> state) {
  switch (state) {
    case SomeVeryVeryVeryLongNamedStateLoading():
    case SomeVeryVeryVeryLongNamedStateSuccess():
    case SomeVeryVeryVeryLongNamedStateError():
  }
}

[dart-github-bot]

Summary: This issue proposes adding support for nested class declarations in Dart, similar to Kotlin. The goal is to improve readability, especially for sealed classes, by eliminating the need for verbose prefixes or suffixes in subclass names.

[julemand101]

This is not a Dart SDK issue but instead an issue for the Dart Specification issue tracker which can be found at: https://github.com/dart-lang/language/issues

Your issue looks like a duplicate of: dart-lang/language#336