Either class in dart to hold Either Left or Right value. useful for holding data OR error.

either.dart

// MIT License
//
// Copyright (c) 2025 Nidal Bakir
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in all
// copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
// SOFTWARE.
//

// Inspiration from functional programming

abstract class Either<L, R> {
  B fold<B>(B Function(L l) ifLeft, B Function(R r) ifRight);

  bool isLeft();
  bool isRight();

  Left<L, R> getLeft();
  Right<L, R> getRight();

  L getLeftValue();
  R getRightValue();
}

class Left<L, R> extends Either<L, R> {
  final L left;

  Left(this.left);

  @override
  B fold<B>(B Function(L l) ifLeft, B Function(R r) ifRight) => ifLeft(left);

  @override
  bool isLeft() => true;

  @override
  bool isRight() => false;

  @override
  Left<L, R> getLeft() => this;

  @override
  Right<L, R> getRight() => throw NotRightException();

  @override
  L getLeftValue() => left;

  @override
  R getRightValue() => throw NotRightException();
}

class Right<L, R> extends Either<L, R> {
  final R right;

  Right(this.right);

  @override
  B fold<B>(B Function(L l) ifLeft, B Function(R r) ifRight) => ifRight(right);

  @override
  bool isLeft() => false;

  @override
  bool isRight() => true;

  @override
  Left<L, R> getLeft() => throw NotLeftException();

  @override
  Right<L, R> getRight() => this;

  @override
  L getLeftValue() => throw NotLeftException();

  @override
  R getRightValue() => right;
}

class NotRightException implements Exception {}

class NotLeftException implements Exception {}

example.dart

// MIT License
//
// Copyright (c) 2025 Nidal Bakir
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in all
// copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
// SOFTWARE.
//

void main(List<String> arguments) {
  final result = foo();
  result.fold(
    (error) => print(error),
    (somedata) => print(somedata),
  );

  if (result.isLeft()) {
    final error = result.getLeftValue();
    // handel error
  } else {
    final data = result.getRightValue();
    // do somethings with data
  }
}

Either<Exception, String> foo() {
  try {
    return Right(bar());
  } on Exception catch (e) {
    return Left(e);
  }
}

String bar() {
  // Doing sometings
  return 'bar';
}