What is Left and Right in the Either module in Flutter? - flutter

Question

I'm looking at some Flutter code that looks like this :
try {
return Right(_doSomethingAndReturnSingleValue());
} on CustomException {
return Left(CustomException());
}
Left and Right are from the core either.dart package, this is the code :
class Left<L, R> extends Either<L, R> {
final L _l;
const Left(this._l);
L get value => _l;
#override B fold<B>(B ifLeft(L l), B ifRight(R r)) => ifLeft(_l);
#override bool operator ==(other) => other is Left && other._l == _l;
#override int get hashCode => _l.hashCode;
}
class Right<L, R> extends Either<L, R> {
final R _r;
const Right(this._r);
R get value => _r;
#override B fold<B>(B ifLeft(L l), B ifRight(R r)) => ifRight(_r);
#override bool operator ==(other) => other is Right && other._r == _r;
#override int get hashCode => _r.hashCode;
}
I'm really struggling to make any sense of what this logic is supposed to do.
Can anyone help me understand what Left() and Right() are for in Dart ?

Answer 1

Left and Right are two generic classes inherited from the same parent, which does almost same thing. The major difference is in the fold method implementation. The left class call the ifLeft callback and right class call the ifRight callback.
For example:
Either<CustomException, String> getSomething() {
try {
return Right(_doSomethingAndReturnSingleValue());
} on CustomException {
return Left(CustomException());
}
}
No matter what happens the above function will return either an object of Either with CustomException (Means Left) or an object of Either with String (Means Right).
Now if you use the function like:
final eitherData = getSomething();
You will be getting an either object (Object of Left or Right). Instead of checking whether that eitherData is of type Left or Right, you can call the fold method on that object like below:
eitherData.fold<Widget>(
(err) => Text('Error Happened: $err'), // ifLeft callback
(data) => Text('Got data: $data'), // ifRight callback
)
As I mentioned earlier based on the object type the corresponding callback will get triggered and you can gracefully handle the success and error cases without writing any if else statements or type checks.