flatMap<R2> method
Applies the function f to the value contained in Right, if it exists,
and returns a new Either containing the result. If this is a Left,
it is returned unchanged.
Implementation
@override
Either<L, R2> flatMap<R2>(Either<L, R2> Function(R right) f) {
// Left stays unchanged during flatMap
return Left<L, R2>(value);
}