What does Either represent and what will result be?

type Either<L, R> =
  | { tag: "left"; value: L }
  | { tag: "right"; value: R };

const left = <L>(value: L): Either<L, never> =>
  ({ tag: "left", value });

const right = <R>(value: R): Either<never, R> =>
  ({ tag: "right", value });

const map = <L, A, B>(
  either: Either<L, A>,
  fn: (a: A) => B
): Either<L, B> =>
  either.tag === "right"
    ? right(fn(either.value))
    : either;

const result = map(right(10), x => x * 2);