TypeScript Either / Pattern Matching

example.ts

import { Either, Left, Right, match } from './patterns.ts';

interface Person {
  name: string;
}

const readValueFromAPI = (): Either<Error, Person> => {
  // ...
  const person: Person = {
    name: 'Test',
  };

  return Right(person); // or Left(new Error('Something happened...'));
};

const str = match(
  readValueFromAPI(),
  (err) => err.message,
  (person) => person.name,
);

patterns.ts

export type Left<T> = { tag: 'left'; value: T };
export type Right<T> = { tag: 'right'; value: T };
export type Either<L, R> = Left<L> | Right<R>;

export const match = <T, L, R>(
  input: Either<L, R>,
  left: (left: L) => T,
  right: (right: R) => T,
) => {
  switch (input.tag) {
    case 'left':
      return left(input.value);
    case 'right':
      return right(input.value);
  }
};

export const isRight = <L, R>(input: Either<L, R>): input is Right<R> => {
  return input.tag === 'right';
};

export const isLeft = <L, R>(input: Either<L, R>): input is Left<L> => {
  return input.tag === 'left';
};

export const Right = <T>(value: T): Right<T> => {
  return {
    tag: 'right',
    value,
  };
};

export const Left = <T>(value: T): Left<T> => {
  return {
    tag: 'left',
    value,
  };
};