Typescript Either TryOut

ts-either.ts

class Either {
  protected $value;

  static of<T>(x: T) {
    return new Right(x);
  }

  constructor(x) {
    this.$value = x;
  }
  
  fold<T>(fn: Function): T {
    return fn(this.$value);
  }
}

class Right<T> extends Either {
  public type: 'right' = 'right';
  constructor(x: T) {
    super(x);
  }

  inspect() {
    return `Right(${this.$value})`;
  }

  toString() {
    return `${this.$value}`;
  }

  map<U>(fn: Function): Right<U> {
    return Either.of(fn(this.$value));
  }
}

class Left<T> extends Either {
  public type: 'left' = 'left';
  constructor(x: T) {
    super(x);
  }

  inspect() {
    return `Left(${this.$value})`;
  }

  toString() {
    return `${this.$value}`;
  }

  map(): Left<T> {
    return this;
  }
}

const either = Either.of<number>(10);

const calculated = either
  .map(n => n * 10)
  .map(n => n + 100);

const handleValue = (val: Left<string> | Right<number>) => {
  if (val.type === 'left') {
    console.error(val.fold(x => x));
  }

  if (val.type === 'right') {
    console.log('the value is ' + val);
  }
}

// 특정 조건을 만족할 때 Left 객체를 리턴하도록 만든다
const left = new Left('Hi, I am Error!');

handleValue(calculated);
handleValue(left);