patrickmichalina/typescript-monads

import { IEitherPattern, IEither } from './either.interface'

export class Either<L, R> implements IEither<L, R> {
  constructor(private readonly left?: L, private readonly right?: R) {
    if (this.neitherExist()) {
      throw new TypeError('Either requires a left or a right')
    }
    if (this.bothExist()) {
      throw new TypeError('Either cannot have both a left and a right')
    }
  }

  private static exists<T>(value?: T): boolean {
    return typeof value !== 'undefined' && value !== null
  }

  private bothExist(): boolean {
    return this.isLeft() && this.isRight()
  }

  private neitherExist(): boolean {
    return !this.isLeft() && !this.isRight()
  }

  public isLeft(): boolean {
    return Either.exists(this.left)
  }

  public isRight(): boolean {
    return Either.exists(this.right)
  }

  public match<T>(pattern: IEitherPattern<L, R, T>): T {
    return this.isRight()
      ? pattern.right(this.right as R)
      : pattern.left(this.left as L)
  }

  public tap<T>(pattern: Partial<IEitherPattern<L, R, T>>): void {
    this.isRight()
      ? typeof pattern.right === 'function' && pattern.right(this.right as R)
      : typeof pattern.left === 'function' && pattern.left(this.left as L)
  }

  public map<T>(fn: (r: R) => T): IEither<L, T> {
    return this.isRight()
      ? new Either<L, T>(undefined, fn(this.right as R))
      : new Either<L, T>(this.left)
  }

  public flatMap<T>(fn: (r: R) => IEither<L, T>): IEither<L, T> {
    return this.isRight()
      ? fn(this.right as R)
      : new Either<L, T>(this.left)
  }
}