namespace Validation {
  export interface StringValidator {
    isValid(s: string): boolean;
  }

  export class EmailValidator implements StringValidator {
    isValid(s: string): boolean {
      return /^\S+@\S+\.\S+$/.test(s);
    }
  }

  // Not exported: invisible outside Validation
  const MIN_LENGTH = 3;
}

const validator = new Validation.EmailValidator();
console.log(validator.isValid("a@b.com")); // true