block10.ts

declare class EmailTag {
   private __kind: 'email'
}

type Email = string & EmailTag;

function isEmail(str: string): str is Email {
  //Very simple validation
  return str.indexOf('@') > 0;
}

type User =  {
  name: string,
  email: Email,
}

function createUser(name: string, email: string): User | {error: string}  {
   if(isEmail(email)) {
       return { name, email };
   } else {   
      return {error: `${email} is not a valid email` };
   }
}