balanza

type Foo = {
    value: string, 
    __phantom: never
}
const f = () => { throw '' }
const x: Foo = { value: 'foo', __phantom: f() }

balanza

type ConfirmedMobile = string & { __phantom_1548946378096: never }
const createConfirmedMobile = (num: String) => createOf<ConfirmedMobile, String>(num)

type CertifiedMobile = string & { __phantom_1548946391842: never }
const createCertifiedMobile = (num: String) => createOf<CertifiedMobile, String>(num)

const confirmed = createConfirmedMobile('2984829202') 
// ^^^ ConfirmedMobile
const certified = createCertifiedMobile('2984829202') 
// ^^^ CertifiedMobile

balanza

// it's just a cast ;)
const createOf = <T extends Base, Base>(original: Base) => <T>original

balanza

type ConfirmedMobile = string & { __random_transparent_property: never }
type UnconfirmedMobile = string & { __another_random_transparent_property: never }
type CertifiedMobile = string & { __again_random_transparent_property: never }
type Mobile =  // ConfirmedMobile | UnconfirmedMobile | CertifiedMobile :)
    ConfirmedMobile 
    | UnconfirmedMobile 
    | CertifiedMobile

const printString = (p: string) => console.log(`print String ${p}`)
const printConfirmedMobile = (p: ConfirmedMobile) => console.log(`print ConfirmedMobile ${p}`)

balanza

type ConfirmedMobile = string
type UnconfirmedMobile = string
type CertifiedMobile = string
type Mobile =  // just string :(
    ConfirmedMobile 
    | UnconfirmedMobile 
    | CertifiedMobile

balanza

const exhaustionCheck = (e: never) => console.log('this will never be executed')

const printMobileSafe = (mobile: Mobile) => {
  if(mobile.status === 'confirmed') {
    console.log('Your mobile is confirmed!')
  } else if(mobile.status === 'unconfirmed') {
    console.warn('Your mobile is not confirmed!')
  } else {
    exhaustionCheck(mobile) // <-- you'll get a compilation error here!
  }

balanza

const certified: CertifiedMobile = { status: 'certified', value: '543234531' }

printMobile(certified) // warn: Your mobile is not confirmed!

printMobileAlternative(certified) // unhandled exception: illegal state! mobile.status=certified

balanza

// add this line
type CertifiedMobile = Valued<string> & { status: 'certified' }   

// add CertifiedMobile to the unon type
type Mobile = ConfirmedMobile | UnconfirmedMobile | CertifiedMobile

balanza

// optimistic implementation: assume mobile.status will only have those two values
const printMobile = (mobile: Mobile) => {
  if(mobile.status === 'confirmed') {
    console.log('Your mobile is confirmed!')
  } else {
    console.warn('Your mobile is not confirmed!')
  }
}

// defensive implementation: assume mobile.status can be whatever

balanza

type Valued<T> = { value: T }
type ConfirmedMobile = Valued<string> & { status: 'confirmed' } 
type UnconfirmedMobile = Valued<string> & { status: 'unconfirmed' }    
type Mobile = ConfirmedMobile | UnconfirmedMobile
type User {
  /* name, surname, etc */
  mobile: Mobile
}

// an user with confirmed mobile