newtype.ts

newtype.ts

type Newtype<S extends string, T extends any> = T & { readonly symbol: S}
type Currency = Newtype<'currency', number>

const wrap = <S extends string, T>(input: T): Newtype<S,T> => input as unknown as Newtype<S,T>
const unwrap = <S extends string, T>(input: Newtype<S,T>):T => input satisfies T
const over = <S extends string, T>(nt: Newtype<S,T>, f: (t:T) => T):Newtype<S,T> => {
    const t: T = unwrap(nt)
    const ft: T = f(t)
    return wrap(ft)}
const eur: Currency = wrap(1)

const toDollar = (currency: Currency): Currency => {
    return over(currency, c => c * 1.12)

}

const usd = toDollar(eur)
// const usd2 = toDollar(1.21)
console.log(usd)