type USD = number
type EUR = number

const myamount: USD = 1

declare function change(usd: USD): EUR
declare function saveAmount(eur: EUR): void

saveAmount(change(myamount)) // ok
saveAmount(myamount) // opss... this is also ok because both EUR and USD are type alias of number!

import { Newtype, iso } from 'newtype-ts'

interface EUR extends Newtype<{ readonly EUR: unique symbol }, number> {}

// isoEUR: Iso<EUR, number>
const isoEUR = iso<EUR>()

// myamount: EUR
const myamount = isoEUR.wrap(0.85)

// n: number = 0.85
const n = isoEUR.unwrap(myamount)

declare function saveAmount(eur: EUR): void

saveAmount(0.85) // static error: Argument of type '0.85' is not assignable to parameter of type 'EUR'
saveAmount(myamount) // ok

import { Newtype, prism } from 'newtype-ts'

interface Integer extends Newtype<{ readonly Integer: unique symbol }, number> {}

const isInteger = (n: number) => Number.isInteger(n)

// prismInteger: Prism<number, Integer>
const prismInteger = prism<Integer>(isInteger)

// oi: Option<Integer>
const oi = prismInteger.getOption(2)

declare function f(i: Integer): void

f(2) // static error: Argument of type '2' is not assignable to parameter of type 'Integer'
oi.map(f) // ok

import { NonZero, prismNonZero } from 'newtype-ts/lib/NonZero'

// a total function
const safeDivide = (numerator: number, denominator: NonZero): number => {
  return numerator / prismNonZero.reverseGet(denominator)
}

// result: Option<number>
const result = prismNonZero.getOption(2).map(denominator => safeDivide(2, denominator))

const double = n => n * 2
const doubleEUR = eurIso.modify(double)

const double = (n: number): number => n * 2

// doubleEUR: (s: EUR) => EUR
const doubleEUR = eurIso.modify(double)