pickaroo.gleam

// pickaroo: Money

pub type Currency {
  USD
  EUR
  PLN
}

pub type Money {
  Money(cents: Int, currency: Currency)
}

pub type MoneyError {
  CurrenciesMismatch(c1: Currency, c2: Currency)
}

pub fn add(m1: Money, m2: Money) {
  case m1, m2 {
    Money(cents1, currency), Money(cents2, currency) -> Ok(Money(cents1 + cents2, currency))
    Money(currency: c1), Money(currency: c2) -> Error(CurrenciesMismatch(c1, c2))
  }
}

pickaroo_test.gleam

import pickaroo
import gleam/expect

pub fn add_ok_test() {
  pickaroo.add(pickaroo.Money(100, pickaroo.USD), pickaroo.Money(200, pickaroo.USD))
  |> expect.equal(_, Ok(pickaroo.Money(300, pickaroo.USD)))
}

pub fn add_mismatch_test() {
  pickaroo.add(pickaroo.Money(100, pickaroo.USD), pickaroo.Money(200, pickaroo.EUR))
  |> expect.equal(_, Error(pickaroo.CurrenciesMismatch(pickaroo.USD, pickaroo.EUR)))
}