money.erl

add(M1, M2) ->
    case {M1, M2} of
        {{money, Cents1, Currency}, {money, Cents2, Currency1}} ->
            {ok, {money, Cents1 + Cents2, Currency1}};

        {{money, _, C1}, {money, _, C2}} ->
            {error, {currencies_mismatch, C1, C2}}
    end.

money.gleam

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(_, c1), Money(_, c2) -> Error(CurrenciesMismatch(c1, c2))
  }
}