type currency = string;
type money = real * currency;

fun convert (amount, to) =
    let val toeur = fn
        (x, "eur") => x
      | (x, "usd") => x / 1.05 
      | (x, "ounce gold") => x * 1113.0
    in
        ( case to of
            "eur" => toeur amount
          | "usd" => toeur amount * 1.05
          | "ounce gold" => toeur amount / 1113.0
        , to)
   end;