Using module signatures

Let's imagine that we're building an invoice-management system and we have an Invoice module that defines an invoice type along with a function that other modules can use to create values of that type. This arrangement is shown in the following code:

/* Invoice.re */
type t = {
  name: string,
  email: string,
  date: Js.Date.t,
  total: float
};

let make = (~name, ~email, ~date, ~total) => {
  name,
  email,
  date,
  total
};

Let's also suppose that we have another module that is responsible for sending emails to customers, as shown in the following code:

/* Email.re */
let send = invoice: Invoice.t => ...
let invoice =
  Invoice.make(
    ~name="Raphael",
    ~email="[e-mail]",
    ~date=Js.Date.make(),
    ~total=15.0,
  );
send(invoice);

Since the Invoice.t type is exposed, the invoice can be manipulated by Email...