The UDL file

We describe in a UDL (a type of IDL, Interface Definition Language) file what is exposed and available to foreign-language bindings. In this case, we are only playing with primitive types (u32) and not custom data structures but we still want to expose the add method.
Let's create a math.udl file in the math crate's src/ folder:

namespace math {
  u32 add(u32 a, u32 b);
};

Here you can note multiple things: - The namespace directive: it will be the name of your Kotlin/Swift package. It must be present in any udl file, even if there ain't any exposed function (e.g. namespace foo {}). - The add function is in the namespace block. That's because on the Rust side it is a top-level function, we will see later how to to handle methods. - Rust's u32 is also UDL's u32, but it is not always true! See the Built-in Types chapter for more information on mapping types between Rust and UDL.

Note: If any of the things you expose in the udl file do not have an equivalent in your Rust crate, you will get a hard error. Try changing the u32 result type to u64 and see what happens!

Note It's also possible to use Rust procmacros to describe your interface and you can avoid UDL files entirely if you choose. Unfortunately the docs aren't quite as good for that yet though.