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! (TODO table correspondance)

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!