External
external, o "FFI" (foreign function interface) o simplemente "interop" (de "interoperabilidad") es cómo Reason se comunica con otros lenguajes, como C o JavaScript.
Imagina escribir un enlace let, pero omitiendo su cuerpo y escribiendo su tipo (obligatorio):
external myCFunction: int => string = "theCFunction";
Note: externals can only be at the top level, or inside a module definition. You can't declare them in e.g. a function body.
Uso
You'd use an external value/function as if it was a normal let binding.
For more infos on how to link up your C functions within your Reason application, have a look at the Dealing with foreign libraries section in the dune build system docs.