Take a look at the current specs. There is a section on Fifi, the FFI implementation.
GitHub - yenrab/silica: A language designed to closely model current chip designs rather than the PDP-11 · GitHub Is the url to the project. Currently working on self compilation prior to completing the rest of a robust set of first-class data structures.
By first class data structures I mean they are first-class citizens as are the functions and structures used to build them. OrderedSet and OrderedMap are complete along with List.