Erlang FFI

One of the big benefits of targeting the BEAM is being able to leverage a rich and mature ecosystem.

We can do so by binding Erlang functions to Mond names with extern let. We said earlier that Mond does not use type signatures, this was a little white lie. extern declarations are the only place where Mond uses type signatures.

(extern let system-time ~ (Unit -> Int) erlang/system_time)

pub extern let makes the binding importable by other modules — this is how large parts of the standard library are implemented e.g.

(pub extern let println ~ (String -> Unit) io/format)

We can do something similar for opaque foreign-backed types.

(pub extern type Pid)
(pub extern type ['k 'v] Map maps/map)

The trailing module/type target on extern type is optional metadata. Use it when it helps document the foreign runtime type, but plain opaque declarations such as (pub extern type Pid) are also valid.