In Ocaml-world it is customary to write .mli files that specify the types of exported functions and modules. Those are then checked by the compiler against the .ml file with the implementation. In the .ml file you indeed use type inference over annotations almost all of the time.
The Ocaml compiler is largely written in Ocaml.
C and C++ are not used very much at Jane Street as far as I know.
In Ocaml-world it is customary to write .mli files that specify the types of exported functions and modules. Those are then checked by the compiler against the .ml file with the implementation. In the .ml file you indeed use type inference over annotations almost all of the time.
The Ocaml compiler is largely written in Ocaml. C and C++ are not used very much at Jane Street as far as I know.