The standard compiler can do a cold compile of a single file in milliseconds. When the compiler is that fast you _can_ just wrap it up and it'll still be faster than TypeScript.

But to answer your actual question: The current standard (reason-language-server) does simply wrap the compiler, and it's more than fast enough, but the community is working on a new merlin-based language server (ocaml-lsp) which does support partial compilation.