For what it’s worth it looks like you’ve really thought through this problem and I really like a more idiomatic approach to web servers in Reason, especially when it has the possibility to compile to native in the future. I know in the past OCaml has been a poor server-side language due to a lack of proper threading? The bigger issue might have been that it just wasn’t popular enough for people to just make something good.
Although this is perhaps outside the scope of your project I’ve loved the Phoenix framework on Elixir, which is a sort of functional rails. It’s really well put together.
Overall: Keep up the good work