That code is probably some of the hardest you'll encounter in Ocaml, but for me its quite obvious what it does and easy to read because I've worked with GADTs before. If you haven't then its you'll need to study and understand them to understand the code.

I actually really like the syntax of OCaml, its very easy to write and when you're used to it, easy to read (easier than reasonml IMO).

Double semicolons are afaik only used in the repl.