The standard stdlib has all useful data structures, mostly the differences are in providing more convenient functions and monadic interfaces that you might have

The standard stdlib has all useful data structures, mostly the differences are in providing more convenient functions and monadic interfaces that you might have to write yourself, if you didn't use the alternate libraries. There is nothing to "learn" about using another stdlib since most signatures are the same. And they can use a different stdlib if they want. The migration from standard stdlib to alternate ones is fairly trivial(some signatures might differ but typed language means the compiler will catch them for you), so you can start using the stdlib and if you feel like writing some trivial things that should be in the stdlib or you need some advanced data structures you can use the alternate ones. Also you need `lwt`(most people prefer this)/`async` library for writing concurrent programs like a webserver. `Cohttp` is a good library for doing http based client/servers. I strongly recommend `Real world ocaml` and the reference manual is excellent at looking up some new type-level features. Also reading well-known libraries code is an excellent way to learn a language.