Getting Started OCaml: Setting up utop with Base
It is not obvious how one uses the Jane Street OCaml packages in utop, at least under Linux in 2019.
The following assumes that you have managed to get OCaml installed and executing eval $(opam config env) sets up everything correctly in your environment.
The key is to run #require "base";; in utop. The following sequence should work:
#require "core";;
open Base;;
List.map ~f:String.length ["Hello"; "World!"];;
The List.map above is Base.List.map, as desired.
If you don't run #require "core";; you will get the message: Error: Unbound module Base.