OCaml-stk is a Graphical User Interfce toolkit for OCaml based on libSDL 2, through the Tsdl bindings.
Lwt is used to provide pseudo-parallelism in event handling.
Stk is still in development but already provides various widgets.
The Stk_iconv library provides bindings to GNU libiconv, to convert strings to and from UTF-8, which is the format of strings in OCaml-stk.