dune file starts with
(* -*- tuareg -*- *), then it is
interpreted as an OCaml script that generates the
dune file as described
in the rest of this section. The code in the script will have access to a
module containing details about the build context it’s executed in.
The OCaml syntax gives you an escape hatch for when the S-expression
syntax is not enough. It isn’t clear whether the OCaml syntax will be
supported in the long term, as it doesn’t work well with incremental
builds. It is possible that it will be replaced by just an
stanza where one can include a generated file.
Consequently you must not build complex systems based on it.