Hi all,

I'm attempting to convert an existing (reasonably small) HTTP server
written in OCaml to run on mirage's xen backend. I've gotten the compiler
reasonably happy by replacing uses of Unix / Lwt_unix with appropriate
mirage-compatible modules. But when it comes to link time, it seems that my
application is still indirectly depending on Unix:

+ ocamlbuild -quiet -cflag -warn-error -cflag +a-3 -build-dir
[...]/_build/mirage-xen -use-ocamlfind -lflags -g,-linkpkg,-dontlink,unix
main.native.o
+ ocamlfind ocamlopt -g -linkpkg -dontlink unix -output-obj -syntax camlp4o
-package 'fat-filesystem, io-page, mirage-block-xen.front, batteries,
yojson, safepass, sha, lwt, lwt.syntax, mirage-console.xen, mirage-types,
mirage-types.lwt, mirage-net-xen, mirage-types,cohttp,cohttp.lwt,
mirage-http, lwt, mirage.runtime,
tcpip.stack-direct,tcpip.stack-socket,mirage-clock-xen,conduit.mirage'
src/passe.cmx src/passe_server.cmx src/unikernel.cmx main.cmx -o
main.native.o
File "_none_", line 1:
Error: No implementations provided for the following modules:
         Unix referenced from
/home/tim/.opam/mirage_4_01_0/lib/lwt/lwt-unix.cmxa(Lwt_engine),
           /home/tim/.opam/mirage_4_01_0/lib/lwt/lwt-unix.cmxa(Lwt_unix),

/home/tim/.opam/mirage_4_01_0/lib/ipaddr/ipaddr_unix.cmxa(Ipaddr_unix)
Command exited with code 2.

I can't figure out why those modules are being linked in, though. I have
grepped all files in _build/mirage-xen/**/*.ml.depends (as well as my own
source code), and nothing in there matches "unix", nor mentions any of the
modules listed above. The only lwt_* matches are:

Cohttp_lwt_body
Lwt_mutex
Lwt_stream

Which all seem like they shouldn't depend on unix either directly or
indirectly.

Is there some way to see a tree / list of "what module in my source code is
transitively depending on Unix, and via what modules is that dependency
inherited?" Or even if there's no way to see a tree, is there a way to do
it one step at a time (e.g "what is depending directly on Lwt_unix", then
repeat the process for those modules listed until I see something that
surprises me)

Failing that, does anyone have tricks for weeding indirect `Unix` usage out
of a codebase which is not directly using it?

Thanks,
 - Tim Cuthbertson