I am trying to use sexplib to serialize some data structures, as many of you
advised, and I am somewhat stuck trying to compile.
This is what I get:

ocamlfind ocamlc -package
unix,str,vec,mapmin,hashtbl_bounded,fileinfo,intvmap,extlib,mysql,sexplib -I
../../batch/analysis -I +camlp4 -pp camlp4orf -g -c online_db.ml
File "online_db.ml", line 44, characters 22-26:
Parse error: [semi] expected after [str_item] (in [implem])
Preprocessor error

where my line 44 is simply:

type foo_t = int list with sexp

I have the impression that the problem is in how I compile.  Specifically,
if I do not use the "-pp camlp4orf" part, I get a "syntax error", as the
file is not getting preprocessed.
If I use the above -pp line, I get the error I reported.  I am not sure what
is the proper way to preprocess the file, and unfortunately, sexplib does
not explain how to compile code against it.
I am sure this is obvious for everybody, but it is the first time I use
camlp4, so some help would be appreciated.

All the best,

Luca