The organization here is a little bit weird. This note explains why.
The core of the pretty printer is printer.ml, and its companion signature file printer.mli depends on it.
In both printer.ml and printer.mli, we need to specify module types (e.g., PrinterT).
However, if we define PrinterT in printer.ml, then we also need to define it again in printer.mli.
Why don't want to duplicate the module types over and over again.
My solution is to define the module types in a new file signature.mli,
and then both printer.ml and printer.mli can refer to it.
Note that signature is specified as modules_without_implementation under dune, and things work out.
Stuff that are depended on by signature.mli and printer.ml are defined in util.ml.