Undefined module when using Oasis to build an OCaml project

It's me, again.. I am working on an OCaml project and I would like to use Oasis to build the whole code base. Here is how my codebase is organized.

src/
    core/
      init.ml
      type.ml
      utils.ml
    plugin/
      main.ml

I firstly only build the library with the following _oasis file:

Library "engine"                                                       
Path: src/core                                                         
Modules:                                                               
  Init,                                                                
  Type,                                                                
  Utils                                                            
BuildDepends:   deriving, deriving.syntax, core, batteries             
XMETADescription: core engine

And it works fine. There is no error and I can find a library engine.a in the _build/src/core folder.

However, when I try to include the library in the main.ml with the following way:

Module T = Engine.Type
...

And compile with the following _oasis file:

Library "engine"                                                       
Path: src/core                                                         
Modules:                                                               
  Init,                                                                
  Type,                                                                
  Utils                                                            
BuildDepends:   deriving, deriving.syntax, core, batteries             
XMETADescription: core engine


Executable "main"                                                      
  Path:           src/plugin                                         
  MainIs:         main.ml                                              
  CompiledObject: best                                                   
  Install:        false                                                  
  BuildDepends:   core, batteries, engine

I got an error:

Unbound module Engine

Am I doing anything wrong here? Could anyone give me some help? Thank you!

Solution

Your _oasis module defines a library with the name engine but it does not define an Engine module. So your Init, Type and Utils modules are exposed and should be accessible directly without any prefix.

If you want to pack those modules into a parent you can:

Manually pack each module into one big engine.ml file
Use Pack: true in the Library section of your _oasis file which would pack the included modules into a module called Engine
Use module aliases (see the OCaml manual for more information)