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Hidden includes

This RFC proposes to add to the compiler command line a new kind of includes (-I) called hidden includes (-Ihidden). Currently -I has two impacts:

  • User: All modules corresponding to .cmi files found in included directories are added in the initial scope.
  • Typer: Type definition present in the .cmi files found in included directories can be used by the typer.

So if Foo defines val v: Bar.t and Bar defines type t = int. The user can type Foo.v + 1. Currently if the user only depend on the library foo the build system also needs to include the library bar in the include directories.

The problem arise when the user uses directly the Bar module for example by writting (Foo.v + 1 : Bar.t). If the library foo stop depending on bar, for example val v: int the compilation of the user code will fail because bar is not anymore a transitive dependency. We would like the user code to be forced to directly depend on bar if it uses Bar.

In order to do that we propose to add a variant of -I for example called -Ihidden that is seen by the typer but not by the user, i.e. not added in the initial scope. If foo is a direct dependency of the user code and bar only a dependency of foo a build system can make the following call to the compiler:

ocamlopt -I <foo>/ -Ihidden <bar>/

If both foo and bar are dependencies of the user code it would be the usual:

ocamlopt -I <foo>/ -I <bar>/

Proof of concept

There is currently no proof of concept. Yet the idea would be for Load_path to consider -I and -Ihidden in the same way except for a new a function Load_path.get_not_hidden that would only give directories from -I. It would be used during the creation of the initial environment in Typemod:

  let units =
    List.map Env.persistent_structures_of_dir (Load_path.get_not_hidden ())
  in

Extensibility

  • Private modules are not in the scope of this RFC. However it is not clear if removing the .cmi of the private modules, is supported by the typer. Perhaps another type of include -Iprivate could be used to give the location of the .cmi of the private modules. The typer should not use the type equalities, since it is the goal of a private module, but perhaps it could simplify the support of private modules.

  • It has been proposed to not use directories for including modules but directly the list of .cmi that should be made accessible. It could help to avoid creating specific directories for sorting .cmi files and be very precise on what the compilation depends. However such feature is orthogonal, could be generalized to any io made by the compiler and is not required for correct dependencies.

scope /dir/foo.cmi\000
hidden /dir/bar.cmi\000

Summary

This RFC proposes to add a variant -Ihidden of -I, which don't modify the initial scope of the environment but allows the typer to lookup type definition:

option Initial scope Type Lookup
-I X X
-Ihidden X

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