Re: [Caml-list] Type That's Concrete From Within A Library Abstract From Without

[Ivan Gotovchits <ivg@ieee.org>]

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Hi Jun,

You can achieve this by implying an extra layer of indirection, i.e., by
having two levels of interfaces. For example,

   * A.ml - implementation of module A
   * A.mli - private interface of module A
   * B.ml  - implementation of module B that may rely on anything in A.mli
   * Std.ml - a set of modules that you would like to import, e.g., `module
A = A`, `module B = B`
   * Std.mli - public interface specification


Next, you deploy `std.cmxa` and `std.cmi` but keep `a.cmi` and `b.cmi` to
yourself. This will prevent users from accessing your private modules A and
B directly. (In oasis you can use PrivateModules stanza for this)

Now you will have `Std.A` and `Std.B` that exposes as much as you want. Not
sure whether it will work with the `-pack`, but you can use this approach
instead of it. This is how we address the same issue in [BAP][1]

Cheers,
Ivan

[1]: https://github.com/BinaryAnalysisPlatform/bap


On Thu, Apr 26, 2018 at 10:18 AM, Jun Inoue <jun.lambda@gmail.com> wrote:

> Dear list,
>
> Is there a way to make a type concrete inside a library, yet opaque to
> library users, preferably in a way that works with -pack?  This is a
> nagging issue in our sundials package
> (http://inria-parkas.github.io/sundialsml/).
>
> Basically, we have a type declared in one module of the library that
> is pattern-matched upon in other modules, like:
>
> (* private.ml *)
> type opaque_type = Foo | Bar
>
> (* public.ml *)
> let f : opaque_type -> int = function
>   | Foo -> 0
>   | Bar -> 1
>
> There are a few constraints:
> - We don't want users to be able to pattern-match on opaque_type.
> - We need multiple modules in the library to pattern-match on
> opaque-type (so moving opaque_typ e to public.ml is not an option).
> - To avoid namespace pollution, we want to pack the whole library
> (with ocamlc -pack) as a single Sundials module, so the user sees a
> Sundials.Public module instead of just Public.
>
> Is this possible?  Right now, we just collect public.cmo and
> private.cmo into sundials.cma and throw away private.cmi.  But this
> doesn't work with packing:
>
> $ ocamlc -pack -o sundials.cmo private.cmo public.cmo
>
> demands that there be a private.cmi.
>
> --
> Jun Inoue
>
> --
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