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

[Jun Inoue <jun.lambda@gmail.com>]

Hi Ivan,

That's basically our current approach, but it doesn't solve the
namespace pollution problem.  In your example, when someone installs a
file named b.cmi (whose interface is unrelated to your b.ml), the name
conflict prevents loading the std.cma file at all:

$ ocaml
        OCaml version 4.04.0

# #show B;;
module B : sig val foo : int end
# #load "std.cma";;
The files std.cma and b.cmi disagree over interface B

So the technique makes B inaccessible but doesn't remove it from the
namespace.  This is why we want to -pack things, because our analogue
of b.ml is named matrix.ml, and there's no other sensible name for it.

This technique doesn't work with -pack because that option demands all
.cmi's, including b.cmi.  I guess we could rename matrix.ml to
matrix_internal_dont_touch.ml, but we wanted to know if there's a
cleaner approach.  I wish we could supply a .mli file to the product
of -pack, but that also doesn't work...

On Fri, Apr 27, 2018 at 12:06 AM, Ivan Gotovchits <ivg@ieee.org> wrote:
> 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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>
>



-- 
Jun Inoue

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