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

["Nicolás Ojeda Bär" <nicolas.ojeda.bar@lexifi.com>]

Dear Jun,

This is a limitation of module packs. However, there is a much better
alternative to module packs in the form of module aliases which will
allow you to do what you want.

You can head to
https://caml.inria.fr/pub/docs/manual-ocaml/extn.html#sec249 for a
more lengthy explanation, but briefly, in your example:

1. rename private.ml to sundials__private.ml and public.ml to
sundials__public.ml
2. create a file sundials.ml with contents:

  module Private = Sundials__private
  module Public = Sundials__public

3. compile sundials.ml with

  ocamlc -no-alias-deps -c sundials.ml

(you may want to disable warning 49 when doing this)

4. compile the individual files in the library with

  ocamlc -no-alias-deps -open Sundials -c sundials__private.ml
  ocamlc -no-alias-deps -open Sundials -c sundials__public.ml

5. You can now delete (or not install) sundials__private.cmi and ship
the other files

  sundials.cmo
  sundials.cmi
  sundials__public.cmo
  sundials__public.cmi
  sundials__private.cmo

to hide the private module from "outside" library users.

Note that if you use jbuilder/dune, then your library will be built
like this (steps 1-4) by default.

Hope it helps,

Best wishes,
Nicolás

On Thu, Apr 26, 2018 at 4:18 PM, 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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