[ocaml-platform] How to inform OPAM's "system" switch about OCaml libraries installed by RPMs?

[Evgeny Roubinchtein]

This is very helpful.  Thank you for a detailed reply.

-- 
Best,
Evgeny ("Zhenya")

On Fri, Apr 13, 2018 at 2:26 AM, <louis.gesbert at ocamlpro.com> wrote:

> On jeudi 12 avril 2018 15:16:48 CEST Evgeny Roubinchtein wrote:
> > Dear OCaml platform users and developers,
> >
> > I am working in an environment where OCaml (but not OPAM) is installed
> > system-wide via RPMs, along with a few OCaml libraries.  All of the
> > libraries have corresponding OPAM packages.   I would like to create an
> > OPAM  switch that is an alias of the "system" switch, but to also inform
> > OPAM about the packages already installed system-wide.  Simply creating a
> > switch a as an alias of "system" seems to "not notice" any of the
> libraries
> > installed via RPMs.  Do you have any suggestions?
> >
> > The set of RPMs installed on the system changes at glacial speeds, so
> > having a dumb script into which I hard-code some information about the
> > packages installed by system-wide RPMs is perfectly acceptable in my
> case,
> > if a more elegant solution does not exist (yet): I just would like to
> know
> > what information I would need to hard-code, and I am hoping that asking
> > here can save me a few hours of hunting through the OPAM source code.
> >
> > In case it matters, this is OPAM 2.0 pre-release.
> >
> > Thank you in advance!
>
> Hi
>
> This is unfortunately not supported at the moment, as it would be quite
> difficult in general to map system packages (from all the OSes and
> ditributions
> out there) to opam packages, including version numbers, fixes, etc.
>
> If you really want to avoid recompilations, you could try a few
> workarounds,
> though:
>
> - find the corresponding opam packages and their versions, pin them (with
> `-n`)
> and pretend to install them with `--fake`. This will register them as
> installed, but without any processing. It is pretty brittle, though, and
> be
> careful that all fake-installed packages really are on the system.
>
> - or, much cleaner, but also more work, define your own package repository
> (it's pretty simple), get the definitions of the packages you are
> interested in
> from the official opam-repository, and remove their `url` definitions, as
> well as
> build, install and remove instructions to make them dummy packages. Then
> the
> clients should use this repository on top on the official repository
> (`opam repo
> add`) to get the dummy packages. Then, on the client side, make sure to
> use
> `opam pin` to remain on the provided versions of the packages.
>
> There may still be issues with `ocamlfind`, which needs to find the system
> packages besides the opam ones (and here, we can't filter between what is
> installed opam-wise or not). Using `setenv:` in the concerned packages
> might
> help.
>
> Overall, I wouldn't recommend it: this requires quite some work, and there
> is
> no guarantee that it will work reliably (package mapping is never 1-1).
> This
> is the reason why the approach taken by opam is to rebuild all packages
> from
> their source, using its own package definitions. We are experimenting with
> ways
> to have binary caches of packages built by opam, though.
>
> Best,
> Louis Gesbert - OCamlPro
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