Adding Trilinos to dealii package

[Paul A. Patience]

Hi,

I would like to eventually package the Lethe CFD library [1], but it
requires a version of dealii compiled with Trilinos, which is not
currently available in Guix.

PETSc and Trilinos (and many other libraries available in Guix) can be
compiled with several different options.
What is the standard way to deal with this in Guix?
>From what I can tell, in general the packages get a reasonable default
and people who require different options make a channel and inherit from
the base package, and in some really practical cases we get a variant of
the package with a similar name, e.g., dealii-openmpi.

The potential problem with PETSc and Trilinos when it comes to dealii is
that some of the optional modules they can be compiled with are the
same, which causes trouble when dealii is configured with both of
them [2].
(From what I can tell, the module in question is not configured in the
Guix package for PETSc, so this point may be moot.)

I'm wondering what the best way would be to add Trilinos to the dealii
packages (assuming I manage to package Trilinos as well).
In other words, which would be best among the following:

- Build Trilinos with a reasonable configuration for dealii [3] and
  update the dealii and dealii-openmpi packages to include it (if the
  problem mentioned above turns out not to be an issue).
  This assumes people won't mind having to build Trilinos as a
  dependency for dealii even if they don't need it.

- Build Trilinos as above but add separate packages for the resulting
  dealii, e.g., dealii-trilinos, dealii-trilinos-openmpi.
  This seems like it would get awkward quite quickly, which is why
  assuming my thoughts on the purposes of channels are correct, this is
  the inferior option.

- Abandon all hope and just maintain a channel for dealii configured
  with Trilinos (the inferiorer option :)).

- Some other possibility I have not thought of.

Thank you and best regards,
Paul

[1]: https://github.com/lethe-cfd/lethe
[2]: https://www.dealii.org/current/external-libs/petsc.html
[3]: https://www.dealii.org/current/external-libs/trilinos.html