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docs: add strong preferences to faq (#49556)

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Harmen Stoppels 2025-03-19 08:53:14 +01:00 committed by GitHub
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18
lib/spack/docs/frequently_asked_questions.rst
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@ -30,7 +30,7 @@ than always choosing the latest versions or default variants.
.. note::
As a rule of thumb: requirements + constraints > reuse > preferences > defaults.
As a rule of thumb: requirements + constraints > strong preferences > reuse > preferences > defaults.
The following set of criteria (from lowest to highest precedence) explain
common cases where concretization output may seem surprising at first.
@ -56,7 +56,19 @@ common cases where concretization output may seem surprising at first.
concretizer:
reuse: dependencies # other options are 'true' and 'false'
3. :ref:`Package requirements <package-requirements>` configured in ``packages.yaml``,
3. :ref:`Strong preferences <package-strong-preferences>` configured in ``packages.yaml``
are higher priority than reuse, and can be used to strongly prefer a specific version
or variant, without erroring out if it's not possible. Strong preferences are specified
as follows:
.. code-block:: yaml
packages:
foo:
prefer:
- "@1.1: ~mpi"
4. :ref:`Package requirements <package-requirements>` configured in ``packages.yaml``,
and constraints from the command line as well as ``package.py`` files override all
of the above. Requirements are specified as follows:
@ -66,6 +78,8 @@ common cases where concretization output may seem surprising at first.
foo:
require:
- "@1.2: +mpi"
conflicts:
- "@1.4"
Requirements and constraints restrict the set of possible solutions, while reuse
behavior and preferences influence what an optimal solution looks like.

2
lib/spack/docs/packages_yaml.rst
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@ -486,6 +486,8 @@ present. For instance with a configuration like:
you will use ``mvapich2~cuda %gcc`` as an ``mpi`` provider.
.. _package-strong-preferences:
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Conflicts and strong preferences
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^