diff options
Diffstat (limited to 'lib/spack/docs/packaging_guide.rst')
| -rw-r--r-- | lib/spack/docs/packaging_guide.rst | 31 |
1 files changed, 31 insertions, 0 deletions
diff --git a/lib/spack/docs/packaging_guide.rst b/lib/spack/docs/packaging_guide.rst index eb62fa8bf8..4633ef25a8 100644 --- a/lib/spack/docs/packaging_guide.rst +++ b/lib/spack/docs/packaging_guide.rst @@ -1423,6 +1423,37 @@ other similar operations: ).with_default('auto').with_non_feature_values('auto'), ) +""""""""""""""""""""""""""" +Conditional Possible Values +""""""""""""""""""""""""""" + +There are cases where a variant may take multiple values, and the list of allowed values +expand over time. Think for instance at the C++ standard with which we might compile +Boost, which can take one of multiple possible values with the latest standards +only available from a certain version on. + +To model a similar situation we can use *conditional possible values* in the variant declaration: + +.. code-block:: python + + variant( + 'cxxstd', default='98', + values=( + '98', '11', '14', + # C++17 is not supported by Boost < 1.63.0. + conditional('17', when='@1.63.0:'), + # C++20/2a is not support by Boost < 1.73.0 + conditional('2a', '2b', when='@1.73.0:') + ), + multi=False, + description='Use the specified C++ standard when building.', + ) + +The snippet above allows ``98``, ``11`` and ``14`` as unconditional possible values for the +``cxxstd`` variant, while ``17`` requires a version greater or equal to ``1.63.0`` +and both ``2a`` and ``2b`` require a version greater or equal to ``1.73.0``. + + ^^^^^^^^^^^^^^^^^^^^ Conditional Variants ^^^^^^^^^^^^^^^^^^^^ |