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Add constants to BC promise #5672

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Add constants to BC promise #5672

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15 changes: 15 additions & 0 deletions contributing/code/bc.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -169,6 +169,10 @@ Add type hint to an argument No
Remove type hint of an argument No
Change argument type No
Change return type No
**Constants**
Add constant Yes
Remove constant No
Change value of a constant Yes [1]_ [5]_
============================================== ==============

Changing Classes
Expand Down Expand Up @@ -253,6 +257,10 @@ Change return type Yes
**Static Methods**
Turn non static into static No
Turn static into non static No
**Constants**
Add constant Yes
Remove constant No
Change value of a constant Yes [1]_ [5]_
================================================== ==============

.. [1] Should be avoided. When done, this change must be documented in the
Expand All @@ -267,6 +275,13 @@ Turn static into non static No
.. [4] When changing the parent class, the original parent class must remain an
ancestor of the class.

.. [5] The value of a constant may only be changed when the constants aren't
used in configuration (e.g. Yaml and XML files), as these do not support
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Yaml should be removed until symfony/symfony#18626 is merged in 3.2

constants and have to hardcode the value. For instance, event name
constants can't change the value without introducing a BC break.
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or when the value can be used in serialize/unserialize

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That can apply to any constant, isn't it?

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I think Nicolas' point is probably that if a value will likely be used in serialize/unserialize, like if there were some constant that was commonly put onto User classes (since those are serialized). But this does highlight that this is slippery. Perhaps that this should be re-worded that changing constant values is discouraged, but is allowed if we're quite sure that it's not being used in configuration files or in objects that are serialized. Anyways, the "serialized" is a good point, because it might make us think about this in the future and realize that changing some constant value would be bad in one case :)

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I think we should simply add:

Additionally, if a constant will likely be used in objects that are serialized, the value of
a constant should not be changed.

To your point Wouter, technically any constant could be used in a configuration file too - but there are a few that are very commonly used (i.e. it's a "main supported use-case") - and it's those that we should avoid).

Additionally, if a constant will likely be used in objects that are
serialized, the value of a constant should not be changed.

.. _Semantic Versioning: http://semver.org/
.. _scalar type: http://php.net/manual/en/function.is-scalar.php
.. _boolean values: http://php.net/manual/en/function.boolval.php
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