Propellor is versioned using the Haskell [Package Version Policy](https://www.haskell.org/haskellwiki/Package_versioning_policy).

This means that propellor 2.10.x contains some changes to its API;
code written for propellor 2.9.x may need to be changed. Conversely,
there are no breaking changes between 2.10.1 and 2.10.2.

Whenever possible, breaking changes are made in a way that either changes a
data type, or a function name, so that code that used the old API version
won't compile under the new one.

Sometimes, changes to the behavior of properties also merit a major version
change. For example, propellor 0.8.0 completely revamped the privdata
storage, and users had to manually transition.
