Validation mechanism has been removed As a result of cleaning up Slice from various utility functionality we decided to remove the whole validation mechanism in Slice 4.0.
If you would like to use this mechanism with Slice 4.0, feel free to take classes responsible for that from Slice 3.2.0 source code: https://github.com/Cognifide/Slice/tree/3.2.0 and create your own validation library. The code from version 3.2 is fully compatible with Slice 4.0.
More details on how to use validation you can find in Slice 3.2 documentation.