An Evaluator does not know about the Context object which limits use-cases. For example, the use-case given by Gregg in solr-user – http://markmail.org/thread/pu3z27lreefbbkin
We can add the evaluate method and pass in the Context object. It is a non back-compatible change but since the Evaluator is marked as experimental and it's API has never been documented on the wiki, it should be fine.
We should also rename it to Function to be consistent with the dataimporter.functions namespace it is used with.