I think "functional database" can be seen as a wider data pattern than just a datastore that exclusively implements that pattern. It's a design principle where

I think "functional database" can be seen as a wider data pattern than just a datastore that exclusively implements that pattern. It's a design principle where a piece of data once created is never changed. That leads to event sourcing or Twitters "lambda architecture". The compiler is a database in the sense that it has a memory across incremental invocations. So the 'storage' happens to be in memory but could be on disk.