The JDK stores all Java-specific project information directly in the projects IJavaNature. EclipseFP uses a HaskellProjectManager singleton that maps IProjects to IHaskellProjects. It would be cleaner to just store this stuff in the IHaskellNature implementation.
However, HaskellProjectManager performs some more tasks on the side, so this might not be trivial or even desirable. Needs more investigation.