Take for example octahydropentalene (bicyclo[3,3,0]octane) and spiro(3.4)octane:
I would think that they are chain isomers of sort but according to nomenclature counting rules, they are both 8-carbon main-chains.
However they could also be considered ring-chain isomers but I thought that is only classified satisfactorily by a main chain being closed and open.
I wouldn't think they are functional isomers although it may be true. Since alkadienes and alkynes (this is also a bit grey to me since I couldn't find a good source for it) are considered functional isomers.
Also I would appreciate any primary source that determines the type of isomers two compounds are called and the rules which determine their relation.