UML has many diagrams. Only class diagrams are done directly for OOP conception. Object and Composite structure diagrams are not so strict already.
All other diagrams can be used without and apart from the OOP. Sequence, Timing, Activity, Communcation, Use case, Interaction Overview, State Machine, Package, Deployment, and Component diagrams work OK.
So, the only diagram not for the direct use is the class diagram, that you simply needn't in the functional programming.