I'm not sure I agree. Functional Reactive Programming is a declarative way of doing user interaction, but it's still a computer science research topic (Elm used to claim to be FRP, but has now decided to move away from its old allegedly FRP-like style). I need to play around with the latest FRP research libraries some time!

In addition there is also machines/pipes/conduits, which all mix imperative I/O with declarative style code in an interesting and new way.