Sounds like a combination between flow-based programming and declarative programming. Perhaps a possible approach would be to write a flow-based programming lan

Sounds like a combination between flow-based programming and declarative programming. Perhaps a possible approach would be to write a flow-based programming language with some methods marked as "pure" which statelessly map input to output and others able to be written in a more traditional, imperative, less functional style? In a flow-based architecture, it seems to me that by design, almost every component in the flow would be testable.