Types just describe your data structures, and type relations describe mappings between data structures. In this sense, functional programming is actually the purest form of data-first programming: It discourages inventing algorithms at any point in the process, instead asking you to think exclusively about what data structure you have and what data structure you want to end up with. You may have to step between multiple data structures to get from A to B, but the focus is always on the data, never on the algorithms.

From what I've seen, this is where people usually struggle when it comes to learning functional programming. People are so wrapped up in the algorithms that they have a hard time thinking in data in the way that is required to do functional programming effectively.