A general "web app"'s germane parts are:
- The part that receives the connection
- The part that sends back a response
- Interacting with other unspecified systems through IPC, RPC or whatever (databases mainly)
The shit in between, calculating a derivative or setting up a fancy data structure of some kind or something, is interesting but how much of that do we actually do as programmers? I'm not being obtuse - intentionally anyway - I'm actually curious what interesting things functional programmers do because I'm not seeing much of it.
Edit: my point is, you say "Anything else is logic." to which I respond "What's left?"