I'm not massively keen on the former (IMO having to write .map etc helps you get a better "feel" for what parts will have the biggest impact on performance), bu

I'm not massively keen on the former (IMO having to write .map etc helps you get a better "feel" for what parts will have the biggest impact on performance), but the latter looks an awful lot like lenses, which are pretty great. Of course, a lens is really just a pair of functions in disguise, so it wouldn't prevent you from sorting my more complex functions if needed.