[gfodor]

"fundamental as function composition" is obviously subjective, but I think the point is that it seems you can make a fairly strong argument that in lisps, there is a question of what does the form (map f) eval to, and transducers put forth an answer that seems to be logically correct and superior to any alternatives. Hence, it seems fundamental.

[tel]

I would suggest that there's a good chance that lenses (in particular, one subset of the general theory called Folds) are vastly more general.