Category theory, computational complexity, and combinatorics connections?
Dave Clarke's comment is quite important. Generally fusion doesn't change the O(-) efficiency.  However, of particular interest is Liu, Cheng, and Hudak's work on Causal Commutative Arrows. Programs written with them are necessarily optimizable, in part through stream fusion, to a single loop free of dynamic memory allocation and intermediate structures: http://haskell.cs.yale.edu/?post_type=publication&amp;p=72