Sliced bananas on opaque data (The expression lemma). Ralf Lämmel and Ondrej Rypacek.
Algebraic data types and catamorphisms (folds) play a central role in functional programming as they allow programmers to define recursive data structures and operations on them uniformly by structural recursion. Likewise, in object-oriented (OO) programming, recursive hierarchies of object types with virtual methods play a central role for the same reason. There is a semantical correspondence between these two situations which we reveal and formalize categorically. To this end, we assume a coalgebraic model of OO programming with functional objects. In practical terms, the development prepares for refactorings that turn sufficiently disciplined functional folds into OO programs of a designated shape (and v.v.).
I haven't even glanced at the paper yet, but it looks extremely interesting, and it's directly related to some recent discussion. This blog post from Ondrej is also relevant.