After a recent discussion on this topic on the Haskell Reddit I posted that I've begun to the see the Expression Problem in terms of data and codata [1]. In ve

After a recent discussion on this topic on the Haskell Reddit I posted that I've begun to the see the Expression Problem in terms of data and codata [1]. In very brief, data tends to be completely transparent in its construction, finite (or tending toward finiteness), possessing obvious structural equality, and can be easily transformed in myriad ways via induction. Codata on the other hand comes from the other side. It is completely transparent in its destruction (observation, think object properties), tends to be infinite (think how the process of a state machine is infinite), possesses obvious entity identity, and can be easily created in myriad ways via coinduction. In most programming settings, these are just perspectives as they can easily coincide, but you can observe the difference when you program in a strongly normalizing like Agda. I feel that OOP has built most of its abstraction on the assumption that codata-oriented (object-oriented, kind of) modeling works well. For this reason, it's easy to talk about black-boxed domain models which are only predictably observed and classified, but it gets hairy to talk about equality of two trees---you're forced to think about pointer equality even when structural equality might be a more natural fit. FP doesn't necessarily pick one side or the other, but it certainly makes data more accessible (algebraic data types being as terse as they can be) leading to the Alan Perlis "100 functions" quote. Greatly powerful things can come about when the two sides are combined or emphasized for their strengths. In particular, the `pipes` ecosystem being built in Haskell right now straddles the line by being perhaps best modeled as a hybrid data/codata type. This means that you can both somewhat easily construct pipes inductively while also observing the infinite process (codata) they represent. I don't think anything I'm saying is particularly new to researchers, but I personally have been finding it incredibly instrumental when talking about how OOP and FP differ. [1] http://www.reddit.com/r/haskell/comments/1quhrl/from_object_oriented_programming_to_functional/ http://www.reddit.com/r/haskell/comments/1quhrl/from_object_...