I don’t understand what you mean by “tension.” In an OO setting, you register specialized implementations with some type of interface / protocol / dispatcher. I

I don’t understand what you mean by “tension.” In an OO setting, you register specialized implementations with some type of interface / protocol / dispatcher. In functional programming you create functions that allow a data type’s specialized implementation to be type matched against a typeclass. In declarative programming you flatten out attributes so that a pre-existing set of polymorphic operators apply, and you translate the complexity of lookups (that match the implementation) into the complexity of specifying everything in a splayed-out form. It’s all the same complexity, just different trade-offs for how it’s represented. I wouldn’t describe this as “tension” because that makes it sound as though declarative programming has some fundamental limitation or missing capability regarding encapsulation, but it doesn’t. It’s just making a different trade-off of the same thing.