"Architecting programs as folds or traversals can be powerful." This is my own idiosyncratic view but lately I've been fiddling with the proposition that the d

"Architecting programs as folds or traversals can be powerful." This is my own idiosyncratic view but lately I've been fiddling with the proposition that the defining characteristic of functional programming languages is the definition and pervasive utilization of recursion schemes via composition as the primary organizational method for code. That is to say, such programs do not just borrow a few recursion schemes here or there (like chaining a map with a filter and calling it a "functional program"), but that the entire architecture is oriented around operating on data via such recursion schemes, stacked on top of each other. So many of the previous touchstones keep getting moved into non-functional languages, like "functions as first-class citizens" (almost everything has this now) and "immutability" (less common, but common enough we can say with confidence now that having immutable values does not turn a language "functional"), but the destination languages remain quite distinct from something like Haskell. From that point of view, lens is a development you'd expect any such language to eventually come up with and make extensive use of. It isn't the only way to compose recursion schemes together, but it's certainly one of the basic tools. (An interesting element of this particular heterodox view is that this enables one to imagine a "pure functional language" that is not based on total immutability, though whether that's a good idea is another question.)