By "primitive data types" I didn't mean to exclude built-in collections of primitive data types. Point taken, though: I probably should have said "built-in data

By "primitive data types" I didn't mean to exclude built-in collections of primitive data types. Point taken, though: I probably should have said "built-in data types", or some such. I didn't even mean to exclude algebraic data types, as I explicitly mentioned. Though I think that the question of whether algebraic datatypes should be allowed is an interesting one. Also, clearly allowed would be any objects provided by the standard library, since returning those would not couple your code to the code of the API. As Rich Hickey expresses it, I believe, you should only return from an API data that you could send over the wire without sharing any of the API's code. If you stick to this rule, then, for instance, it is much easier to make your application distributed. (Though I'm not sure to what degree Hickey and The Law of Demeter would agree on everything. E.g., returning some hairy nested dictionary of dictionaries of dictionaries of strings to represent a book, or what have you. I don't know what Rich Hickey would say about that.)