Functions are not always pure, no. Clojure is not so strict about I/O, meaning that function could for instance call out to make a database request. But I think

Functions are not always pure, no. Clojure is not so strict about I/O, meaning that function could for instance call out to make a database request. But I think that is unrelated to immutability or ownership of data? Immutability guarantees if I call getFoo(x) I can trust that nothing happens to x (assuming x is proper Clojure data and not some unsafe other thing like a Java Object). And getFoo can likewise return anything without there being any risk that anyone else modifies that thing. It does not matter who owns x or who owns the return value from getFoo, since no one is able to modify those things anyway. * Database request was a bad example as, as far as I know, there is nothing built-in for supporting that, so that would already be in unsafe Java territory. But there are non-pure functions built into the Clojure standard library that getFoo could call to return different results on the same input, like slurp or deref, and probably many other ones (I have not used Clojure for a long time).