alt Hacker Newsfoo is a name. It's not at all clear what you mean by "the foo" ... the called function can modify the object referenced by the symbol foo unless it's immutable. If this is your complaint, then solve it with documentation ... I never write a function, in any language, that modifies anything--via parameter or in an outer scope--without documenting it as doing so.
> I don't care about the reference to foo (that's a low-level detail that should be confined to systems languages, not application languages)
This is not true at all. There's a big difference, for instance, between assigning a reference and assigning an object ... the former results in two names referring to the same object, whereas in the latter case they refer to copies. I had a recent case where a bug was introduced when converting Perl to Python because Perl arrays have value semantics whereas Python lists have reference semantics.
There seem to be disagreements here due entirely to erroneous or poor use of terminology, which is frustrating ... I won't participate further.
>> I sorely miss it in Python, JS and other languages. They keep me guessing whether a function will mutate the parent structure, or a local copy in those languages!
> Python at least is very clear about this ... everything, lists, class instances, dicts, tuples, strings, ints, floats ... are all passed by object reference. (Of course it's not relevant for tuples and scalars, which are immutable.)
Then let me just FTFY based on what you've said later:
Python will not be very clear about this ... everything, lists, class instances, dicts, tuples, strings, ints, floats, they all require the programmer to read and write documentation every time.