[jcparkyn]

It's only semantically a pass-by-value, in reality a reference is passed and the data is only copied if needed (i.e. value is mutated while shared).

[zahlman]

So the language has reference semantics, and (per the edit) for every object (like in Python)?

(Ah, no, your example elsewhere in the thread suggests that the referred-to structures get implicitly copied all over the place.)

[jcparkyn]

Nope, it's value semantics (unlike Python), the references are just an internal optimization. Implicit copies happen when a list/dict with more than one reference is mutated.