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).

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zahlman 2 hours ago | parent [-]

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.)

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jcparkyn 2 hours ago | parent [-]

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.

	▲	zahlman an hour ago \| parent [-]
		> the references are just an internal optimization Optimization specifically for function calls, or... ? Because if you're doing all this copy-on-write anyway, the indirection seems to be a needless cost in other contexts.