Copies
The Rust implementation, pinned to de1b6c9e. Every builtin signature.
dup returns a logical copy of a value. Arrays and hashes share accounted storage until a write requires a copy, so a nested update through the copy leaves the original unchanged:
a = { items: [1] }
b = a.dup
b["items"].push(2)
[a, b] # [{items: [1]}, {items: [1, 2]}]
Collections already have value semantics, so binding one to a second name makes the same independent copy; dup makes it explicit. For a class instance dup returns the same object, since instances have identity. Scalars, enums, type literals, classes and builtin exports return themselves.
Match data and rescued errors keep their protection and special string rendering through dup, including transfer through Rust host values: a nested write such as m.dup.captures.push("x") is rejected. Captures assigned to a separate variable are independent collection values and can be updated.
dup takes no arguments, keywords or block. User-defined class methods named dup take precedence, subject to visibility. It retains the same tracked storage and work counters as an ordinary binding.
clone, freeze and frozen? were removed by ADR-008: clone was a second spelling of dup, and with no mutable freeze flag freeze returned its receiver and frozen? always answered true. vibes fix rewrites them.