with-meta

(with-meta obj metadata)

Answers a copy of obj carrying the map metadata (nil for none), which meta reads back. The original keeps its own metadata, and = ignores metadata. Maps, vectors, lists, sets, records, reify values, lazy seqs and functions carry it; a string, number, keyword or other value signals, like the oracle. A symbol answers itself without metadata.

Metadata changes dispatch only for a protocol declared :extend-via-metadata true (see defprotocol). A value derived from the copy (assoc, conj, ...) starts without metadata, where the oracle keeps it.

Reader metadata (^:private, ^:dynamic, ^{...} attr maps, type hints) on a name or a local parses and drops; only binding reads ^:dynamic. On a vector, map or set literal it attaches, like the oracle's reader: ^:k [1] carries {:k true}.

Under *print-meta* true, pr, prn, pr-str and str write non-empty metadata ahead of the value (^{:k 1} [1 2], a lone :tag as ^String [1]); print and println never do.

^{:k 1} [1 2]