# vary-meta Apply `$f` to the value's current metadata (or nil when it has none) and attach the result — equivalent to `[with-meta $value [$f [meta $value]]]`. Convenient for threading functional metadata updates. ## Syntax

[vary-meta $value $f]

## Parameters - `$value` — A metadata-capable value: map, tagged map, vector, set, string, or keyword. - `$f` — Callable receiving the current metadata map (or nil) and returning the new metadata map, or nil to strip it. ## Examples

def v [with-meta :deepseek %{n 1}]
[meta [vary-meta $v {|m| [assoc $m :n ($m~n + 1)]}]]   # → %{:n 2}

[meta [vary-meta :plain {|m| %{seen true}}]]           # → %{seen true}

## Returns A new value sharing the payload, with `$f`'s result attached as metadata (a nil result strips it). `%Err{:message "vary-meta: second argument must be callable"}` when `$f` is not callable; immediate values (numbers, nil, booleans) return unchanged. ## See Also `with-meta`, `meta`, `without-meta`