kix.keep

kix.keep container

Marks a container “never collapse to absent”. Use it in constructors that build a presence-meaningful field (an emptyDir volume source, a label selector) from optional arguments that may all be null.

Argument and result

Input	Result
Attrset	The same attrset plus an additive _kixKeep marker. Resolved at mkResource; the marker never reaches rawManifest, the identity JSON, or the rendered output.
List	Normalized eagerly: null elements dropped; [ ] returned if nothing survives. No marker, because Nix merges replace lists wholesale, so a kept list cannot be re-filled with nulls later.
Anything else	Throws.

Normalization background

Rendering applies the collapse rule, bottom-up:

• null values are dropped.
• A container that had members and lost all of them collapses to null and is omitted; the collapse cascades upward through wrappers.
• A container written literally empty ({ }, [ ]) renders as {}/[].
• Null list elements are dropped; an element that collapses entirely leaves its list.

kix.keep overrides the second point for one container: it renders {} even when every member was null.

Behavior under merges

Operation	Result
lib.recursiveUpdate kept { field = value; }	Field merges in; keep is preserved.
lib.recursiveUpdate kept { field = null; }	Keep is preserved; the container still renders {} if everything collapses.
attrs // { key = replacement; }	Wholesale replacement discards the keep, matching the author’s intent: the value was replaced.
kix.keep (kix.keep x)	Same as applying it once.

When it is not needed

• An explicit empty value (emptyDir = { };) renders as-is. No keep required.
• A constructor that filters its own nulls (lib.optionalAttrs) produces a literal empty and needs no keep. kix.mount.emptyDir works this way.
• CRD schema subtrees rarely need it: the collapse rule keeps their literal empties (properties = { };) as written.

Related

• Keep intentionally empty values for the task-oriented guide.
• Built-in reliability rules: the volumeHasSource rule fails the build on volumes that would render without a source.