The merge rules say objects merge, arrays replace, undefined is skipped and null clears a key. The first two I have hit in practice. The last two I cannot reproduce — putting mask: null in an environment file gives me a schema error, not a cleared key.
Am I using it in the wrong place?
Accepted answer
Yes — the two rules live in different places, and neither is where you looked.
null clears a key during a merge that happens before validation. That is the project file: framework defaults are merged with sdods.project.yaml and the result is then parsed. So a null there removes the key from the merged object, and the schema default takes over for it — which usually means you get the default back rather than nothing. In an environment file the order is reversed, the file is parsed first, so mask: null is a type error exactly as you saw.
undefined skipped is the flag layer, and it is what makes flags usable at all: a flag you did not pass is undefined and never overwrites the yaml. Without that rule every unpassed flag would blank out the value beneath it.
Also worth knowing: a bare mask: in YAML with nothing after it is null, not "unset". People write that meaning "leave it alone" and get the other behaviour.
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