Conditional updates

[lornajane]

How about allowing conditional updates. The use cases I'm thinking of:

• if target exists (or doesn't)
• if target is empty (or isn't)

This would let us do things like add missing error responses, but not overwrite any that are already defined.

[handrews]

@lornajane an add operator that only takes effect if nothing is present at the target would allow implementing document-wide default values, which is something we have been asked about repeatedly.

Current Overlays are great for unconditional globals or merging in fields, but defaults are a bit different.

[lornajane]

Yes, agree, we need some sort of weak add or fallback option.