This is pretty cool.

At one point, I had to write some code to deal with messy JSON that was highly nested, and being able to use the null conditional operator (?.) to read things deep in the tree without crashing was very useful from both a sanity and also readability perspective.

This seems like a more general approach, assuming missing.blah_property would also return missing.

User extensible types on core functionality is where Julia really shines! Looking forward to see how the downstream libraries use it.