  With data lakes and schema-free NoSQL document stores, extracting a
descriptive schema from JSON data collections is an acute challenge. In this
paper, we target the discovery of tagged unions, a JSON Schema design pattern
where the value of one property of an object (the tag) conditionally implies
subschemas for sibling properties. We formalize these implications as
conditional functional dependencies and capture them using the JSON Schema
operators if-then-else. We further motivate our heuristics to avoid
overfitting. Experiments with our prototype implementation are promising, and
show that this form of tagged unions can successfully be detected in real-world
GeoJSON and TopoJSON datasets. In discussing future work, we outline how our
approach can be extended further.
