As a compromise, I think it would be helpful to be able to define structs that have a known layout in memory but no dynamic identity. They would be treated like primitive types in Java, but with the crucial difference that users could define their own. That way users could write Julia code that stores and accesses data in the same format they need for interoperating with whatever native libraries they use, instead of serializing and deserializing between Julia objects and C struct arrays (or using int or byte arrays in their Julia code and giving up most of the advantages of a modern programming language.)