While everyone is talking about Parameters.jl I may as well add that I wrote this recently:
https://github.com/rafaqz/Defaults.jl
It’s kind of a minimalist version of Parameters.jl, with easy override of field values and the ability to add default constructors to 3rd party structs. It’s 46 lines, with a single 180 line dependency. But not in the registry yet, and probably not for ages at the rate I’m actually registering my packages right now…