Don't worry, I'm not re-opening the currently debated issue . It occurred to me a while ago that it should be possible to support this feature without any changes to the language itself.
Instead I've created the macros `@_abstract`, `@_type`, and `@_immutable`. The idea is that `@_abstract` does two things: 1. It adds the fields to a global registry and creates an abstract class with the given name 2. When a @_type or @_immutable is created, it copies the fields from the abstract registry and adds its own The current working code can be found at: https://github.com/abeschneider/AbstractFields.jl A quick example: @_abstract Point begin x::Float64 y::Float64 end @_type 3DPoint Point begin z::Float64 end # create a 3DPoint (abstract's values get filled first) point = 3DPoint(1.0, 1.0, 1.0) One of the main advantages of using these macros is that you know if you have a function that takes a `Point`, it will have `x` and `y` defined. As has been pointed out, this isn't strictly necessary, as you could provide an interface with `getx`, `gety`, `setx`, `sety` for each subclass of `Point`. However, this can result in a lot of extra boilerplate code. Anyways, I'm not sure if this is useful/interesting to anyone else, but it's something I've wanted for my own code.
