Abstract type in Julia and member variables

Question

I am very confused as to whether abstract types in Julia can have member variables (like virtual classes in C++). I am sure the docs mention it but I cannot find it!

I tried something like:

abstract AbstractR
  source
end

And now I have a concrete implementation as:

type RR <: AbstractR
end

function loadSource(reg::RR, x::AbstractString)
  println("Hello")
end

However, when I try to load the module, I have:

ERROR: LoadError: LoadError: LoadError: UndefVarError: source not defined

So is it that abstract type is just a type name and nothing else? Also, why did the language designers decide to not support member variables (if that is indeed the case). Also, I do not then really see the point of having an abstract type anyway...

EDIT

Module file

module TestProj
export AbstractR
export RR
include("generic.jl")
include("rr.jl")
end

generic.jl

abstract AbstractR
  source
end

rr.jl

type RR <: AbstractR
end

function loadSource(reg::RR, x::AbstractString)
  println("Hello")
end

Alexander Morley Alexander Morley · Accepted Answer · 2017-03-31T09:18:40

At least the way I understand it is that abstract types are intended to serve as "nodes" and as such are not themselves instantiateable (sp?). As such they allow you organise concrete types into hierarchies & provide an easy way for methods to dispatch on any of a group of concrete types.

I think the docs explain this here.

(With this the error msg should make sense as well?)

Abstract type in Julia and member variables

2 Answers