Julia parametric constructor and incomplete initialization

Question

I'm new to programming in Julia (I'm coming from Java) and some Julia concepts are still difficult to understand to me. What I intend is to replicate this Java code:

public class Archive<T extends Solution> {
  public List<T> solutions;

  public Archive() {
    solutions = new ArrayList<>() ;
  }
}

Then, I have defined the following Julia struct:

mutable struct Archive{T <: Solution}
  solutions::Vector{T}
end

and I would like to define a constructor to inialize empty archives. In the case of an external constructor, I think that the code would look like this:

function Archive()
  return Archive([])
end

Would it be possible to modify the constructor somehow to include the parametric type T?.

How would you like to use `T` in the constructor? Also, note that the first `struct` definition does not work on Julia (at least on version 1.8). If you add details and make code more runnable, the answers could be more on point. — Dan Getz, Oct 19 '22 at 15:16
Now things are clearer. For the definition, you would want `struct Archive{T<:Solution} solutions::Vector{T} end`. It's usually best to leave things immutable, and you could still `push!` and modify elements of the `solutions` vector. In addition, in Julia the compiler infers types for you, so, there is no need to over-specify types (a common instinct for people from statically typed languages). — Dan Getz, Oct 19 '22 at 16:27

score 3 · Answer 1 · answered Oct 19 '22 at 16:36

This is most likely the constructor you want:

Archive(T::Type{<:Solution}=Solution) =Archive(T[])

Then you can write Archive() or Archive(SomeType) if SomeType is a subtype of Solution.

Note that your definition:

function Archive()
  return Archive([])
end

Is incorrect unless Solution is Any (and most likely it is not). You will get an error as [] is Vector{Any} and you require Archive to store subtypes of Vector{<:Solution}.

Julia parametric constructor and incomplete initialization

1 Answers1