(Re-)exporting from extension

General Usage
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1

Hi everyone,

is it possible to re-export names defined in a package extension from within the main package itself?
For example, the hypothetical main package “Optimizer” defines an optimization structure allowing for gradient based optimization, but the user would have to provide derivatives by hand. I could now think of offering optional ForwardDiff support via Requires.jl or the new extension system.
With the Requires approach, I was able to load the sub-module like so:

function __init__()
  require ForwardDiff="f6369f11-7733-5829-9624-2563aa707210" begin
    include("../ext/ForwardDiffBackendExt.jl")
    using .ForwardDiffBackendExt: ForwardDiffBackend
    export ForwardDiffBackend
  end
end

Now, if ForwardDiff has been loaded, a user could conveniently use ForwardDiffBackend.
Can I achieve something similar with extensions? That is, load the extension in the main package and maybe even export names from it?
I tried

function __init__()
     m = Base.get_extension(@__MODULE__, :ForwardDiffBackendExt)
     global ForwardDiffBackend = isnothing(m) ? m : m.ForwardDiffBackend
 end

But this gives

ERROR: LoadError: InitError: UndefVarError: `ForwardDiffBackend` not defined

and

Error: Error during loading of extension ForwardDiffBackendExt of Optimizer, use `Base.retry_load_extensions()` to retry.

This issue seems related:

2

Related docs PR, if anyone wants to improve / merge it:

2 Likes
3

Thanks! From looking at it, what I have in mind is not supported/recommended as of yet (?)
Current workaround is a getter-function that I export instead.