Have extensions trigger from another extension getting loaded

[KristofferC]

A feature request regarding extensions (from @termi-official).

Being able to write:

[extensions]
ExtBase = [...]
ExtAdvanced = ["ExtBase", ...]

which would cause ExtAdvanced to be loaded if ExtBase (and the rest of the weak deps dependencies for it) are loaded. You should then be able to get a handle to ExtBase inside ExtAdvanced. This is so you can extend methods in the parent for types defined inside ExtBase.

[termi-official]

To give a more specific example to elaborate the specific use-case: I have some numerical package with works per default without MPI. The first extension provide the some basic MPI functionality, while the transient extension provides a bridge to specific solvers and matrix data types. However, since these bridges are rather small I think it would be better suited to have it as an extension instead of as separate packages.

[KristofferC]

I wonder if this isn't enough to solve this:

name = "Foo"

[extensions]
ExtBase = ["A", "B"]
ExtAdvanced = ["A", "B", "C"]

ExtAdvanced.jl:

module ExtAdvanced

using Foo, A, B, C
const ExtBase = Base.get_extension(Foo, :ExtBase)

# code

end

We would just have to make sure that ExtBase gets loaded before ExtAdvanced.

[termi-official]

This should also be fine, if there is an efficient way to do it and if it does not interfere with other requirements for extensions.