Does importing a submodule do anything?

General Usage
,
1

The following codes do the same thing:

module A
    x = 3
end
@show A.x # 3

and

module A
    x = 3
end
import .A
@show A.x # 3

Does import .A do anything at all in the second example? If not, why is it allowed?

2

No, I think it does not nothing, as it is bringing to the scope a name that is already in the current scope.

I’m not sure if that is needed in any situation, maybe it is allowed because not allowing it would require a more complicated implementation of import in general. Do you see any important pitfall on allowing it?

1 Like
3

Reviving an old topic.

module MySubModule
    f()::Nothing = println("MySubModule.f(): Some code")

end # MySubModule

import .MySubModule # MySubModule defined in scope

MySubModule.f()

This works with and without the import. I think, just as stated above, that MySubModule is already defined in scope, so bringing it in scope with import again is a no-operation.

Surprisingly, I found exactly this in the documentation:

In contrast,

julia> import .NiceStuff

brings only the module name into scope.

Is this documentation from an era in which import NiceStuff made sense? Or should the documentation be updated?

4

I suspect it’s just for consistency with the ongoing example. NiceStuff is an ad-hoc module defined interactively — tutorial-like — and so it is definitionally not a package that needs to be brought into scope. That could indeed be noted! Maybe something like this:

… brings only the module name into scope. In this case, the NiceStuff module was interactively defined and so its name is already in scope, so this doesn’t do anything here.

1 Like
5

But if it is not defined interactively, say in a script, the same scope considerations hold, and also there the import .SubModule can be omitted. Does Julia have any trickery to show a list of identifiers that are in scope, I wonder now. If such a submodule is on the list, we would know a little bit more.

6

At any top-level, you can see this with names:

julia> module NiceStuff
       end
Main.NiceStuff

julia> names(@__MODULE__)
5-element Vector{Symbol}:
 :Base
 :Core
 :InteractiveUtils
 :Main
 :NiceStuff
1 Like