import/using search order?

[toivoh]

In the example below are three (top level) modules, ModA, Meta, and ModB. ModB attempts to use the other two.
The trouble is that ModB has an implicit using Base, which shadows the top level Meta module:

module ModA
export myvar_A
myvar_A=5
end

module Meta
export myvar_Meta
myvar_Meta=5
end

module ModB
using ModA, Meta
@show myvar_A
@show myvar_Meta
end

Running this in the repl gives me

myvar_A => 5
ERROR: myvar_Meta not defined

This behavior caused me the trouble in toivoh/Debug.jl#35: Debug.jl has a sub-module Debug.Meta. When Base.Meta was added, it shadowed Debug.Meta for the other Debug sub-modules. In general, it seems to me that the current behavior with implicit using Base risks to break existing code each time an export is added to Base.

My suggestion is to change name lookup within a module according to:

• No implicit using Base in any module
• If name lookup fails within a non-bare module, a lookup is made in Base as a fallback.

An alternative would be to change the interpretation of using to provide a fallback only if an identifier is not found in any other way.

[toivoh]

After thinking about this some more, I realized that the current behavior is not a bug. But I do think that a change is motivated not to let the implicit using Base shadow other identifiers, risking to break code each time an export is added to Base. I've updated the issue accordingly.

[JeffBezanson]

This is really all about the search order for using and import. I agree it seems wrong for import X to snag an X that happens to be visible via using. I'm trying out the idea of simply ignoring using for the purposes of evaluating import paths, and so far it seems reasonable. cc @StefanKarpinski

[JeffBezanson]

There are some cases where code has relied on the current behavior, e.g. import Sort.lt, where Sort is visible via the implicit using Base. That seems reasonable by itself, but of course it's unacceptable that this prevents you from using a top-level module called Sort. It should probably have to be written import Base.Sort.lt.

[vtjnash]

should this be closed now?

[JeffBezanson]

Ok, I believe I have the final design for this on the jb/relativeimport branch.

[JeffBezanson]

merged into master.