[swift-evolution] [Pitch] Let's talk about submodules

[Brent Royal-Gordon]

> On Feb 21, 2017, at 8:33 PM, Matthew Johnson <matthew at anandabits.com> wrote:
> 
>> A submodule may not import any direct parent module (parent, grandparent, etc.), but may import any other submodule in the same module. This list shows permitted imports for a project with four modules/submodules:
>> 
>> 	ModKit
>> 		- ModKit.Foo
>> 		- ModKit.Foo.Bar
>> 		- ModKit.Quux
>> 	ModKit.Foo
>> 		- ModKit.Foo.Bar
>> 		- ModKit.Quux
>> 	ModKit.Foo.Bar
>> 		- ModKit.Quux
>> 	ModKit.Quux
>> 		- ModKit.Foo
>> 		- ModKit.Foo.Bar
> 
> Am I understanding this correctly?  It looks like ModKit.Foo.Bar cannot see or import any symbols declared in ModKit.Foo or ModKit.  Is that correct?  Descendents cannot see or import symbols declared in ancestors (including public symbols) but they can import any submodule that is not their ancestor (and therefore see public symbols in any submodule that’s not an ancestor by way of importing that submodule)?  

Yes, that's correct.

> I think I understand why you might have specified this design but am interested in hearing you elaborate a bit further on the rationale.

A few reasons:

1. It creates a stricter layering between parent modules and submodules: submodules are for lower layers, while parent modules are for higher layers. Layering reduces spaghetti, and less spaghetti is generally a good thing.

2. It limits the amount of code the compiler has to consider at once.

3. It helps avoid (but doesn't completely prevent) circular dependencies between submodules, ensuring there's an order in which they can be compiled separately.

On the other hand, a design which considered all submodules at once would certainly be more flexible. You could call back and forth between submodules and their parents freely, and organize your code more flexibly. But I'm not sure it can be made to work with this "build system decides what's in a submodule" approach—I'm trying to imagine how you would tell the compiler about the submodule mappings for fifty files without that information being in the source code, and I'm not liking any of the solutions I'm coming up with.

-- 
Brent Royal-Gordon
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