[swift-evolution] [Draft] Really Simple Submodules

Michel Fortin michel.fortin at michelf.ca
Mon Mar 6 09:14:45 CST 2017 

How do you define 'outsider'? What is the threat model you want to be protected from?

I believe it is someone on your team or a contributor not following the established policies for the project, in particular the policies related to encapsulation. Those policies are necessarily going to vary from project to project and from team to team. It is to accommodate these varied cases and policies that the other submodule proposals have plenty of different ways submodules can be organized and used, and also why they are complex.

Perhaps instead of baking those policies as language features the team should use a pre-commit linter tool enforcing them. This proposal should make it simpler to write such a linter.

> On 6 mars 2017, at 2:47, Rien <Rien at balancingrock.nl> wrote:
> 
> +1 for the simplicity. I like that.
> 
> However I dislike the non-hierachical approach: It should not be possible for an ‘outsider’ to declare itself part of a submodule if the original developer of that submodule did not want that. I understand that the submodules are not exported through the module (though wait for the requests to do so if this were ever implemented) but if large teams want to use submodules to structure their code base, some level of hierarchy is imo necessary.
> 
> Regards,
> Rien
> 
> Site: http://balancingrock.nl
> Blog: http://swiftrien.blogspot.com
> Github: http://github.com/Balancingrock
> Project: http://swiftfire.nl
> 
> 
> 
> 
> 
>> On 05 Mar 2017, at 23:16, Michel Fortin via swift-evolution <swift-evolution at swift.org> wrote:
>> 
>> Sorry for introducing yet another submodule proposal out of the blue.
>> 
>> I'm a bit surprised at how far-reaching the various submodule proposals floated on this list have been. Directories, access controls, @exported imports... For comparison's sake here's one that is *really* simple and short I wrote today. Best of all: it requires no new access modifier. 
>> 
>> I also expect everyone to scream at it because it does not include all the desirable features of a submodule system. At the very least I'll have redefined the meaning of lightweight in that discussion. Good reading.
>> 
>> Also available here: 
>> https://gist.github.com/michelf/779b1bc26a778051b6231b5639665e18
>> 
>> 
>> ## Motivation
>> 
>> The goal of this proposal is to provide lightweight intra-module boundaries so you can avoid exposing every `internal` declaration within a big module to all other files in the module.
>> 
>> Not a goal: addressing the file-level access fileprivate/private or scoped/protected debates. This is left to other proposals.
>> 
>> 
>> ## Summary
>> 
>> This proposal adds the declarations `submodule` and `import submodule`. It also limits the visibility of `internal` to files with a matching `submodule` or `import submodule` declaration.
>> 
>> Submodules are never exposed outside of the module. They only change the visibility of internal declarations, so there is no point in exposing them publicly.
>> 
>> Submodules are not bound to directories, nor are they necessarily hierarchical.
>> 
>> This change is purely additive and introduces no source compatibility issue.
>> 
>> 
>> ## Details
>> 
>> A `submodule <name>` declaration at the beginning of a file contains an identifier with the submodule name:
>> 
>> 	submodule Foo
>> 
>> 	internal func foo() {}
>> 	public func pub() {}
>> 
>> `internal` declarations within that file are only visible to other files sharing the same submodule name. The submodule only protects `internal` declarations: `public` declarations in the file are visible everywhere (in other submodules and in other modules).
>> 
>> A file can be part of more than one submodule:
>> 
>> 	submodule Foo
>> 	submodule Bar
>> 
>> 	internal func achoo() {
>> 		foo() // available in Foo (from other file)
>> 	}
>> 
>> This makes the internal `achoo` function visible within both the `Foo` and `Bar` submodules. Also note that it makes internal members of both submodules `Foo` and `Bar` visible within the file.
>> 
>> A file can access internal declarations of a submodule without having to expose its own internal functions to the submodule with `import submodule`:
>> 
>> 	submodule Test
>> 	import submodule Foo
>> 
>> 	internal func test() {
>> 		foo() // internal, imported from Foo
>> 		achoo() // internal, imported from Foo
>> 		pub() // public, so always visible
>> 	}
>> 
>> Finally, when a file has no submodule declaration, its internal declarations are visible everywhere in the module and all its submodules:
>> 
>> 	--- Hello.swift ---
>> 	// no submodule declaration
>> 	internal func hello() {}
>> 
>> 	--- World.swift ---
>> 	submodule World
>> 	internal func test() {
>> 		hello() // visible!
>> 	}
>> 
>> 
>> ## Nitpicky Details (Conflicting Declarations)
>> 
>> Declaring `internal` things that share the same name in two separate submodules is not a conflict:
>> 
>> 	--- Foo1.swift ---
>> 	submodule Foo1
>> 	class Foo {} // added to Foo1
>> 
>> 	--- Foo2.swift ---
>> 	submodule Foo2
>> 	submodule Foo3
>> 	class Foo {} // added to Foo2 and Foo3
>> 
>> (Note: It would be a conflict if one of them was `public`, because `public` declarations are always visible everywhere inside (and outside of) the module.)
>> 
>> Attempting to use both from another submodule will create ambiguity errors. You can disambiguate using the submodule name as a prefix:
>> 
>> 	--- Main.swift ---
>> 	import submodule Foo1
>> 	import submodule Foo2
>> 	import submodule Foo3
>> 	let f0 = Foo() // ambiguity error
>> 	let f1 = Foo1.Foo() // good
>> 	let f2 = Foo2.Foo() // good
>> 	let f3 = Foo3.Foo() // good
>> 
>> Best to avoid this for your own sanity however.
>> 
>> 
>> ## Alternatives Considered
>> 
>> ### Conflicting Declarations
>> 
>> Instead of allowing conflicting symbols in different submodules, we could continue to disallow conflicting `internal` declarations even when they belong to different submodules. This would make the design simpler, as it is closer to how `internal` currently works and prevent ambiguity errors from arising when importing multiple submodules. The behavior would be a little surprising however.
>> 
>> We could also simplify by removing the ability to use the submodule name as a prefix to disambiguate. This has the advantage that if you put a type inside of a submodule with the same name, no conflict can arise between the name of the type and the name of the submodule. Disambiguation would have to be done by renaming one of the conflicting declarations. Since this ambiguity can only affect `internal` declarations (submodules only group internal declarations), requiring a rename will never break any public API. But forcing a rename is not a very elegant solution.
>> 
>> ### `import` syntax
>> 
>> Renaming `import submodule` to `import internal`. Having "internal" in the name could make it clearer that we are giving access to internal declarations of the submodule. But it also make the import less relatable to the `submodule` declaration in other files.
>> 
>> 
>> ## Future Directions
>> 
>> ### Submodule-Private
>> 
>> While a submodule-private access modifier could have been added to this proposal, the belief is that this proposal can live without it, and not having this greatly reduce the little details to explore and thus simplifies the design.
>> 
>> In many cases you can work around this by putting "private" stuff in a separate submodule (somewhat similar to private headers in C land). For instance:
>> 
>> 	--- Stuff.swift ---
>> 	submodule Stuff
>> 	submdoule StuffImpl
>> 
>> 	func pack() { packImpl() }
>> 
>> 	--- StuffImpl.swift ---
>> 	submodule StuffImpl
>> 
>> 	func packImpl() { ... }
>> 
>> This will not work for stored properties however. A future proposal could suggest allowing stored properties in extensions to help with this.
>> 
>> And a future proposal could also add submodule-private to limit visibility of some declarations to only those files that are part of a specific module. 
>> 
>> 
>> -- 
>> Michel Fortin
>> https://michelf.ca
>> 
>> _______________________________________________
>> swift-evolution mailing list
>> swift-evolution at swift.org
>> https://lists.swift.org/mailman/listinfo/swift-evolution
> 

-- 
Michel Fortin
https://michelf.ca

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