[swift-evolution] [Draft] Really Simple Submodules

[Michel Fortin]

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