Yet Another Take on Swift Sub-modules

sub-modules.md

Sub-modules

A sub-module solution in Swift should have the following properties:

• Extremely light-weight
• Low API surface area
• Adopt progressive disclosure
• Integrate with Access Control features to enable a level of encapsulation & hiding between the Module and File level
• Be permeable when desired

Discussion

As we get deeper into building real applications & frameworks with Swift, we begin to realize that having a way to express relationships between types is desireable. Currently, Swift only allows us to express these relationships at two levels, the Module and the File.

The Module boundary is acceptable for small, focused frameworks, while the File boundary is acceptable for small, focused Types, but both levels can be unweildy when dealing with certain cases where a cluster of internally related types needs to know about each other but may only want to publish a narrow set of APIs to the surrounding code, or in large complex applications which are necessarily structured as a single Module. In these cases, we wind up with large monolithic Modules or (even worse) large monolithic Files.

I have seen this proliferation of Huge Sprawling Files (HSFs) in my own code, and seek a way to combat this rising tide.

Goals

It is a goal of this proposal to:

• Suggest a mechanism for organizing code between the Module and File levels that is as lightweight and low-friction as possible
• Provide mechanisms for authors to create both "hard" and "soft" API boundaries between the Module and File levels of their code

Anti-Goals

It is not a goal of this proposal to:

• Move Swift away from filesystem-based organization
• Significantly alter the current Access Control philosophy of Swift

Proposal Notes

Please take the following proposal wholely as a Straw-Man... I would be equally satisfied with any solution which meets the critera described at the top of this document.

Unless specified otherwise, all spellings proposed below are to be considered straw-men, and merely illustrative of the concepts.

Proposed Solution

Two things are clear to me after using Swift and following the Swift Evolution list since their respective publications:

1. Swift has a preference for file-based organization
2. Vocal Swift Users dislike fileprivate and want to revert to Swift2-style private

Because of #1, this proposal does not seek to change Swift's inherent file-system organization, and instead will expand on it.

Since I personally fall into the camp described by #2, and most of the community response to this has been "Lets wait to deal with that until sub-modules", I'm making this proposal assuming that solving that quagmire is in-scope for this propsoal.

Changes to Access Control Modifiers

As part of this proposal, I suggest the following changes to Swift 3's Access Control modifiers:

• Revert private to Swift 2's meaning: "hidden outside the file"
• Remove fileprivate as redundant

This is potentially a source-breaking change. However, it is interesting to note that this change is not required for the following proposal to function.

Changes that are necessary are:

• Change the spelling of internal to module (making module the new default)
• Introduce a new modifier internal to mean "Internal to the current sub-module and its child-sub-modules"

These changes are not source-breaking because the new internal modifier acts exactly as the old internal modifier unless it is used within a sub-module. The specific spelling of this new internal modifier is necessary to maintain backwards source compatibility.

The new module modifier allows authors to make APIs permeable between sub-modules while still hidden outside the owning Module if desired.

All other Access Control modifiers behave the same as they currently do irrespective of sub-module boundaries, so:

• public => Visible outside the Module
• open => Sub-classable outside the Module

Making a Sub-module

To create a sub-module within a Module (or sub-module) is simple: The author creates a directory, and places a "sub-module declaration file" within the directory:

//  __submodule.swift
//  MyModule

submodule SubA

Then any files within that directory are part of the sub-module:

//  Foo.swift
//  MyModule.SubA

struct Foo {
    private var mine: Bool
    internal var sub: Bool
    module var mod: Bool
}

public struct Bar {
    module var mod: Bool
    public var pub: Bool
}

This creates a sub-module called "SubA" within the module "MyModule". All files within the directory in which this file appears are understood to be contained by this sub-module.

If in the future we choose to add additional complexity (versioning, #availability, etc) to the sub-module syntax, the sub-module declaration gives a natural home for this configuration.

It's important to note some benefits of this approach:

• Using the "special file" means that not all Directories are automatically submodules
• Any given source file may only be a member of 1 submodule at a time
• Use of filesystem structure to denote sub-modules plays nicely with source control
• The sub-module structure is instantly clear whether using an IDE (which can be taught to parse the __submodule.swift files to decorate the UI), or simple text-editor (assuming a convention of naming the Directory the same as the sub-module, which is a linter problem)

Using Sub-modules

Referencing a sub-module should be natural and clear at this point:

From Within the Parent Module/Sub-module

Sub-modules are simply code-organization & namespacing tools within modules. As such, when referenced from within their parent Module, there is no need for imports

//  in MyModule

let foo = SubA.Foo()
foo.mine = true // Compiler error because it's private
foo.sub = true  // Compiler error because it's internal to the sub-module
foo.mod = true  // OK

From Outside the Parent Module/Sub-module

When referenced from outside their parent Module, one imports the whole module in the standard way:

import MyModule

let foo = SubA.Foo() // Compiler error because it's internal to the Module

let bar = SubA.Bar() // OK
bar.mod = true  // Compiler error because it's internal to the Module
bar.pub = true  // OK

What this Proposal Deliberately Omits

This proposal deliberately omits several concepts which may be integral to various use-cases for sub-modules, primarily because they can be treated as purely additive concepts and I don't wish to weigh down the consideration of the overall approach with a larger API surface area that might be debated separately. I.e: Keep it as small as possible for now, then if it's any good, iterate on the design.

Inter-Sub-Module Access Control

One might ask given a sub-module structure like:

MyModule
  |
  +--- SubA
        |
        +--- SubB

"How can SubB hide properties from MyModule without hiding them from SubA?"

This is a valid question, and not answered by this proposal for two reasons:

• This trivial case could be solved by simply adding a new modifier submodule if we so desired, but:
• In the absence of any direct response, the status-quo provides a work-around: Omit the sub-sub-module structure and use the file-access constraints of private
• This overall problem probably should be solved by addressing larger questions in the Access Control scheme of Swift, irrespective of the sub-module mechanism

Expressiveness of Sub-module Imports

One might ask: "Why can't I import only a specific sub-module or alias a sub-module?"

I have ignored this aspect of submodules because the question of import expressiveness is a separate issue in my mind. The fact that we cannot say:

import MyModule as Foo

Has no relationship to the lack of sub-modules in Swift.

If the community deems it an important enough use-case to warrant altering import behavior, so be it, but that can be treated as purely additive to this proposal.

But it should be understood that this approach to sub-modules is not designed to provide an expressive "exports" capability. It is primarily interested in organizing code within a Module