[swift-evolution] Yet Another Take on Swift Sub-modules

[Karim Nassar]

> On Mar 5, 2017, at 10:10 AM, Rien <Rien at Balancingrock.nl> wrote:
> 
>> 
>> On 05 Mar 2017, at 15:52, Karim Nassar via swift-evolution <swift-evolution at swift.org> wrote:
>> 
>> 
>>> On Mar 4, 2017, at 2:54 PM, Matthew Johnson <matthew at anandabits.com> wrote:
>>> 
>>>> 
>>>> On Mar 4, 2017, at 9:56 AM, Karim Nassar <karim at karimnassar.com> wrote:
>>>> 
>>>> 
>>>>> On Mar 3, 2017, at 5:21 PM, Matthew Johnson <matthew at anandabits.com> wrote:
>>>>> 
>>>>>> 
>>>>>> On Mar 3, 2017, at 9:24 AM, Karim Nassar via swift-evolution <swift-evolution at swift.org> wrote:
>>>>>> 
>>>>>> 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”
>>>>> 
>>>>> Can you give concrete examples of use cases where a descendent submodule needs access to symbols declared by an ancestor?  I gave some thought to this when drafting my proposal and came to the conclusion that this runs against the grain of layering and is likely to be a bad idea in practice.  If there are use cases I didn’t consider I am very interested in learning about them.
>>>> 
>>>> On further reflection and examination of my notes, I think you’re right, and that the `internal` encapsulation should be purely horizontal. Will adjust to reflect that.
>>>> 
>>>>>> ### 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
>>>>> 
>>>>> Why the double underscore prefix?  To make it sort to the top in a file browser?
>>>>> 
>>>>> Is this file allowed to have any Swift code?  Or is it limited to submodule-related declarations only?  If the latter, why not use an extension such as `.submodule` or `.swiftmodule` to differentiate it from ordinary Swift files and allow the submodule to be named by the name of this file?
>>>> 
>>>> So, my reasoning was that by requiring a specific standard name for the declaration file, we guarantee that any directory can only describe one submodule. Prefixing the proposed name with underscores was simply a way of preventing naming collision with potential “real code” files (and yes, as a side-effect alpha-floating it to the top). Since the `submodule` declaration might expand to include statements & configuration about the sub-module, I see no reason to prohibit Swift code from existing in that sub-module declaration file… Disallowing/controlling that seems to be a style/linter concern.
>>>> 
>>>> However, as I mentioned above, all the specific spellings (except `internal`)  for the different concepts in this proposal are straw-men awaiting input. I’d say the addition of a new type of file extension raises some concerns for me, but there’s already been a lot of push back on the general idea of using filesystem structures to organize sub-modules, so the whole idea may be moot. 
>>> 
>>> I’ve actually been starting to come around to the idea of using the file system.  Not so much because I really like it, but because I have been considering further some of the drawbacks of other approaches.  
>>> 
>>> One big reason is that a submodule should form a scope and scopes should consist of code that is physically adjacent.  In practice this means it should reside in close proximity in the file system.  Allowing any file in a project to be a part of any submodule partly defeats the purpose of using them to structure a project internally.  If we’re going to be organizing the files in a submodule physically anyway maybe we should just take advantage of that fact and prevent a stray file in a distant part of the file system from being part of the submodule.
>>> 
>>> The second reason is that there is a big problem with placing a simple `submodule Foo` declaration at the top of each file in a submodule.  We all make typos from time to time.  This method makes it too easy to accidentally type `submodule Fooo` and end up in a submodule you didn’t intend.  This mistake would likely be caught relatively quickly but it seems silly to have a system subject to this kind of mistake.  This means we would end arbitrarily placing the declaration in one file and saying `extends submodule Foo` in the rest of the files.  Your design avoids the need to arbitrarily choose where to place the declaration and avoids the need for a declaration in the rest of the files.
>> 
>> I’m not dead-set on this approach, but as you say, it solves a *lot* of problems that other approaches introduce. I do recognize the reasonableness of the main argument against, that file location shouldn’t have such a dramatic affect on behavior… *but* the fact is (whether by convention or expediency) we already *do* have filesystem location dependencies on our code projects… 
>> 
>> * One can’t willy-nilly move files around the disk and expect a project to build… if you move it somewhere the compiler doesn’t know to look for it, you’re going to break things
>> * One can’t just move a file out of your SCM repo root, or you’ll “lose” the file
> 
> True, but if other files do not refer to the lost file, you don’t even know for which file to look.
> (imo this is already a weak point in swift, but this approach to submodules would make it -much?- worse)
> 
> If you were to include filenames in the “submodule declaration file” at least this omission in swift would be partly covered, and you would not need to move the other files into special places. (You could though)
> 

But isn’t this exactly the same case if you’re using Swift without an IDE? It’s true that Xcode keeps track of files in your project, and there’s no reason it couldn’t also keep track of files in a submodule (in fact it would… it just might not know they belong in the sub-module without some work from the Xcode dev team).

But as a thought-experiment, here’s how I’m thinking of this problem:

Swift (as a language) has established that a file *is* a unit of code. Not the only one of course, but it is one, as evidenced by access controls (fileprivate, Swift2 private) which bound on the file.

I (and many others) want to be able to describe another unit of code larger than a file and smaller than a Module to help organize the file-units we have which go together within a Module, but which shouldn’t (for various reasons) describe a complete Module in and of themselves.

For the moment—though we actually propose to call this unit a sub-module—we'll call this new unit a Foo. Let's describe properties of a Foo:

* A Foo should serve to organize one or more File code units by:
  - ensuring a File (the code-unit) may belong to one and only one Foo
  - making clear (with or without an IDE) to which Foo a given file might belong
* A Foo should have a unique name
* A Foo should be able to nest under another Foo, and the name of that nesting should be clear & unique
* A Foo should be able to be easily created, split, and merged as the codebase evolves
* A Foo should be able to be moved, copied, and SCMed as a unit, without reference to anything outside the Foo

Having now described the critical properties of this new unit called “Foo” have we not also described a filesystem directory?

Naturally, we don’t want *all* directories to become Foos (that would be overly restrictive to authors, and backwards-incompatible), so we need a way to distinguish a normal directory from one that is acting as a Foo. I originally proposed a special “sub-module declaration file”, but there may be other ways. For example, we might establish that a directory whose name matches `[SubModuleName].swift-sub-module` becomes a sub-module of designated name, however this can cause problems due to the disjoint sets of “Legal Filesystem Directory Characters” and “Legal Swift Identifier Characters”. It also doesn’t give us a place to “grow” the declaration in the future if we choose to add additional properties to the sub-module declaration.


—Karim





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