> On May 20, 2016, at 11:15 AM, Tony Allevato <[email protected]> wrote:
>
>
>
> On Fri, May 20, 2016 at 8:48 AM Matthew Johnson <[email protected]
> <mailto:[email protected]>> wrote:
>> On May 20, 2016, at 10:27 AM, Tony Allevato via swift-evolution
>> <[email protected] <mailto:[email protected]>> wrote:
>>
>> Another use case to consider: code generation. There, namespaces can be
>> vital; they let you isolate code that you may have no real control over (for
>> example, data models that correspond to remote services) from the rest of
>> your code to avoid collisions. In some cases that can be achieved by putting
>> the shared code in its own module, but if that data description format has
>> hierarchical namespaces/packages of its own (Google's protocol buffers, for
>> example), then the lack of namespaces forces the code generator to come up
>> with contrived schemes for naming nested entities (like the Objective-C
>> implementation, which uses underscore_delimited_names). The result is that
>> Swift code using those services would look *very* unnatural.
>
> What benefits do namespaces provide that empty enums and / or submodules do
> not? This isn’t clear to me and I think it is an essential part of the case
> that must be made.
>
> Nobody is arguing that we don’t need a way to isolate names. The question is
> whether namespaces are the right mechanism for doing that or not.
>
> I'm certainly flexible on the way that organization manifests itself. In a
> general sense, using structs-with-private-initializers or enums seems like
> abuse of those constructs. Generic types also can't be nested yet (but this
> is well-known and on the list of things to fix).
Yes, it’s definitely an abuse, but one with a reasonable degree of utility. I
bring it up primarily to drive the discussion of what people want namespaces to
do that this does not do today. I don’t actually think they are a sufficient
solution. :-)
>
> Some other concerns from my own experience in the domain of code generation:
>
> I need to be able to define namespaces/organization in the source code, not
> as an artifact of the build system, which may prevent submodules from working
> as intended. Unless we want to be so prescriptive as to say, this is the One
> True Way that you will build your Swift code and nothing else matters, in
> which case I could probably generate a companion Swift package manifest.
I would expect submodules to be able to do this. The compilation target is the
module. Submodules are defined in source within the module, but otherwise
provide the same benefits of encapsulation that modules do. For example,
`internal` visibility would be restricted to the submodule (which raises the
question of whether we would want the ability to distinguish “submodule only”
vs “whole module” visibility).
Namespaces and submodules are similar enough that we probably shouldn’t have
both. There are a couple of primary differences between them IMO. The big one
is that namespaces only deal with encapsulating names, whereas modules and
submodules deal with encapsulation more generally.
The other is that depending on the design of namespaces, it might be possible
to introduce names into the same namespace from *different* modules. That is a
questionable capability in my mind and I would prefer to have the more
generalized encapsulation of submodules.
>
> Would submodules support circular references between them? If module Foo.Bar
> has a type that references something in Foo.Baz, and Foo.Baz has a type that
> references something in Foo.Bar, will that import work (they don't even have
> to be circular references between two *types*, just different types within
> two *modules*)? If it's like regular top-level modules now, that's not
> possible, because A can't import B if B imports A.
This is a good question. This is kind of thing I was looking for. I would
hope this is possible since they are compiled together.
>
> Structs/enums as faux namespaces also fall apart if you try to link multiple
> build artifacts into the same library or executable. Let's say I'm using a
> code generator to generate types from a data model that defines a namespace
> Foo. Let's go on to say that, due to the way the dependencies of my project
> are structured, the code for type Foo.Bar is generated separately from
> Foo.Baz. Each run of the code generator needs to be independent, so in order
> to do that, it defines an empty Foo struct and extend it with the types
> inside that namespace that are generated in that run. So, in one run I have
> this:
>
> generated1/Foo.swift:
> public struct Foo { private init() {} }
>
> generated1/Foo.Bar.swift:
> extension Foo {
> struct Bar { ... }
> }
>
> and in another run, I have this:
>
> generated2/Foo.swift:
> public struct Foo { private init() {} }
>
> generated2/Foo.Baz.swift:
> extension Foo {
> struct Baz { ... }
> }
>
> I have two copies of the Foo struct, so I can't easily compile these sources
> into object files and then pull them all together to link. Even though Foo
> doesn't contain anything other than nested types, the compiler outputs
> metadata symbols for Foo itself that causes duplicate link errors.
>
> Swift as it is today already gets me *almost* the whole way there—merging the
> .swiftmodules produced from the builds above works fine and results in a
> module that defines both Bar and Baz inside Foo. Essentially, my use case
> would be solved if I could somehow say "Foo is not actually a type, it's used
> for organization and name mangling only, so please don't include any symbols
> for it alone in the output binary". Then everything would link correctly.
If you absolutely need the two runs to be totally independent I can see why it
is frustrating to be so close. What is the reason that you can’t have them
share the struct “namespace” declarations in a third folder? It’s not a good
long-term solution but might help you move forward with what we have available
today.
namespaces/Foo.swift:
public struct Foo { private init() {} }
>
>
>
>>
>>
>> On Fri, May 20, 2016 at 8:08 AM T.J. Usiyan via swift-evolution
>> <[email protected] <mailto:[email protected]>> wrote:
>> +1 for namespaces.
>>
>> On Fri, May 20, 2016 at 10:52 AM, Haravikk via swift-evolution
>> <[email protected] <mailto:[email protected]>> wrote:
>>
>> > On 20 May 2016, at 14:51, Krystof Vasa via swift-evolution
>> > <[email protected] <mailto:[email protected]>> wrote:
>> >
>> > When you have namespaces Car and Animal and each contains a class called
>> > List, IMHO there should be classes CarList and AnimalList. It's more
>> > verbose, but you imediately know which class is being used in opposite of
>> > just using List.
>>
>> Why not use Car.List and Animal.List when its unclear from context? With
>> Swift’s type inference you don’t often need to specify types anyway so your
>> editor will know which list type you’re using based on how you obtained it.
>>
>> That said it does depend on the purpose of each List; do they have any
>> commonality? They could for example both be generic List implementations,
>> but were never factored out into a common module. If however they are
>> specialised constructs specific to cars or animals, then the prefix may make
>> most sense.
>>
>> For example, in the libdispatch thread the naming of Dispatch.DispatchQueue
>> was queried, however this isn’t a general purpose queue type, it’s more
>> specialised than that, so a name of “Queue” doesn’t convoy enough
>> information, just as List might not. But it depends on what it actually
>> does, which a basic example tends not to include ;)
>>
>>
>> Anyway, I’m +1 for namespaces everywhere, some names can be common. For
>> example Node could be related to trees, physics engines and all sorts of
>> constructs. “Node” may be a perfectly fine name for these. That said, these
>> are sometimes tied to specific types in which case nesting them may make
>> more sense, which I believe is already being addressed (currently we can’t
>> nest generic types)? It’s certainly not as simple as it can appear!
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