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> On Nov 30, 2017, at 00:24, Douglas Gregor via swift-evolution 
> <> wrote:
>> On Nov 26, 2017, at 10:04 PM, Chris Lattner via swift-evolution 
>> <> wrote:
>> I’d like to formally propose the inclusion of user-defined dynamic member 
>> lookup types.
>> Here is my latest draft of the proposal:
>> An implementation of this design is available here:
>> The implementation is straight-forward and (IMO) non-invasive in the 
>> compiler.
> I think better interoperability with Python (and other OO languages in 
> widespread use) is a good goal, and I agree that the implementation of the 
> feature described is straight-forward and not terribly invasive in the 
> compiler.
> However, I do not think this proposal is going in the right direction for 
> Swift. I have objections on several different grounds.
> Philosophy
> Swift is, unabashedly, a strong statically-typed language. We don’t allow 
> implicit down casting, we require “as?” so you have to cope with the 
> possibility of failure (or use “as!” and think hard about the “!”). Even the 
> gaping hole that is AnyObject dispatch still requires the existence of an 
> @objc declaration and produces an optional lookup result, so the user must 
> contend with the potential for dynamic failure. Whenever we discuss adding 
> more dynamic features to Swift, there’s a strong focus on maintaining that 
> strong static type system.
> IMO, this proposal is a significant departure from the fundamental character 
> of Swift, because it allows access to possibly-nonexistent members (as well 
> as calls with incorrect arguments, in the related proposal) without any 
> indication that the operation might fail. It’s easy to fall through these 
> cracks for any type that supports DynamicMemberLookupProtocol—a 
> single-character typo when using a DynamicMemberLookupProtocol-capable type 
> means you’ve fallen out of the safety that Swift provides. I think that’s a 
> poor experience for the Python interoperability case, but more on that in the 
> Tooling section below.
> While we shouldn’t necessarily avoid a feature simply because it can be used 
> distastefully, consider something like this:
>       public extension NSObject :  DynamicMemberLookupProtocol, 
> DynamicCallableProtocol { … }
> that goes directly to the Objective-C runtime to resolve member lookups and 
> calls—avoiding @objc, bridging headers, and so on. It’s almost frighteningly 
> convenient, and one could imagine some mixed Objective-C/Swift code bases 
> where this would save a lot of typing (of code)… at the cost of losing static 
> typing in the language. The presence of that one extension means I can no 
> longer rely on the safety guarantees Swift normally provides, for any project 
> that imports that extension and uses a subclass of NSObject. At best, we as a 
> community decide “don’t do that”; at worse, some nontrivial fraction of the 
> community decides that the benefits outweigh the costs (for this type or some 
> other), and we can no longer say that Swift is a strong statically-typed 
> language without adding “unless you’re using something that adopts 
> DynamicMemberLookupProtocol”.
> Tooling
> The Python interoperability enabled by this proposal *does* look fairly nice 
> when you look at a small, correctly-written example. However, absolutely none 
> of the tooling assistance we rely on when writing such code will work for 
> Python interoperability. Examples:
> * As noted earlier, if you typo’d a name of a Python entity or passed the 
> wrong number of arguments to it, the compiler will not tell you: it’ll be a 
> runtime failure in the Python interpreter. I guess that’s what you’d get if 
> you were writing the code in Python, but Swift is supposed to be *better* 
> than Python if we’re to convince a community to use Swift instead.
> * Code completion won’t work, because Swift has no visibility into 
> declarations written in Python
> * Indexing/jump-to-definition/lookup documentation/generated interface won’t 
> ever work. None of the IDE features supported by SourceKit will work, which 
> will be a significant regression for users coming from a Python-capable IDE.
> Statically-typed languages should be a boon for tooling, but if a user coming 
> from Python to Swift *because* it’s supposed to be a better development 
> experience actually sees a significantly worse development experience, we’re 
> not going to win them over. It’ll just feel inconsistent.
> Dynamic Typing Features
> It’s possible that the right evolutionary path for Swift involves some notion 
> of dynamic typing, which would have a lot of the properties sought by this 
> proposal (and the DynamicCallableProtocol one). If that is true—and I’m not 
> at all convinced that it is—we shouldn’t accidentally fall into a suboptimal 
> design by taking small, easy, steps. If we’re to include dynamic-typing 
> facilities, we should look at more existing practice—C# ‘dynamic' is one such 
> approach, but more promising would be some form of gradual typing a la 
> TypeScript that let’s one more smoothly (and probably explicitly) shift 
> between strong and weak typing.
> How Should Python Interoperability Work?
> Going back to the central motivator for this proposal, I think that providing 
> something akin to the Clang Importer provides the best interoperability 
> experience: it would turn Python declarations into *real* Swift declarations, 
> so that we get the various tooling benefits of having a strong 
> statically-typed language. Sure, the argument types will all by PyObject or 
> PyVal, but the names are there for code completion (and indexing, etc.) to 
> work, and one could certainly imagine growing the importer to support 
> Python’s typing annotations. But the important part here is that it doesn’t 
> change the language model at all—it’s a compiler feature, separate from the 
> language. Yes, the Clang importer is a big gnarly beast—but if the goal is to 
> support N such importers, we can refactor and share common infrastructure to 
> make them similar, perhaps introducing some kind of type provider 
> infrastructure to allow one to write new importers as Swift modules.
> In truth, you don’t even need the compiler to be involved. The dynamic 
> “subscript” operation could be implemented in a Swift library, and one could 
> write a Python program to process a Python module and emit Swift wrappers 
> that call into that subscript operation. You’ll get all of the tooling 
> benefits with no compiler changes, and can tweak the wrapper generation 
> however much you want, using typing annotations or other Python-specific 
> information to create better wrappers over time.


In particular, I’d rather take the time to design a foundation for Swift 
interoperating with any given language, than have one mechanism for Python, 
another for Ruby, yet another for C++, etc.

- Dave Sweeris
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