> On 1 Dec 2017, at 00:54, Xiaodi Wu via swift-evolution > <swift-evolution@swift.org> wrote: > >> On Thu, Nov 30, 2017 at 2:24 AM, Douglas Gregor via swift-evolution >> <swift-evolution@swift.org> wrote: >> >> >>> On Nov 26, 2017, at 10:04 PM, Chris Lattner via swift-evolution >>> <swift-evolution@swift.org> wrote: >>> >>> I’d like to formally propose the inclusion of user-defined dynamic member >>> lookup types. >>> >>> Here is my latest draft of the proposal: >>> https://gist.github.com/lattner/b016e1cf86c43732c8d82f90e5ae5438 >>> https://github.com/apple/swift-evolution/pull/768 >>> >>> An implementation of this design is available here: >>> https://github.com/apple/swift/pull/13076 >>> >>> 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”. > > There are several commenters below to whom I would have liked to respond in > the fullness of time, but time constraints would make doing so prohibitive. > Since your message set off an abundance of discussion, I'll reply to the > points you make here and, along the way, ask your forbearance to bring up and > respond to some related concerns raised by others. > > I agree that the prospect above seems not ideal at all. On reading Chris's > proposal, it never occurred to me that the intention was to support such > retroactive conformance to these special protocols. Admittedly, such > retroactive conformance is possible with all protocols--with the notable > exception of those that require compiler synthesis of requirements. But > Chris's protocols seemed magical enough (in the gut feeling sense) that I > naturally assumed that retroactive conformance was never on the table. We > would be justified in making that prohibition here, I think, although I'm not > sure if Chris as proposal author feels the same way. > > Alternatively--and perhaps more elegantly--we could address this concern > head-on by having, instead of `DynamicMemberLookupProtocol` and > `DynamicCallable`, a magical class `DynamicObject` which all dynamic types > must inherit from. It would then be clear by design that Swift types cannot > be _retroactively dynamic_ in the sense that Chris proposes. I *think* the > vast majority of bridged use cases can tolerate being `final class` types > instead of `struct` types. I could be wrong though. > > Now, as to the possibility of failure: I agree also that eliding the > possibility of lookup failure at the callsite requires further consideration. > Some might agree that restricting dynamic features only to subclasses of a > `DynamicObject` might be clear enough that we do not need to go further in > this regard. I think all will agree that inventing new (to Swift) notations > like "->" or "::" to denote dynamic lookup is rather awkward and not > ergonomic. I wonder if it would be instead sufficient to require dynamic > member lookup to return values of type `T!` (as in, IUOs). IUOs are, after > all, designed to deal with similar situations in bridging from Obj-C, and are > explicitly "transitional technology."
I love the idea of a DynamicObject type instead of a protocol. >> 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. > > As I wrote in the earlier thread, I think this is the wrong way to reason > about the use case--and a counterproductive one that effectively rejects the > legitimacy of dynamic language interop support instead of working towards an > optimal solution for it. Along those lines, some replies to your message > literally question whether the user case is worthwhile to support at all, > which I think entirely misses the mark. > > As Chris writes in his proposal, there are several areas (such as data > science) where, to put it plainly, Python or another dynamic language is > significantly better than Swift due to a much larger ecosystem of libraries, > tools, and user communities, with sometimes decades of lead time. It is > nonsensical to talk about how Swift is "supposed to be better" in any way > whatsoever in that context. As diehard Swift users, we may be confident that > the virtues of Swift's syntax, static typing, compiler smarts, protocol-based > design, or whatever else offer opportunities for, say, data science libraries > and tools to be better in the future, eventually. But to make this even > possible involves first making Swift a viable language in which to work with > current data science tools. Your top bullet point here reasons that one key > flaw of Chris's proposal is that he has not somehow figured out how to make > dynamic Python calls give compile-time Swift errors. If this is the minimum > bar for Python interop, then as far as I can tell, it seems isomorphic to > rejecting interop with dynamic languages altogether. > > It goes without saying that, in bridging between languages X and Y, much of > X's native tooling will be inoperable, and much of Y's native tooling will > not work. The solution is to build additional tools where necessary, not to > argue that interop shouldn't be implemented in the first place. I agree with everything that Xiaodi said. I believe that while it’s noble to want to bring more type information (ala TypeScript) to dynamic languages, we still need a simple way to dynamically call *at runtime* into dynamic languages. That may cause runtime errors, but that’s what dynamic languages are all about. I think it’s illusory to think that their nature can be profoundly changed. Even TypeScript, which brings all this type information to JavaScript still needs a way to call into non-typed interfaces: the compiler will be helpless in protecting the user from some runtime errors, but that’s the price you pay. >> 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. > > > I doubt seriously that there is any viable path to interoperating with a much > more established and extensive ecosystem which begins by requiring that the > more dominant ecosystem support Swift-specific annotations or tooling. It > would seem that, if we're to implement a Swift library to do what you > describe, it'd have to be one with extensive knowledge of both Python and > Swift, being able to parse entire Python modules as well as create entire > Swift ones. Since we haven't even designed a way of writing code-generating > macros for Swift in Swift, I struggle to see how this hypothetical tool is > going to be ever built, or whether sufficient people exist with the expertise > to do so, given that you'd need deep expertise working in both languages in > order to write the tool that permits you to work in both languages. > _______________________________________________ > swift-evolution mailing list > swift-evolution@swift.org > https://lists.swift.org/mailman/listinfo/swift-evolution
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