> On Nov 30, 2017, at 1:01 PM, Zach Wolfe <zacharyreidwo...@gmail.com> wrote: > > Doug and others have brought up some great points, and I think Doug’s idea of > a common infrastructure for importing declarations from other languages is > _extremely_ attractive for the long-term future of Swift. > > However, unlike this proposal, that will (I imagine as a non-compiler > engineer) be a colossal undertaking, and as such it’s not going to make it > into Swift 5, or possibly even 6 or 7. I understand, then, Chris’s (and > other’s) desire to start interfacing with Python code now, not later. For me, > far and away the biggest problem with this proposal (and the only outright > deal-breaker) is that dynamic member lookups do not differentiate themselves > in any way from statically-checked member lookups syntactically. I don’t > object as strongly as others to the idea of adding this kind of dynamism to > the language, but if it’s going to be there, it should not be possible to > slightly misspell a static member name and end up with an unexpectedly > dynamic member that may or may not fail at compile-time.
As noted at the end of my message, I think one can write a Swift library to make working with the Python runtime much easier, and write a wrapper generator that produces Swift APIs from Python code that use said Swift library. - Doug > > On Nov 30, 2017, at 2:24 AM, Douglas Gregor via swift-evolution > <swift-evolution@swift.org <mailto:swift-evolution@swift.org>> wrote: > >> >> >>> On Nov 26, 2017, at 10:04 PM, Chris Lattner via swift-evolution >>> <swift-evolution@swift.org <mailto: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://gist.github.com/lattner/b016e1cf86c43732c8d82f90e5ae5438> >>> https://github.com/apple/swift-evolution/pull/768 >>> <https://github.com/apple/swift-evolution/pull/768> >>> >>> An implementation of this design is available here: >>> https://github.com/apple/swift/pull/13076 >>> <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”. >> >> 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 >> <https://docs.python.org/3/library/typing.html>. 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. >> >> - Doug >> >> _______________________________________________ >> swift-evolution mailing list >> swift-evolution@swift.org <mailto:swift-evolution@swift.org> >> https://lists.swift.org/mailman/listinfo/swift-evolution >> <https://lists.swift.org/mailman/listinfo/swift-evolution>
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