> On Nov 30, 2017, at 3:40 PM, Douglas Gregor via swift-evolution 
> <swift-evolution@swift.org> wrote:
>> On Nov 30, 2017, at 1:01 PM, Zach Wolfe <zacharyreidwo...@gmail.com 
>> <mailto: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.

That might be true for Python; I’m quite certain it’s not the case for Ruby or 
JS as they exist in the wild. However…

At the risk of stating a universally unpopular opinion, beyond interop with 
dynamic languages such as Python, there’s also value in dynamic member lookup 
for •purely Swift-centric• reasons.

I say this with mixed feelings. I share Douglas’s concerns about breaking 
Swift’s safety bubble. I like it that the language generally makes it clear •at 
the point of use• which language features have runtime pitfalls that the 
compiler cannot check. The points where this is not true of the language (e.g. 
array subscripts) always feel a bit gritty between the teeth.

There can, however, be great value in constructing type membership at runtime. 
The ergonomic access to JSON that’s been discussed in this proposal is nothing 
to sneeze at. Dynamic languages make for lovely DSLs in a way that fully 
statically type checked languages never do. Things like Rails’s 
programmatically generated named routes (e.g. “user_post_path(id)” because 
routing configuration created a user resource with nested posts) are possible 
to approximate with statically typed approaches — but only clumsily, never with 
the same expressiveness and concision.

I don’t mean to sneeze at the downside of dynamic typing either. They too are 
nothing to sneeze at. I’ve spent many years on both sides of the dynamic 
divide, and all I see is difficult tradeoffs.

Swift so far has said that dynamism isn’t worth the tradeoffs; it’s 
aggressively avoided even robust runtime reflection, much less dynamic 
dispatch. That may indeed be the right call for the language — but I’m not 
easily convinced there isn’t room to open the door wider.

I think of Chris’s post back in the early days of this list about the 
“programmer model” the language creates, and wonder if there’s a way we can’t 
make a model that leaves the door open to this kind of dynamic dispatch where 
it makes sense for the people and situation at hand, while still staying true 
to the language’s spirit of “compile-time safety by default, exceptions to that 
by intent at point of use.”

Opening the door to dynamism might take some of the flavor of the way the 
language currently handles unsafe memory access: it’s possible and even 
reasonably ergonomic, but you always know when you’ve crossed the Rubicon into 



>       - 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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