[swift-evolution] Proposal: Introduce User-defined "Dynamic Member Lookup" Types

Chris Lattner clattner at nondot.org
Fri Dec 1 00:05:37 CST 2017

Hi Doug,

Thank you for the detailed email.  I have been traveling today, so I haven’t had a chance to respond until now.  I haven’t read the down-thread emails, so I apologize if any of this was already discussed:

> 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.

Fantastic, I’m really pleased to hear that!  I only care about solving the problem, so if we can find a good technical solution to the problems than I’ll be happy.

A funny thing about swift-evolution is that it is populated with lots of people who already use Swift and want to see incremental improvements, but the people who *aren’t* using Swift yet (but should be) aren’t represented here.  As you know, I’m perhaps the biggest proponent of Swift spreading into new domains and earning the love of new users.

> However, I do not think this proposal is going in the right direction for Swift. I have objections on several different grounds.

Thanks for being up front about this.  It turns out that a majority of the points you raise were brought up early in the pitch phases of the discussion, but I screwed up by not capturing that discussion into the alternatives section of the proposal.

Better late than never I guess: I just significantly revised the proposal, adding a new “Alternative Python Interoperability Approaches” section.  I’d appreciate it if you could read it and comment if you find any part disagreeable:
https://gist.github.com/lattner/b016e1cf86c43732c8d82f90e5ae5438#alternative-python-interoperability-approaches <https://gist.github.com/lattner/b016e1cf86c43732c8d82f90e5ae5438#alternative-python-interoperability-approaches>

Many of the points you make are discussed there, but I’ll respond point-by-point below as well:

> Philosophy
> Swift is, unabashedly, a strong statically-typed language.

That’s factually incorrect.

> 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.

AnyObject dispatch is one of the gaping holes for sure.  You don’t mention it, but its lookup results are represented as *ImplicitlyUnwrappedOptional* results, which do not force the user to contend with dynamic failure.  Further, unlike my proposal (which is fully type safe), AnyObject lookup is not type safe at all.  It is also far more invasively intertwined throughout the compiler.

More problematically for your argument: your preferred approach requires the introduction of (something like) DynamicMemberLookupProtocol or something like AnyObject-For-Python, so your proposal would be additive on top of solving the core problem I’m trying to solve.  It isn’t an alternative approach at all.

> Whenever we discuss adding more dynamic features to Swift, there’s a strong focus on maintaining that strong static type system.

Which this does, by providing full type safety - unlike AnyObject lookup.

> 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.

The only way your claim is correct is if someone implements the protocol wrong.  What you describe is true of AnyObject lookup though, so I understand how you could be confused by that.

> 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.

Since you seem to be latching on to this, I’ll say it again: the proposal is fully type safe :-)

That said, the “simple typo” problem is fundamental to the problem of working with a dynamically typed language: unless you can eradicate all “fundamental dynamism” from the language, you cannot prevent this.

That said, since this is a problem inherent with these languages, it is something people are already very familiar with, and something that everyone using those APIs has had to deal with.  This is also not our problem to solve, and people in the Python community have been discussing and working on it over a large part of its 25 year history.  I find your belief that we could solve this for Python better than the Python community itself has both idealistic and a bit naive.
> 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.

Yes, this feature (like just about everything) can be misused in Swift.  If someone actually did that, they’d find out rapidly that it is a really bad thing to do for lots of reasons, some of which you point out.

That said, I find it far more likely that someone would pervasively use IUOs everywhere or use AnyObject for everything.  This is something that real users really do, and while it is unfortunate, it is impractical to prevent all forms of abuse.  

> 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.

Yes, welcome to the realities of modern Python development!

> 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.

By your argument we should ban AnyObject lookup as well, given its inconsistency with the rest of the language.

I’m glad you mention the idea of users coming from Python to Swift: by many estimates, the Python community is *two* orders of magnitude or more larger than the Swift community - and lets not mention other dynamic languages like Javascript.  It really is high value to make the path from Python to Swift nicer, and if we do so, I believe people will find lots of reasons to write new Swift code to get all the advantages that you mention (and more).

> 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.

Given that you haven’t followed the discussion on the many threads we’ve had on this, and haven’t proposed a workable approach to this problem, I’m not sure upon what basis your fears and uncertainty and doubt are founded.

> 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.

This could be an theoretically interesting refinement to this proposal but I’m personally very skeptical that this is every going to happen.  I’ve put the rationale into the alternatives section of the proposal.  I don’t explain it in the proposal in this way directly, but I believe it is far more likely for a Pythonista transplant into Swift to rewrite their code in Swift than it is to use Python type annotations.

> Sure, the argument types will all by PyObject or PyVal,

That’s the root of the problem.  Python has the “fully dynamic” equivalent of “id” in Objective-C, so we need to represent that somehow.  Even if we followed the implementation approach of the Clang importer, we would need some way to represent this dynamic case.  That type needs features like DynamicMemberLookup or AnyObject.  In my opinion, the DynamicMemberLookup approach is better in every way than AnyObject is.  

Which approach do you think is the best way to handle the untyped “actually dynamic” case?  

> 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>.

You’re basing this on the flawed assumption that local variables will pervasively have types, which I can’t imagine being the case.  Even on "typable” API, I wouldn’t expect people to commonly get code completion results for reasons now explained in the proposal.

> 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.

Sure, doing something like this could be an interesting direction to consider after getting basic fully dynamic support in.  

In earlier threads, we had extensive discussion about type providers, and discussed that they don’t solve the problem for the same reasons that Clang importer doesn’t solve the problem: you still need the way to “provide” the fully dynamic type.

If you have to do that, it is perfectly reasonable to get the basic dynamic case in, then weigh the cost/benefit advantage of getting more static typing in if available somehow.  While importing progressive typing annotations could be practical for Javascript interop (for example) I’m pretty skeptical it would be  widely used for Python (again, rationale laid out in the proposal).

> 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’d love for you to sketch out how any of this works with an acceptable user experience, because I don't see anything concrete here.  

You seem to be thinking that there is some world where Python isn’t actually a dynamic language, or where the Python community switches to pervasive adoption of type hints (something that is unlikely to happen for a ton of reasons).  Python is less statically typable than Objective-C is, even ignoring the constraints on the problem, and yet Objective-C has AnyObject - why do you think that Python wouldn’t need it?  Since it needs it, then we need to design it: how do you think it should work?


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