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

Douglas Gregor dgregor at apple.com
Thu Nov 30 02:24:23 CST 2017

> On Nov 26, 2017, at 10:04 PM, Chris Lattner via swift-evolution <swift-evolution at 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.

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”.

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

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