[swift-evolution] [Review] SE-0111: Remove type system significance of function argument labels

Jordan Rose jordan_rose at apple.com
Thu Jun 30 22:33:05 CDT 2016

> On Jun 30, 2016, at 13:36, Taras Zakharko via swift-evolution <swift-evolution at swift.org> wrote:
>> On 30 Jun 2016, at 22:11, Austin Zheng <austinzheng at gmail.com <mailto:austinzheng at gmail.com>> wrote:
>> As for the label semantics, Swift's current behavior is actively misleading, please see the example in the prior email. There are no meaningful semantics in the label, because implicit conversion between differently-labeled function types means that there is no way to usefully enforce these invariants to begin with.
> That is a good point. I admit to not knowing this (I strongly expected that labels would be semantically meaningful). But what exactly is the status of the argument labels than in Swift? Just a documentation device  for the programmer and a  hint for the compiler to do function dispatch? But if the compiler indeed does dispatch on argument labels, then they are not completely void of semantics, are they?  As I mentioned before, I think the problem here is much deeper. 
> The state of affairs I would prefer is something along these lines:
> 1. Labels are semantically meaningful
> 2. There is an explicit casting system for function signatures
> 3. This casting system should be in close correspondence to tuples. The "function argument lists look sort of like tuples“ is a very compelling reason actually, because of the principle of the least surprise. If I have two things in the language that look very similar, then its very confusing if they  exhibit very different behaviour. Again, I am not proposing that one goes back to model functions in terms of tuples. But as long as there is a surface resemblance (and an obvious morphisms between the two), at least some aspects of their design should be kept in sync. 
> But again, this touches on some deep design decisions for the language, so I — as an amateur — don’t feel in my plate discussing this here. I believe that there currently might be some inconsistencies in the language design that should be sealed with (but maybe they are no inconsistencies at all and I simply have false expectations). 

Language history, a.k.a. story time!

We started out with the “perfect” model of a function type being a map from a tuple to a tuple. Different argument labels were just overloads. It really was quite a simple model, other than not having 1-tuples. Well, and variadics and default values and trailing closures didn’t make sense anywhere but in functions, but still. Very simple.

(And inout. And autoclosure. And maybe a few more.)

Then we hit a snag: naming guidelines. We wanted argument labels to be something people felt comfortable using, something that would be encouraged over a sea of unlabeled arguments. But even before the Swift 3 naming conventions were hammered out, the natural names for argument labels didn’t seem to match the names you’d want to use in the function. So we split the names of parameters off from the names of tuple elements.

(This was precipitated by wanting to import Objective-C methods, but I think it would have come up regardless.)

As seen earlier in the thread, argument labels don’t make for good tuple element labels. Especially with the Swift 3 guidelines, argument labels usually don’t make sense without the context provided by the base name, and two methods that happen to share argument labels might not actually be very similar, while two methods that are duals of each other might have different argument labels due to, well, English (e.g. 'add(to:)' vs. 'remove(from:)’).

The real blow, however, came with that very first idea: that we could treat methods with different argument labels as simple overloads in type. This led to poor diagnostics where the compiler couldn’t decide whether to believe the types or the argument labels, and might tell you you have the wrong argument labels rather than a type mismatch. For pretty much every Apple API, this was the wrong decision. On top of all that, it was really hard to refer to a method when you didn’t want to call it. (Most methods with the same base name still have unique labels, so you don’t need the types to disambiguate.)

So we introduced the notion of “full names”, which are the things you see written as ‘move(from:to:)` (and which are represented by DeclName in the compiler). Almost immediately diagnostics got better, testing optional protocol requirements got shorter, and a lot of compiler implementation got simpler.

And then we kind of got stuck here. We have full names used throughout the compiler, but tuple labels still appear in types. They’re still used in mangling. We got rid of the “tuple splat” feature, but still model out-of-order arguments as “tuple shuffles”. And we allow a number of conversions that look like they should be invalid, but aren’t.

(And it’s important that we continue allowing them, or at least some of them, because we want to be able to pass existing functions to things like map and reduce without worrying about conflicting labels.)

So we’ve given up the perfect ideal of tuple-to-tuple. But we did it because we value other things more than that ideal: variadics, default values, trailing closures, inout, autoclosure, distinct argument labels and parameter names, referencing a function by full name, and diagnostics that better match the user’s likely intent (particularly given the naming guidelines and existing libraries). I think that’s a worthwhile trade.


P.S. Anyone is allowed to think this is not a worthwhile trade! But part of the purpose of this story is to show that we’re already 90% of the way towards making tuples and function arguments completely separate, even if they have similar syntax. This proposal gets us to maybe 95%.

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