[swift-evolution] [Review] SE-0005 Better Translation of Objective-C APIs Into Swift

Matthew Johnson matthew at anandabits.com
Mon Feb 1 15:22:55 CST 2016 

+1 to most of what Nate says here.  

I was about to reply in much more brief manner with the same general thoughts.  I’m glad Nate took the time to write a more complete response.  I don’t want to see a rush to retain reference types where value types are a better long-term design.  The `NS` prefix gives a subtle indication that the type is not the final solution for Swift.

The only comment I want to add is that the argument is mostly specific to Foundation.  In most cases removing the prefix for higher level frameworks is appropriate as the types would by reference types in Swift anyway and are not near-tern candidates for a Swifty redesign.  

Perhaps the best solution is an annotation in Objective-C similar to the null annotations.  In Foundation the default would be to leave the `NS` prefix (at least on classes).  Everywhere else the default would be to remove the prefix.  In specific cases an annotation could be used to override the default.

-Matthew

> On Feb 1, 2016, at 3:10 PM, Nate Cook via swift-evolution <swift-evolution at swift.org> wrote:
> 
> This is a concurring opinion with Drew's review, agreeing that we should reconsider removing the "NS" prefix but providing my own reasons. The proposal as a whole is exciting and far-reaching, but this particular change is misguided. My objections are:
> 
> 1) The change will elide the different semantic expectations for Foundation and Swift standard library types.
> 
> Prefix-less Foundation types will blur the different norms and expectations for Foundation types vs what we have in the Swift standard library, particularly in regards to value vs reference semantics of collection types. 
> 
> The Swift standard library defines Array, Set, and Dictionary as collection types with value semantics that operate quite differently from their peers in Foundation. This change will introduce OrderedSet, CountedSet, HashTable, MapTable, and others, all of which use reference semantics and therefore don't provide the same set of guarantees about ownership and immutability.
> 
> As an example, the seemingly similar Set and CountedSet types produce different results from nearly identical code. Swift's Set type has value semantics, so changes to a copy don't affect the original Set instance:
> 
> let simpleSet = Set(["one", "two", "three"])
> var copiedSimpleSet = simpleSet
> copiedSimpleSet.remove("one")
> 
> copiedSimpleSet.contains("one")  // false
> simpleSet.contains("one")        // true
> 
> CountedSet (née NSCountedSet), on the other hand, uses reference semantics, so changes to a copy of an instance. This is true whether the copy is an explicit one made via assignment or the implicit copy made when calling a function with CountedSet as a parameter. This example shows how nearly code nearly identical to the above produces very different results:
> 
> let countedSet = CountedSet(array: ["one", "two", "three"])
> var copiedCountedSet = countedSet
> copiedCountedSet.remove("one")
> 
> copiedCountedSet.contains("one") // false
> countedSet.contains("one")       // false!
> 
> Collections constructed from immutable/mutable class clusters (like OrderedSet and MutableOrderedSet) pose the same problems, since you may be using a mutable instance of the "immutable" collection. We clearly are already dealing with this difference today, but eliminating the "NS" prefix implies that Foundation types are on the same level as the standard library. This makes the confusion more pronounced and significantly increases both the learning curve of the Swift & Foundation APIs and the risk of errors when using different collection types.
> 
> 2) The change may stifle the development of more Swift-oriented APIs.
> 
> Foundation types were developed in and for a language that uses reference semantics and subclassing, rather than value semantics and a protocol-oriented approach. The designs therefore use and reinforce norms that relate better to Objective-C than to Swift—class clusters of non-final, immutable types with mutable subclasses, immutable types with separate but related mutable counterparts, etc.
> 
> A Swift-native CountedSet (and other collections) would surely have value semantics built in. What about other types—do the new Calendar/Date/DateComponents types look like the system that we would design from the ground up in Swift? How about URL and URLComponents? Dropping the "NS" prefix would make it more difficult to gradually expand the standard library to encompass bridging versions of these and other common types.
> 
> 3) The change will make it harder to find information about the revised APIs.
> 
> Excellent search-ability is an unintended benefit of the "NS" prefix, and can be understood as a way that these types avoid collisions in the vast namespace-less sea of Internet search results. Searching for help with URL and Calendar will be much more difficult than their NS'ed counterparts.
> 
> Especially given the challenges that this proposal will pose for code sourced from tutorials, blog posts, Stack Overflow answers, etc., keeping at least the class names as sign posts seems valuable.
> 
> 4) We'll still be using prefixes after the change.
> 
> While the removal of "NS" is far-reaching, prefixes will still be a common occurrence in code written in Swift. UIKit and AppKit, along with all the various frameworks, will still retain their prefixes, so removing prefixes in the case of Foundation types will be more the exception than the norm. As such, any benefit of the removal would be mitigated by the continued use of prefixes for the rest of the first-party types we rely on.
> 
> 
> In sum, the change would make the language and its APIs more confusing and more difficult to use today, would make it more difficult to migrate to modern designs in the future, and would ultimately provide a very minor benefit. I encourage the Swift core team to reconsider the "Strip the "NS" prefix from Foundation APIs" portion of the proposal.
> 
> Nate
> 
> 
>> On Jan 30, 2016, at 5:01 PM, Drew Crawford via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> 
>> I am in favor of this proposal on balance, and I leave the bulk of this to people who interop with Objective-C more often than I do.
>> 
>> I would like to confine my remarks to one corner where I think we are making a very serious mistake.
>> 
>>> The removal of the "NS" prefix for the Foundation module (or other specifically identified modules) is a mechanical translation for all global symbols defined within that module that can be performed in the Clang importer. 
>> 
>> 
>> As I understand it (and I am no Cocoa historian) the NS prefix was originally introduced because Objective-C lacks namespacing.
>> 
>> The thinking seems to be that since Swift has proper namespacing, this historicism is no longer necessary.  I find this argument very flimsy.
>> 
>> Of course Swift has stronger namespacing if one's frame of reference is Objective-C or C.  But for those of us coming from other language backgrounds, namespacing means something much stronger than Swift's concept of it.  I don't mean to suggest that Swift's design is wrong exactly (less is sometimes more), but I do mean to say that if we propose to undo a decision that worked for several decades and break every Swift project in existence on the theory that Swift's namespacing is strong enough we had better be right.
>> 
>> For those unfamiliar, I will explain some of the namespacing tools Swift lacks relative to other languages.  First, many languages have a "hierarchical" namespace system, where one can say
>> 
>> import Foundation.Net.URL
>> let s = Session(...)
>> 
>> instead of for example
>> 
>> import Foundation
>> let s = NSURLSession(...)
>> 
>> Some form of this is used in Rust, Python, and C#, as far as I know.  I believe Swift has some counterfeit version of this, as the book mentions you can import a "submodule <https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Declarations.html#//apple_ref/swift/grammar/import-declaration>", but I do not know what that is, do not know how to make one, have never seen anyone use one, the book suggests it goes only 2 levels deep, and perhaps as a consequences of some of these problems nobody thought of using this for Foundation.
>> 
>> A closely related difference is the use of so-called "selective" imports, where we import only a single symbol (or a list of explicitly-identified symbols) into the scope.  We might express this as
>> 
>> from Foundation.Net.URL import Session, HTTP //import two classes only
>> let s = Session(...)
>> 
>> Again I think Swift technically supports some way to avoid importing a whole gigantic namespace like Foundation, but I am not aware of any actual uses of this feature, and certainly the convention is not to write code this way.  Meanwhile, let's check in with the Python community, who standardized the following guidance on these "wildcard" imports as part of their language evolution process:
>> 
>>> Wildcard imports ( from <module> import * ) should be avoided, as they make it unclear which names are present in the namespace, confusing both readers and many automated tools. There is one defensible use case for a wildcard import...
>> 
>> When a language has a robust namespacing system, which we do not, there are many follow-on consequences.  One is that an import statement is much more of a scalpel than a bludgeon; each import statement only introduces a handful of new names (even if it is a so-called "wildcard" import that grabs all children of some namespace, most of those children are themselves namespaces), unlike importing Foundation which contains thousands of direct child types that are injected into the local scope.
>> 
>> Another consequence is that class names become quite short, shadow each other, and nobody bats an eye.  I searched the C# standard library for "Session", and found some 12 classes with that name:
>> 
>> <Screen Shot 2016-01-30 at 3.44.23 PM.png>
>> 
>> These "standard library" classes not only potentially shadow programmer-defined types, they also shadow each other.  But because the C# language has a robust namespacing system, the chances of there being more than one thing called "Session" in scope in your program (or for that matter, when developing the standard library itself) is quite small, so it's a non-issue.
>> 
>> Now we return to the question of dropping the NS prefix, which will rename thousands of classes in a typical program that has `import Foundation`, in a way that potentially (let's be honest.  More like "probably") shadows one or more programmer-defined classes.  Our review criteria is:
>> 
>>> Is the problem being addressed significant enough to warrant a change to Swift?
>> 
>> 
>> No, the elimination of 2 characters is not significant enough of a problem to break all Swift programs, let alone to introduce literally thousands of new opportunities for shadowing.
>> 
>> To its credit, the proposal acknowledges this, and offers a concession:
>> 
>>> Note that this removal can create conflicts with the standard library. For example, NSString and NSArray will become String and Array, respectively, and Foundation's versions will shadow the standard library's versions.  In cases where the Swift 3 names of standard library entities conflict with prefix-stripped Foundation entities, we retain the NS prefix. These Foundation entities are: NSArray, NSDictionary, NSInteger, NSRange, NSSet, and NSString.
>> 
>> But of course this needlessly draws a distinction between NSString et al and the "normal" Foundation types, and what's more it draws that distinction based on the present composition of the Swift standard library and the present composition of Foundation.  But we have already decided not to guarantee the source compatibility of the standard library, so what happens when that composition changes?  Will we then go back and tweak which classes get NS prefixed to them again?
>> 
>> In my view, if Swift's namespacing is not good enough to let us drop the NS in NSString it is not good enough to drop any prefix.  If we believe that a programmer will struggle to distinguish between Swift String and Foundation String then we should expect them to struggle for any two classes in any two frameworks, and this is special pleading on the part of Foundation.  C#'s libraries declare *twelve* different `Session`s and nobody bats an eye, but we have two types share a name and everybody loses their minds?  Our namespacing is not good enough to kill the prefix, period.
>> 
>> We should either drop these prefixes or we should not; because the claimed motivation–that we have "good enough" namespacing in the language now–is either true or it is not.  This proposal admits that it is not, and tries to drop the prefixes anyway.  I believe that is a mistake.
>> 
>> I certainly support the goal of eliminating these prefixes, they are ugly, they need to be killed, and namespacing is the right solution.  But we must not jump out of the plane until we are very sure our parachute is in order.  In Swift 3 it is not.
>> 
>> I do think the bulk of the proposal is fine, and I apologize for using quite stark language for such a small paragraph in an otherwise reasonable proposal, but I think the problem buried in here is quite serious and is being overlooked.
>> 
>> Drew
>> 
>> 
>>> On Jan 22, 2016, at 3:02 PM, Douglas Gregor via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>> 
>>> Hello Swift community,
>>> 
>>> The review of SE-0005"Better Translation of Objective-C APIs Into Swift" begins now and runs through January 31, 2016. The proposal is available here:
>>> 
>>> https://github.com/apple/swift-evolution/blob/master/proposals/0005-objective-c-name-translation.md <https://github.com/apple/swift-evolution/blob/master/proposals/0005-objective-c-name-translation.md>
>>> Reviews are an important part of the Swift evolution process. All reviews should be sent to the swift-evolution mailing list at
>>> 
>>> https://lists.swift.org/mailman/listinfo/swift-evolution <https://lists.swift.org/mailman/listinfo/swift-evolution>
>>> or, if you would like to keep your feedback private, directly to the review manager. When replying, please try to keep the proposal link at the top of the message:
>>> 
>>> Proposal link:
>>> 
>>> https://github.com/apple/swift-evolution/blob/master/proposals/0005-objective-c-name-translation.md <https://github.com/apple/swift-evolution/blob/master/proposals/0005-objective-c-name-translation.md>
>>> Reply text
>>> 
>>> Other replies
>>>  <https://github.com/apple/swift-evolution#what-goes-into-a-review-1>What goes into a review?
>>> 
>>> The goal of the review process is to improve the proposal under review through constructive criticism and, eventually, determine the direction of Swift. When writing your review, here are some questions you might want to answer in your review:
>>> 
>>> What is your evaluation of the proposal?
>>> Is the problem being addressed significant enough to warrant a change to Swift?
>>> Does this proposal fit well with the feel and direction of Swift?
>>> If you have used other languages or libraries with a similar feature, how do you feel that this proposal compares to those?
>>> How much effort did you put into your review? A glance, a quick reading, or an in-depth study?
>>> More information about the Swift evolution process is available at
>>> 
>>> https://github.com/apple/swift-evolution/blob/master/process.md <https://github.com/apple/swift-evolution/blob/master/process.md>
>>> Thank you,
>>> 
>>> -Doug Gregor
>>> 
>>> Review Manager
>>> 
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>> 
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