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

[Douglas Gregor]

> On Jan 25, 2016, at 6:55 AM, Radosław Pietruszewski <radexpl at gmail.com> wrote:
> 
> Hello all,
> 
> I’m overwhelmingly *for* this proposal. I think removing needless verbosity and keeping the signal-to-noise ratio high is one of the most immediately appealing aspects of Swift, as well as a great general improvement to the programming experience.
> 
> And so unswiftified (yes, it’s a word now) APIs from Objective-C stick out like a sore thumb. Not only are they harder to read and write, they visually overwhelm the less verbose, information-dense Swift-first code.
> 
> Just like previous 1.0—2.0 attempts at bridging the gap (with NSError params being translated to Swift errors, factory methods translated to initializers, etc.), automating this will be an error-prone process, and almost bound to be a bit annoying at first, before all the glitches and poor translations are smoothed out. And yet I feel like just like the previous automated translations were overwhelmingly a great thing, so will the result of this proposal.
> 
> * * *
> 
>>    Add First Argument Labels
>>    
>>    - func enumerateObjectsWith(_: NSEnumerationOptions = [], using: (AnyObject, UnsafeMutablePointer) -> Void)
>>    + func enumerateObjects(options _: NSEnumerationOptions = [], using: (AnyObject, UnsafeMutablePointer) -> Void)
> 
> Good! The Guidelines recommend an explicit first parameter label for arguments with a default value, but this is a good change also for another reason, a use case not included in the Guidelines (I have more to say about this in the SE-0023 thread):
> 
> “Options” is the description of the parameter, not the method itself. Even if (for whatever reason!) `options` didn’t have a default value and the word “Options” wasn’t omitted in the translation,
> 
>    enumerateObjects(options: …)
> 
> would be clearer than
> 
>    enumerateObjectsWithOptions(…)
> 
> It’s not even about the extra word, about the four useless characters, it’s simply that “WithOptions” doesn’t describe the operation at all. It’s a word that conveys no information (“with”), and “options”, which describes the first parameter. In Objective-C, there’s no such thing as parameter labels, it’s all one name, so “With” is used as a separator. But in Swift, making the first parameter’s label explicit just makes more sense.

That’s an interesting thought! If “with” is truly used as a convention for separating the description of the operation from the description of the first parameter, that’s something that can be codified in the Clang importer. I was curious, so I hacked it up. Here’s a diff of the Cocoa APIs that shows what things would look like if we treated “with” as a separator:

	https://github.com/apple/swift-3-api-guidelines-review/pull/5/files

It’s a diff against SE-0005, and it introduces a significant number of first argument labels. Indeed, you’ll need to grab the patch to see them all:

	https://github.com/apple/swift-3-api-guidelines-review/pull/5.patch

A brief survey shows that some cases seem to be lining up with the guideline proposals that have been under discussion. For example, the patch includes:

-  func fillWith(blendMode: CGBlendMode, alpha: CGFloat)
-  func strokeWith(blendMode: CGBlendMode, alpha: CGFloat)
+  func fill(blendMode blendMode: CGBlendMode, alpha: CGFloat)
+  func stroke(blendMode blendMode: CGBlendMode, alpha: CGFloat)

-  func encodeWith(aCoder: Coder)
+  func encode(coder aCoder: Coder)

which you might recognize, because it’s the example you used:

> And with that in mind, I object to these translations:
> 
>    func fillWith(_: CGBlendMode, alpha: CGFloat)
>    func strokeWith(_: CGBlendMode, alpha: CGFloat)
>    func encodeWith(_: Coder)
> 
> Even though these don’t have default values, I believe this version to be clearer and make more sense, even if slightly more verbose:
> 
>    func fill(blendMode: CGBlendMode, alpha: CGFloat)
>    func stroke(blendMode: CGBlendMode, alpha: CGFloat)
>    func encode(coder: Coder)

Another random interesting example I encountered:

-  func addArcWithCenter(center: CGPoint, radius: CGFloat, startAngle: CGFloat, endAngle: CGFloat, clockwise: Bool)
+  func addArc(center center: CGPoint, radius: CGFloat, startAngle: CGFloat, endAngle: CGFloat, clockwise: Bool)

which seems to match the idea behind Erica’s "semantic relationship between the parameters is stronger than their relation to the operation” (or Paul Cantrell’s similar notion of "the direct object is several args taken together”, which feels more in line with the way the API guidelines are written).

There’s also this:

-  func tracksWithMediaType(mediaType: String) -> [AVMovieTrack]
+  func tracks(mediaType mediaType: String) -> [AVMovieTrack]

-  func tracksWithMediaCharacteristic(mediaCharacteristic: String) ->
+  func tracks(mediaCharacteristic mediaCharacteristic: String) -> [AVMovieTrack]

which feels reminiscent of Paul’s “resource” example:

    service.resource("/foo")
    service.resource(absoluteURL: "http://bar.com <http://bar.com/>")
    service.resource(absoluteURL: NSURL(string: "http://bar.com <http://bar.com/>"))

where (I think) the argument is that the various methods should all have the same base name because they’re all returning “tracks” or a “resource”, respectively.

There is a ton of data in that patch. I’d be interested to hear whether the resulting Cocoa APIs feel better in Swift—are they following the evolving set of guidelines for first argument labels that are under discussion, and are the resulting APIs clearer/more Swifty? What specific APIs work well and where does this “with-as-separator” heuristic break down?

	- Doug


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