[swift-evolution] When to use argument labels (a new approach)

Dave Abrahams dabrahams at apple.com
Wed Feb 3 16:06:31 CST 2016

on Tue Feb 02 2016, Charles Kissinger <swift-evolution at swift.org> wrote:

> +1 to all three rules. One question though: is Rule 2 intended to
> apply to cases where there are multiple “attribute” parameters? Would
> it be:
> a.tracksHavingMediaType(“Wax Cylinder”, genre: “Jazz”)
> or:
> a.tracks(mediaType: “Wax Cylinder” , genre: “Jazz”)
> or, for that matter:
> a.tracksMatching(mediaType: “Wax Cylinder” , genre: “Jazz”)
> ?

Thanks for asking.  We thought about this, and on quick examination, we weren't able to find enough
cases like this in Cocoa to make it obviously worth handling specially
in the importer... and as a result I omitted it from my suggestion,
oops.  The thought was that if you have a word that needs to “distribute”
over multiple arguments, you keep that outside the parens, per

   a.tracksHaving(mediaType: "Wax Cylinder", genre: "Jazz")

we could discuss whether it's better to go with 

   a.tracksHaving(mediaType: "Wax Cylinder", andGenre: "Jazz")

here in order to reinforce the distribution.

[Yes, you could use "matching" here as well. I don't have a strong
preference either way.]

> The “sentence” requirement of Rule 1 doesn’t seem to be met here, but
> the “primary semantics” part would still apply?

I don't know what you mean; AFAICT, both parts apply; you have to apply
them together.  You omit the argument label if and only if you can
describe the primary semantics with a complete sentence starting in the
base name and ending in the first argument.  The primary semantics of
the method don't end with the mediaType: the genre is as much part of
the primary semantics, so we use an argument label.

> —CK
>> On Feb 2, 2016, at 4:32 PM, Dave Abrahams via swift-evolution <swift-evolution at swift.org> wrote:
>> This thread is related to the review of new API guidelines, but it's not
>> a review thread; it's exploratory.  The goal is to come up with
>> guidelines that:
>> * describe when and where to use argument labels
>> * require labels in many of the cases people have asked for them
>> * are understandable by humans
>> * preserve important semantics communicated by existing APIs.
>> Here's what I'm thinking
>> 1. If and only if the first argument could complete a sentence*
>>   beginning in the base name and describing the primary semantics of
>>   the call, it gets no argument label:
>>     a.contains(b)  // b completes the phrase "a contains b"
>>     a.mergeWith(b) // b completes the phrase "merge with b"
>>     a.dismiss(animated: b) // "a, dismiss b" is a sentence but 
>>                            // doesn't describe the semantics at all, 
>>                            // thus we add a label for b.
>>     a.moveTo(x: 300, y: 400) // "a, move to 300" is a sentence 
>>                              // but doesn't describe the primary 
>>                              // semantics, which are to move in both
>>                              // x and y.  Thus, x gets a label.
>>     a.readFrom(u, ofType: b) // "a, read from u" describes
>>                              // the primary semantics, so u gets no
>>                              // label. b is an
>>                              // option that tunes the primary
>>                              // semantics
>>   [Note that this covers all the direct object cases and, I believe,
>>   all the default argument cases too, so maybe that exception can be
>>   dropped.  We still need the exceptions for full-width type
>>   conversions and indistinguishable peers]
>>   Note: when there is a noun in the base name describing the role of the
>>   first argument, we skip it in considering this criterion:
>>      a.addObserver(b) // "a, add b" completes a sentence describing 
>>                       // the semantics.  "Observer" is omitted in 
>>                       // making this determination.
>> * We could say "clause" here but I think making it an *independent*
>>  clause doesn't rule out any important use-cases (see
>>  https://web.cn.edu/kwheeler/gram_clauses_n_phrases.html) and at that
>>  point, you might as well say "sentence," which is a more
>>  universally-understood term.
>> 2. Words that describe attributes of an *already-existing* instance
>>   should go in the base name rather than in a label:
>>      a.tracksHavingMediaType("Wax Cylinder")      // yes
>>      a.removeFirstTrackHavingMediaType("BetaMax") // yes
>>      a.tracks(mediaType: "Wax Cylinder")          // no
>>      a.removeFirstTrack(havingMediaType: "BetaMax") // no
>>   [yes, we could use "With" instead of "Having", but it's more
>>   ambiguous]
>>   Words that describe attributes of an instance *to be created* should
>>   go in argument labels, rather than the base name (for parity with
>>   initializers):
>>      AudioTrack(mediaType: "BetaMax")                   // initializer
>>      trackFactory.newTrack(mediaType: "Wax Cylinder")   // yes
>>      trackFactory.newTrackWithMediaType("Wax Cylinder") // no
>> 3. (this one is separable) When the first argument is the *name* or
>>   *identifier* of the subject in the base name, do not label it or
>>   describe it in the base name.
>>      a.transitionToScene(.GreatHall)               // yes
>>      a.transitionToSceneWithIdentifier(.GreatHall) // no
>>      let p = someFont.glyph("propellor")           // yes
>>      let p = someFont.glyphWithName("propellor")   // no
>>      let p = someFont.glyph(name: "propellor")     // no
>> Thoughts?
>> -- 
>> -Dave
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