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

[Dave Abrahams]

on Wed Feb 03 2016, Radosław Pietruszewski <swift-evolution at swift.org> wrote:

> By the way. I know this isn’t the main topic for the thread, but I
> would adding back a shortcut syntax for making a variable name the
> external label. It’s tiring to have to write:
>
> methodName(moreThanTwoWords moreThanTwoWords: Type) // exaggerated for dramatic effect.
>
> Most methods, in my experience, still don’t need a label for the first
> argument, but in the ones that do, we shouldn’t penalize doing so with
> ugly repetitiveness.

The easiest way to solve this would be to change the default for first
argument labels, but I really don't want to touch that in this thread.
However, I will refer to something Joe Groff points out in
<http://article.gmane.org/gmane.comp.lang.swift.evolution/4096>: the
automatic argument-name-matches-label default you're getting may not
actually be serving the authors of well-written APIs.

> (The Q is what syntax should do this. The old “#argument” obviously
> feels wrong, as “#” is otherwise reserved as “macro-like, or
> compiler-generated”.)
>
> — Radek
>
>> On 03 Feb 2016, at 01:32, 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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-- 
-Dave