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

[Radosław Pietruszewski]

> On 04 Feb 2016, at 02:24, Dave Abrahams via swift-evolution <swift-evolution at swift.org> wrote:
> 
> 
> 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.

True, that would be the easiest — though I fear it’s not the right solution. Though there are seemingly more scenarios in which explicit label is desirable, overall I see a bigger number of methods where it is not.

> 
>> (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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>> 
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> 
> -- 
> -Dave
> 
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