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

Paul Cantrell cantrell at pobox.com
Tue Feb 2 20:59:38 CST 2016

+1 for 1, with an asterisk
mixed feelings about 2
+1 for 3

Notes inline below.

> On Feb 2, 2016, at 6:32 PM, Dave Abrahams via swift-evolution <swift-evolution at swift.org> wrote:
> 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:

Yes. The thoughts of “sentence” and “primary semantics” succinctly capture a lot of good ideas from previous discussion.

>     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

All these examples seem totally solid.

>   [Note that this covers all the direct object cases and, I believe,
>   all the default argument cases too, so maybe that exception can be

Not sure about this one in theory … but yes, I can see how it would play out that way in practice.

>   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.

Not sure you need this complicating exception. “a, add observer b” is still a sentence.

> * 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]

I have reservations about this. It doesn’t generalize perfectly well:

	a.tracksHavingMediaType("Wax Cylinder”, andPianist: “Brahms”)

…strikes me as awkward compared to one of these:

	a.tracksHaving(mediaType: "Wax Cylinder”, pianist: “Brahms”)
	// or 
	a.tracksMatchingCriteria(mediaType: "Wax Cylinder”, pianist: “Brahms”)
	// or even
	a.tracks(mediaType: "Wax Cylinder”, pianist: “Brahms”)

…especially since, in a method of that form, _all_ the arguments are likely to have a default value of nil:

	a.tracks(mediaType: "Wax Cylinder”)
	a.tracks(pianist: “Brahms”)

>   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

Yes, definitely.

> 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

Yes, seems solid.

Cheers, P

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