[swift-evolution] When to use argument labels (a new approach)
Erica Sadun
erica at ericasadun.com
Tue Feb 2 23:07:23 CST 2016
Thoughts? Thoughts:
Swift prizes clarity. Its parameter labeling system emphasizes self-documentation and guides code production. In nearly every case, labels follow three simple rules:
Skip argument labels for a method or function's first parameter
Use argument labels for a method or function's subsequent parameters
Require argument labels for initializers
These base rules enhance Swift legibility. Unlike other languages whose positional argument names have meaning only within the implementation context, Swift's labels convey use and meaning at the calling site. This creates better communication, enhances maintainability, and adheres to the principle that code is written rarely and read and reviewed often.
At times, special circumstances may apply to your code as explored in the following rules:
Skip first argument labels when the first argument completes a sentence established in the base name. If the argument describes a call's primary semantics, it does not require a label:
a.contains(b) // b completes the phrase "a contains b"
a.mergeWith(b) // b completes the phrase "merge with b"
a.readFrom(u, ofType: b) // "a, read from u" describes
// primary semantics so u gets no
// label.
// b is an option that tunes the
// primary semantics
Skip the first argument label when a noun in the base name describes the first argument's role.
a.addObserver(b) // "add b" completes a meaningful sentence that
// defines the intentended semantics. The first
// argument is the "Observer".
Move the first argument label to the base name when it describes a name or identifier that acts as the subject of the base action.
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
Move the first argument label to the base name when it describes argument attributes of existing instances.
a.tracksOfMediaType("Wax Cylinder") // yes
a.removeFirstTrackOfMediaType("BetaMax") // yes
a.tracks(mediaType: "Wax Cylinder") // no
a.removeFirstTrack(havingMediaType: "BetaMax") // no
Use first label arguments when the first parameter is semantically distinct from the base name and does not complete a meaningful "sentence"
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.
Use all argument labels when the relationship between arguments is semantically stronger than the relationship between the first argument and the base name.
moveTo(x: a, y: b)
login(userName: a, password: b)
constructColor(red: r, green: g, blue: b, alpha: a)
Omit labels for argument peers that cannot be usefully distinguished.
min(number1, number2)
zip(sequence1, sequence2)
Use explicit argument labels to describe attributes of an instance that's being created. Your calls should resemble initializers.
AudioTrack(mediaType: "BetaMax") // initializer
trackFactory.newTrack(mediaType: "Wax Cylinder") // yes
trackFactory.newTrackOfMediaType("Wax Cylinder") // no
Use first argument labels that would have normally appeared in the base name when building groups of related calls whose implementations are distinguished specifically by their parameters. Your calls should resemble initializers.
login(userName: a, password: b) // not loginWithUserName(a, password: b)
login(credential: a) // not loginWithCredential(a)
Skip first argument labels for initializers when using full width type conversions, that is when initializing from instances of another type.
extension String {
// Convert `x` into its textual representation
// in the given radix
init(_ x: BigInt, radix: Int = 10)
}
text = "The value is: "
text += String(veryLargeNumber)
text += " and in hexadecimal, it's"
text += String(veryLargeNumber, radix: 16)
Use first argument labels when narrowing initial values to make it conform to restrictions within the new type. The label should describe how the instance will be modified:
extension UInt32 {
init(_ value: Int16) // Widening, so no label
init(truncating bits: UInt64)
init(saturating value: UInt64)
}
-- E
> On Feb 2, 2016, at 5:32 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
>
> _______________________________________________
> swift-evolution mailing list
> swift-evolution at swift.org
> https://lists.swift.org/mailman/listinfo/swift-evolution
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