Hi all,
Here’s a proposal draft to allow one to name any function in Swift. In effect,
it’s continuing the discussion of retrieving getters and setters as functions
started by Michael Henson here:
https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20151207/002168.html
the proposal follows, and is available here as well:
https://github.com/DougGregor/swift-evolution/blob/generalized-naming/proposals/0000-generalized-naming.md
Comments appreciated!
Generalized Naming for Any Function
Proposal: SE-NNNN
<https://github.com/apple/swift-evolution/blob/master/proposals/0000-generalized-naming.md>
Author(s): Doug Gregor <https://github.com/DougGregor>
Status: Awaiting Review
Review manager: TBD
<https://github.com/DougGregor/swift-evolution/blob/generalized-naming/proposals/0000-generalized-naming.md#introduction>Introduction
Swift includes support for first-class functions, such that any function (or
method) can be placed into a value of function type. However, it is not
possible to specifically name every function that is part of a Swift
program---one cannot provide the argument labels when naming a function, nor
are property and subscript getters and setters referenceable. This proposal
introduces a general syntax that allows one to name anything that is a function
within Swift in an extensible manner.
Swift-evolution thread: Michael Henson started a thread about the getter/setter
issue here
<https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20151207/002168.html>,
continued here
<https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20151214/002203.html>.
See the Alternatives considered
<https://github.com/DougGregor/swift-evolution/blob/generalized-naming/proposals/0000-generalized-naming.md#alternatives-considered>
section for commentary on that discussion.
<https://github.com/DougGregor/swift-evolution/blob/generalized-naming/proposals/0000-generalized-naming.md#motivation>Motivation
It's fairly common in Swift for multiple functions or methods to have the same
"base name", but be distinguished by parameter labels. For example, UIView has
three methods with the same base name insertSubview:
extension UIView {
func insertSubview(view: UIView, at index: Int)
func insertSubview(view: UIView, aboveSubview siblingSubview: UIView)
func insertSubview(view: UIView, belowSubview siblingSubview: UIView)
}
When calling these methods, the argument labels distinguish the different
methods, e.g.,
someView.insertSubview(view, at: 3)
someView.insertSubview(view, aboveSubview: otherView)
someView.insertSubview(view, belowSubview: otherView)
However, when referencing the function to create a function value, one cannot
provide the labels:
let fn = someView.insertSubview // ambiguous: could be any of the three methods
In some cases, it is possible to use type annotations to disambiguate:
let fn: (UIView, Int) = someView.insertSubview // ok: uses
insertSubview(_:at:)
let fn: (UIView, UIView) = someView.insertSubview // error: still ambiguous!
To resolve the latter case, one must fall back to creating a closure:
let fn: (UIView, UIView) = { view, otherView in
button.insertSubview(view, otherView)
}
which is painfully tedious. A similar workaround is required to produce a
function value for a getter of a property, e.g.,
extension UIButton {
var currentTitle: String? { ... }
}
var fn: () -> String? = { () in
return button.currentTitle
}
One additional bit of motivation: Swift should probably get some way to ask for
the Objective-C selector for a given method (rather than writing a string
literal). The argument to such an operation would likely be a reference to a
method, which would benefit from being able to name any method, including
getters and setters.
<https://github.com/DougGregor/swift-evolution/blob/generalized-naming/proposals/0000-generalized-naming.md#proposed-solution>Proposed
solution
Swift currently has a back-tick escaping syntax that lets one use keywords for
names, which would otherwise fail to parse. For example,
func `try`() -> Bool { ... }
declares a function named try, even though try is a keyword. I propose to
extend the back-tick syntax to allow compound Swift names (e.g.,
insertSubview(_:aboveSubview:)) and references to the accessors of properties
(e.g., the getter for currentTitle). Specifically,
Compound names can be written entirely within the back-ticks, e.g.,
let fn = someView.`insertSubview(_:at:)`
let fn1 = someView.`insertSubview(_:aboveSubview:)`
The same syntax can also refer to initializers, e.g.,
let buttonFactory = UIButton.`init(type:)`
Getters and setters can be written using dotted syntax within the back-ticks:
let specificTitle = button.`currentTitle.get` // has type () -> String?
let otherTitle = UIButton.`currentTitle.get` // has type (UIButton) -> () ->
String?
let setTintColor = button.`tintColor.set` // has type (UIColor!) -> ()
The same syntax works with subscript getters and setters as well, using the
full name of the subscript:
extension Matrix {
subscript (row row: Int) -> [Double] {
get { ... }
set { ... }
}
}
let getRow = someMatrix.`subscript(row:).get` // has type (Int) -> () ->
[Double]
let setRow = someMatrix.`subscript(row:).set` // has type (Int) -> ([Double])
-> ()
If we introduce property behaviors into Swift, the back-tick syntax could also
be used to refer to behaviors, e.g., accessing the lazy behavior of a property:
self.`myProperty.lazy`.clear()
Base names that are meaningful keywords (init and subscript) can be escaped
with a nested pair of back-ticks:
extension Font {
func `subscript`() -> Font {
// return the subscript version of the given font
}
}
let getSubscript = font.``subscript`()` // has type () -> Font
The "produce the Objective-C selector for the given method" operation will be
the subject of a separate proposal. However, here is one possibility that
illustrations how it uses the proposed syntax here:
let getter: Selector = objc_selector(NSDictionary.`subscript(_:).get`) //
produces objectForKeyedSubscript:
let setter: Selector = objc_selector(NSDictionary.`subscript(_:).set`) //
produces setObject:forKeyedSubscript:
<https://github.com/DougGregor/swift-evolution/blob/generalized-naming/proposals/0000-generalized-naming.md#impact-on-existing-code>Impact
on existing code
This is a purely additive feature that has no impact on existing code. The
syntactic space it uses is already present, and it merely extends the use of
back-ticks from storing a single identifier to more complex names.
<https://github.com/DougGregor/swift-evolution/blob/generalized-naming/proposals/0000-generalized-naming.md#alternatives-considered>Alternatives
considered
Michael Henson proposed
<https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20151207/002168.html>
naming getters and setters using # syntax followed by get or set, e.g.,
let specificTitle = button.currentTitle#get
The use of postfix # is a reasonable alternative here, and more lightweight
than two back-ticks for the simple getter/setter case. The notion could be
extended to allow argument labels for functions, discussed here
<https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20151214/002210.html>.
The proposals in that discussion actually included type annotations as well,
but the syntax seems cleaner---and more directly focused on names---without
them, e.g.,:
let fn = someView.insertSubview#(_:at:)
which works. I didn't go with this syntax because (1) it breaks up Swift method
names such as insertSubview(_:at:)with an # in the middle, and (2) while
useful, this feature doesn't seem important enough to justify overloading
#further.
Joe Groff notes
<https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20151214/003008.html>
that lenses are a better solution than manually retrieving getter/setter
functions when the intent is to actually operate on the properties. That
weakens the case this proposal makes for making getters/setters available as
functions. However, it doesn't address the general naming issue or the desire
to retrieve the Objective-C selector for a getter/setter.
Can we drop the back-ticks? It's very tempting to want to drop the back-ticks
entirely, because something like
let fn = someView.insertSubview(_:at:)
can be correctly parsed as a reference to insertSubview(_:at:). However, it
breaks down at the margins, e.g., with getter/setter references or no-argument
functions:
extension Optional {
func get() -> T { return self! }
}
let fn1 = button.currentTitle.get // getter or Optional<String>.get?
let fn2 = set.removeAllElements() // call or reference?
- Doug
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