[swift-evolution] [Pitch] Introducing `Unwrappable` protocol

[Jaden Geller]

> On Mar 7, 2017, at 8:59 PM, Brent Royal-Gordon via swift-evolution <swift-evolution at swift.org> wrote:
> 
>> On Mar 7, 2017, at 12:14 PM, Erica Sadun via swift-evolution <swift-evolution at swift.org> wrote:
>> 
>> Because of that, I'm going to start over here, hopefully pulling in all the details
>> and allowing the community to provide feedback and direction. The following 
>> gist is an amalgam of work I was discussing with Xiaodi Wu, Chris Lattner, and
>> David Goodine.
>> 
>> https://gist.github.com/erica/aea6a1c55e9e92f843f92e2b16879b0f
> 
> Treating the things separately:
> 
> 1. Introduce an `unwrap` keyword
> 
> I'm really not convinced this pulls its own weight. Without the `let`, it doesn't make the fact that it's shadowing the original (and thus that you cannot modify it) clear; with the `let`, it introduces a new keyword people need to learn for the sake of eliding a repeated variable name.

If Swift supported `if inout x = x { … }`, then `unwrap` could have much more reasonable semantics.

> 
> In the document, you state that `unwrap` "simplifies the status quo and eleminates unintended shadows", but that's not true, because the existing syntax will continue to exist and be supported. Unless we warn about *any* shadowing in an `if let` or `if case`, it will still be possible to accidentally shadow variables using these declarations.
> 
> 2. Introduce an `Unwrappable` protocol
> 
> I like the idea, but I would use a slightly different design which offers more features and lifts this from "bag of syntax" territory into representing a discrete semantic. This particular design includes several elements which depend on other proposed features:
> 
> 	/// Conforming types wrap another type, creating a supertype which may or may not 
> 	/// contain the `Wrapped` type.
> 	/// 
> 	/// `Wrapper` types may use the `!` operator to unconditionally access the wrapped 
> 	/// value or the `if let` and `guard let` statements to conditionally access it. Additionally, 
> 	/// `Wrapped` values will be automatically converted to the `Wrapper`-conforming type 
> 	/// as needed, and the `is`, `as`, `as?`, and `as!` operators will treat the `Wrapped` type 
> 	/// as a subtype of the `Wrapper`-conforming type.
> 	protocol Wrapper {
> 		/// The type that this value wraps.
> 		associatedtype Wrapped
> 		
> 		/// The type of error, if any, thrown when a non-wrapped value is unwrapped.
> 		associatedtype UnwrappingError: Error = Never
> 		
> 		/// Creates an instance of `Self` which wraps the `Wrapped` value.
> 		/// 
> 		/// You can call this initializer explicitly, but Swift will also insert implicit calls when 
> 		/// upcasting from `Wrapped` to `Self`.
> 		init(_ wrapped: Wrapped)
> 		
> 		/// Returns `true` if `Self` contains an instance of `Wrapped` which can be accessed 
> 		/// by calling `unwrapped`.
> 		var isWrapped: Bool { get }
> 		
> 		/// Accesses the `Wrapped` value within this instance.
> 		/// 
> 		/// If `isWrapped` is `true`, this property will always return an instance. If it is `false`, this property 
> 		/// will throw an instance of `UnwrappingError`, or trap if `UnwrappingError` is `Never`.
> 		var unwrapped: Wrapped { get throws<UnwrappingError> }
> 		
> 		/// Accesses the `Wrapped` value within this instance, possibly skipping safety checks.
> 		/// 
> 		/// - Precondition: `isWrapped` is `true`.
> 		var unsafelyUnwrapped: Wrapped { get }
> 	}
> 	
> 	extension Wrapper {
> 		// Default implementation of `unsafelyUnwrapped` just calls `unwrapped`.
> 		var unsafelyUnwrapped: Wrapped {
> 			return try! unwrapped
> 		}
> 	}
> 
> The defaulting of `WrappingError` to `Never` means the error-emitting aspects of this design are additive and can be introduced later, once the necessary supporting features are introduced. The use of separate `isWrapped` and `unwrapped` properties means that `unwrapped` can implement an appropriate behavior on unwrapping failure, instead of being forced to return `nil`.
> 
> (An alternative design would have `wrapped: Wrapped? { get }` and `unwrapped: Wrapped { get throws<UnwrappingError> }` properties, instead of `isWrapped` and `unwrapped`.)
> 
> In this model, your example of:
> 
> 	let value = try unwrap myResult // throws on `failure`
> 
> Would instead be:
> 
> 	let value = try myResult! // throws on `failure`
> 
> (Actually, I'm not sure why you said this would be `unwrap`—it's not shadowing `myResult`, is it?)
> 
> Theoretically, this exact design—or something close to it—could be used to implement subtyping:
> 
> 	extension Int16: Wrapper {
> 		typealias Wrapped = Int8
> 		
> 		init(_ wrapped: Int8) {
> 			self.init(exactly: wrapped)!
> 		}
> 		
> 		var isWrapped: Bool {
> 			return Self(exactly: Int8.min)...Self(exactly: Int8.max).contains(self)
> 		}
> 		
> 		var unwrapped: Int8 {
> 			return Self(exactly: self)!
> 		}
> 	}
> 
> But this would imply that you could not only say `myInt8` where an `Int16` was needed, but also that you could write `myInt16!` where an `Int8` was needed. I'm not sure we want to overload force unwrapping like that. One possibility is that unwrapping is a refinement of subtyping:
> 
> 	// `Downcastable` contains the actual conversion and subtyping logic. Conforming to 
> 	// `Downcastable` gets you `is`, `as`, `as?`, and `as!` support; it also lets you use an 
> 	// instance of `Subtype` in contexts which want a `Supertype`.
> 	protocol Downcastable {
> 		associatedtype Subtype
> 		associatedtype DowncastingError: Error = Never
> 		
> 		init(upcasting subvalue: Subtype)
> 		
> 		var canDowncast: Bool { get }
> 		
> 		var downcasted: Subtype { get throws<DowncastingError> }
> 		
> 		var unsafelyDowncasted: Subtype { get }
> 	}
> 	
> 	// Unwrappable refines Downcastable, providing access to `!`, `if let`, etc.
> 	protocol Unwrappable: Downcastable {}
> 	extension Unwrappable {
> 		var unsafelyUnwrapped: Subtype { return unsafelyDowncasted }
> 	}
> 
> That would allow you to have conversions between `Int8` and `Int16`, but not to use `!` on an `Int16`.
> 
> 3. Apply `unwrap` to non-`Optional` values, and
> 4. Extend `for` and `switch`
> 
> These are pretty straightforward ramifications of having both `unwrap` and `Unwrappable`. I don't like `unwrap`, but if we *do* add it, it should certainly do this.
> 
> 5. Fix Pattern Match Binding
> 
> The `case let .someCase(x, y)` syntax is really convenient when there are a lot of variables to bind. I would suggest a fairly narrow warning: If you use a leading `let`, and some—but not all—of the variables bound by the pattern are shadowing, emit a warning. That would solve the `case let .two(newValue, oldValue)`-where-`oldValue`-should-be-a-match problem.
> 
> 6. Simplify Complex Binding
> 
> I'm not convinced by this. The `case` keyword provides a strong link between `if case` and `switch`/`case`; the `~=` operator doesn't do this. Unless we wanted to redesign `switch`/`case` with matching ergonomics—which, uh, we don't:
> 
> 	switch value {
> 	~ .foo(let x): 
> 		...use x...
> 	...
> 	}
> 
> —I don't think we should go in this direction. `for case` also has similar concerns.
> 
> I think we'd be better off replacing the `~=` operator with something more memorable. For instance:
> 
> 	extension Range {
> 		public func matches(_ value: Bound) -> Bool {
> 			return contains(value)
> 		}
> 	}
> 
> Or:
> 
> 	public func isMatch<Bound: Comparable>(_ value: Bound, toCase pattern: Range<Bound>) -> Bool {
> 		return pattern.contains(value)
> 	}
> 
> -- 
> Brent Royal-Gordon
> Architechies
> 
> _______________________________________________
> swift-evolution mailing list
> swift-evolution at swift.org
> https://lists.swift.org/mailman/listinfo/swift-evolution