[swift-evolution] [Pitch] Introducing `Unwrappable` protocol
David Hart
david at hartbit.com
Wed Mar 8 00:56:42 CST 2017
I will attach my comments to Brent’s answer because I echo many of his thoughts:
> On 8 Mar 2017, at 05:59, 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.
>
> 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.
I’m also not convinced this is worth it. While I see one argument for this feature (following DRY principles) I see many disadvantages:
introduction of an extra keyword
newcomers have to learn that there are two different syntaxes for optional binding
less clear that shadowing is happening
> 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`
Wouldn’t this be confusing for people who associate ! with crashing behaviour (in optional! and try!)
> (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.
Same arguments as for (1)
> 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.
Agreed. The leading let is worth it when matching many variables! I also prefer the warning solution. Forcing let at the variable site would defeat DRY principles.
> 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)
> }
And here’s the only place I disagree with Brent. The way I see it, the `case` keyword does not provide a strong link between `if case` and `switch`/`case`, it simply confuses people who do not expect the see that keyword associated with anything else than switch. It does not scream “pattern matching” to me like ~= does.
Erica, I think that section could contain more examples of the proposed syntax being used with cases:
if .left(let x) ~= either { … }
> --
> Brent Royal-Gordon
> Architechies
>
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> swift-evolution mailing list
> swift-evolution at swift.org
> https://lists.swift.org/mailman/listinfo/swift-evolution
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