[swift-evolution] [Pitch] Use enums as enum underlying types

[Matthew Johnson]

> On Dec 18, 2015, at 11:34 AM, Dennis Lysenko via swift-evolution <swift-evolution at swift.org> wrote:
> 
> Sorry, I got a bit too excited and skimmed over the most important part of the idea. So this is a special type of enum declaration in which you cannot declare any new enum members. I personally have not seen a use for this in my code but I would love to hear others' response to it. It is a very interesting idea though.
> 
> I'm going to go out on a limb with an idea that is in the same vein as this one: What if we favored composition over inheritance here, and made it so that you could transparently refer to members of other enums *without* having another enum as a backing type?
> 
> e.g., you have:
> enum NetworkException {
>   case NoInternetError, SecurityError
> }
> 
> enum ParseException {
>   case FailedResponse(statusCode: Int)
>   case EmptyResponse
>   case MissingField(fieldName: String)
> }
> 
> As two general classes of errors. But for a full API call wrapper, you might want an error class that composes the two, so that when calling the API call from your UI code, you can display a "please check your connection" message for NoInternetError, a "Please log in" error for FailedResponse with statusCode=401, or a "server error" message for any of the rest. 
> 
> I wonder how do you and others feel about that use-case? I have certainly seen it come up a lot in real-world projects that require resilient UI interactions with nontrivial networking operations.
> 
> Here are some quick code samples off the top of my head for how we might go about this (let's say the API operation is "change profile picture":
> 
> enum ChangePictureError {
>   include NetworkException
>   include ParseException
>   case PictureTooLarge
> }

By including all of the cases you make it possible for ChangePictureError to be a supertype of NetworkException and ParseException.  This is a pretty interesting idea.  It might be worth exploring.  

One thing that would need to be considered is that ideally if the actual values was a NetworkException case you would want to be able to call any methods exposed by Network Exception.  A good way to accomplish that might be to add implicit conversion as well as syntactic sugar for nested enums.  So if we have this:

enum ChangePictureError {
  case NetworkException(NetworkException)
  case ParseException(ParseException)
  case PictureTooLarge
}

I can do this:

var error: ChangePictureError // set somewhere, can be set with a NetworkException or a PictureTooLarge
switch error {
case .NoNetworkError:                           // equivaluent to case .NetworkException(.NoNetworkError)
case .NoInternetError:                            // equivaluent to case .NetworkException(. NoInternetError)
case .FailedResponse(let statusCode): // equivaluent to case .ParseException(.FailedResponse(let statusCode))
case .EmptyResponse:                          // equivaluent to case .ParseException(.EmptyResponse)
case .MissingField(let fieldName):         // equivaluent to case .ParseException(. MissingField(let fieldName))
case .PictureTooLarge:
}

The syntactic sugar would only work for case names where there is no overlap.  Case names that overlap would need to be explicitly disambiguated.  The syntactic sugar and implicit conversions could allow for either single-level nesting or arbitrary nesting depth.  An example of arbitrary depth might be ParseException also containing a ValidationError case:

enum ValidationError {
  case OutOfRange
  case InvalidType
}

enum ParseException {
  case ValidationError(ValidationError)
  case FailedResponse(statusCode: Int)
  case EmptyResponse
  case MissingField(fieldName: String)
}

Mostly just thinking out loud here and exploring the idea.  What do others think of this?

> 
> or
> 
> enum ChangePictureError {
>   compose NetworkException.NoInternetError
>   compose ParseException.EmptyResponse
>   compose ParseException.FailedResponse(statusCode: Int)
>   case PictureTooLarge
> }
> 
> Not a proposal by any stretch of the imagination, just a potential direction inspired by your idea, Felix.
> 
> 
> On Fri, Dec 18, 2015 at 12:21 PM Dennis Lysenko <dennis.s.lysenko at gmail.com <mailto:dennis.s.lysenko at gmail.com>> wrote:
> Felix, 
> 
> This seems to be very interestingly tied into your comments about polymorphism in 'throws' type annotations. Would you not feel that allowing enums to be built on top of other enums would promote the kind of egregious proliferation of exception polymorphism that discourages so many from following Java's checked exception model? 
> 
> On Fri, Dec 18, 2015 at 11:29 AM Félix Cloutier <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
> Hi all,
> 
> Swift currently has more or less three conceptual types of enums: discriminated unions, lists of unique tokens, and lists of value of a raw type.
> 
> > // Discriminated unions
> > enum Foo {
> >       case Bar(Int)
> >       case Baz(String)
> > }
> >
> > // Lists of unique tokens (mixable with discriminated unions)
> > enum Foo {
> >       case Frob
> >       case Nicate
> > }
> >
> > // Lists of raw values
> > enum Foo: String {
> >       case Bar = "Bar"
> >       case Baz = "Baz"
> > }
> 
> I think that the last case could be made more interesting if you could use more types as underlying types. For instance, it could probably be extended to support another enum as the backing type. One possible use case would be to have a big fat enum for all the possible errors that your program/library can throw, but refine that list into a shorter enum for functions that don't need it all.
> 
> > enum MyLibError: ErrorType {
> >       case FileNotFound
> >       case UnexpectedEOF
> >       case PermissionDenied
> >       // ... 300 cases later
> >       case FluxCapacitorFailure
> >       case SplineReticulationError
> > }
> >
> > enum FileSystemError: MyLibError {
> >       case FileNotFound = .FileNotFound
> >       case UnexpectedEOF = .UnexpectedEOF
> >       case PermissionDenied = .PermissionDenied
> > }
> 
> This example could be made simpler if the `= .Foo` part was inferred from the name, but you get the idea.
> 
> In this case, it would be helpful (but not required) that FileSystemError was convertible into a MyLibError, so that it could be transparently rethrown in a function that uses the larger enum. I personally don't see why enums with a specified underlying type can't be implicitly converted to it, but this is not currently the case and it probably deserves some discussion as well.
> 
> Is there any interest in that?
> 
> Félix
> 
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