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

Dennis Lysenko dennis.s.lysenko at gmail.com
Fri Dec 18 11:34:57 CST 2015

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


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>

> 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>
> 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
>> _______________________________________________
>> swift-evolution mailing list
>> swift-evolution at swift.org
>> https://lists.swift.org/mailman/listinfo/swift-evolution
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