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

[Dennis Lysenko]

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
>
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