Ceylon demonstrates other uses for the bottom type than just marking @noreturn
methods, e.g. (mixing Ceylon and Swift syntax freely here)
class Iterable<Element, Absent = Null> {
var last: Element | Absent // remember that optionals in Ceylon
are modeled as union type T | Null
...
}
Iterable<String> // a possibly empty iterable where last: Element | Null, i.e.
last: Element?
Iterable<String, Never> // a non-empty iterable where last: Element | Never,
i.e. last: Element
This achieves type safety for `last` (and a couple of other methods not shown
here) with respect to the property of an iterable statically known to be
non-empty.
This is very elegant IMHO.
-Thorsten
> Am 06.06.2016 um 19:46 schrieb Jacob Bandes-Storch via swift-evolution
> <[email protected]>:
>
> What about `PreconditionFailure`? If you obtain an instance of this type, it
> means a precondition has failed.
>
> Jacob
>
> On Mon, Jun 6, 2016 at 9:48 AM, Joe Groff via swift-evolution
> <[email protected] <mailto:[email protected]>> wrote:
> `Never` seems reasonable to me too. I'll add that to the proposal as an
> alternative.
>
> -Joe
>
> > On Jun 5, 2016, at 11:37 AM, T.J. Usiyan via swift-evolution
> > <[email protected] <mailto:[email protected]>> wrote:
> >
> > I vote for Bottom or Never. None does not convey "this should not occur or
> > be evaluated".
> >
> > On Sun, Jun 5, 2016 at 2:26 PM, Антон Жилин <[email protected]
> > <mailto:[email protected]>> wrote:
> > The following names were suggested: NoReturn, Bottom, None, Never.
> > I would pick None, because it looks like opposite to Any and fits nicely in
> > generic types.
> >
> > I would prefer the type to be simple, and be implemented as a case-less
> > enum (not a bottom value, as in Haskell).
> >
> > None should be a usual enum, with no compiler magic except that functions
> > returning None are equivalent to current @noreturn.
> >
> > Example 1.
> > let x: None?
> > // ...
> > let y = x!
> >
> > It will trap in runtime not because we discover scary bottom thing, as in
> > Haskell, but because x had value Optional.none at that moment and we
> > asserted otherwise.
> > We could prove that it is always true in this case, but compiler must be
> > stupid about this.
> >
> > Example 2.
> > Compiler should allow including None in structures. Error will show up in
> > constructor, when we will not be able to initialize the field.
> >
> > Example 3.
> > None in an enum case makes that case never appear in values of such a type.
> > But compiler can not know about that.
> >
> > - Anton
> >
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