[swift-evolution] [Pitch] Synthesized static enum property to iterate over cases

[Tony Allevato]

Thanks for bringing this up, Logan! It's something I've been thinking about
a lot lately after a conversation with some colleagues outside of this
community. Some of my thoughts:

AFAIK, there are two major use cases here: (1) you need the whole
collection of cases, like in your example, and (2) you just need the number
of cases. The latter seems to occur somewhat commonly when people want to
use an enum to define the sections of, say, a UITableView. They just return
the count from numberOfSections(in:) and then switch over the cases in
their cell-providing methods.

Because of #2, it would be nice to avoid instantiating the collection
eagerly. (Also because of examples like Jonathan's, where the enum is
large.) If all the user is ever really doing is iterating over them,
there's no need to keep the entire collection in memory. This leads us to
look at Sequence; we could use something like AnySequence to keep the
current case as our state and a transition function to advance to the next
one. If a user needs to instantiate the full array from that sequence they
can do so, but they have to do it explicitly.

The catch is that Sequence only provides `underestimatedCount`, rather than
`count`. Calling the former would be an awkward API (why is it
underestimated? we know how many cases there are). I suppose we could
create a concrete wrapper for Sequence (PrecountedSequence?) that provides
a `count` property to make that cleaner, and then have
`underestimatedCount` return the same thing if users passed this thing into
a generic operation constrained over Sequence. (The standard library
already has support wrappers like EnumeratedSequence, so maybe this is
appropriate.)

Another question that would need to be answered is, how should the cases be
ordered? Declaration order seems obvious and straightforward, but if you
have a raw-value enum (say, integers), you could have the declaration order
and the numeric order differ. Maybe that's not a problem. Tying the
iteration order to declaration order also means that the behavior of a
program could change simply by reördering the cases. Maybe that's not a big
problem either, but it's something to call out.

If I were designing this, I'd start with the following approach. First, add
a new protocol to the standard library:

```
public protocol ValueEnumerable {
  associatedtype AllValuesSequence: Sequence where
AllValuesSequence.Iterator.Element == Self

  static var allValues: AllValuesSequence { get }
}
```

Then, for enums that declare conformance to that protocol, synthesize the
body of `allValues` to return an appropriate sequence. If we imagine a
model like AnySequence, then the "state" can be the current case, and the
transition function can be a switch/case that returns it and advances to
the next one (finally returning nil).

There's an opportunity for optimization that may or may not be worth it: if
the enum is RawRepresentable with RawValue == Int, AND all the raw values
are in a contiguous range, AND declaration order is numerical order
(assuming we kept that constraint), then the synthesized state machine can
just be a simple integer incrementation and call to `init?(rawValue:)`.
When all the cases have been generated, that will return nil on its own.

So that covers enums without associated values. What about those with
associated values? I would argue that the "number of cases" isn't something
that's very useful here—if we consider that enum cases are really factory
functions for concrete values of the type, then we shouldn't think about
"what are all the cases of this enum" but "what are all the values of this
type". (For enums without associated values, those are synonymous.)

An enum with associated values can potentially have an infinite number of
values. Here's one:

```
enum BinaryTree {
  case subtree(left: BinaryTree, right: BinaryTree)
  case leaf
  case empty
}
```

Even without introducing an Element type in the leaf nodes, there are a
countably infinite number of binary trees. So first off, we wouldn't be
able to generate a meaningful `count` property for that. Since they're
countably infinite, we *could* theoretically lazily generate a sequence of
them! It would be a true statement to say "an enum with associated values
can have all of its values enumerated if all of its associated values are
also ValueEnumerable". But I don't think that's something we could have the
compiler synthesize generally: the logic to tie the sequences together
would be quite complex in the absence of a construct like coroutines/yield,
and what's worse, the compiler would have to do some deeper analysis to
avoid infinite recursion. For example, if it used the naïve approach of
generating the elements in declaration order, it would keep drilling down
into the `subtree` case above over and over; it really needs to hit the
base cases first, and requiring the user to order the cases in a certain
way for it to just work at all is a non-starter.

So, enums with associated values are probably left unsynthesized. But the
interesting thing about having this be a standard protocol is that there
would be nothing stopping a user from conforming to it and implementing it
manually, not only for enums but for other types as well. The potential may
exist for some interesting algorithms by doing that, but I haven't thought
that far ahead.

There are probably some things I'm missing here, but I'd love to hear other
people's thoughts on it.


On Fri, Sep 8, 2017 at 3:40 AM Jonathan Hull via swift-evolution <
swift-evolution at swift.org> wrote:

> +1000
>
> I once made a country code enum, and creating that array was simple, but
> took forever, and was prone to mistakes.
>
> Thanks,
> Jon
>
> > On Sep 8, 2017, at 2:56 AM, Logan Shire via swift-evolution <
> swift-evolution at swift.org> wrote:
> >
> > Googling ‘swift iterate over enum cases’ yields many results of various
> levels of hackery.
> > Obviously it’s trivial to write a computed property that returns an
> enum’s cases as an
> > array, but maintaining that is prone to error. If you add another case,
> you need to make sure
> > you update the array property. For enums without associated types,
> > I propose adding a synthesized static var, ‘cases', to the enum’s type.
> E.g.
> >
> > enum Suit: String {
> >    case spades = "♠"
> >    case hearts = "♥"
> >    case diamonds = "♦"
> >    case clubs = "♣"
> > }
> >
> > let values = (1…13).map { value in
> >    switch value {
> >    case 1: return “A”
> >    case 11: return “J”
> >    case 12: return “Q”
> >    case 13: return “K”
> >    default: return String(value)
> >    }
> > }
> >
> > let cards = values.flatMap { value in Suit.cases.map { “\($0)\(value)"
> } }
> >
> > Yields [“♠A”, “ ♥ A”, …, “♣K”]
> > Thoughts?
> >
> >
> > Thanks!
> > - Logan Shire
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