[swift-evolution] Ad hoc enums / options

Vladimir.S svabox at gmail.com
Wed Jun 1 07:38:06 CDT 2016

On 01.06.2016 11:00, Austin Zheng wrote:
> Tuples are a structural type, they are described entirely by the fact
> that they are a tuple, plus their contained types.
> Enum cases are not individual types; that precedent exists nowhere in
> Swift. You can't (yet) build a structural type out of something that
> isn't a type. The fact that you had to propose something like
> "AdhocEnumFitFill_2383748" as an autogenerated name for the type
> demonstrates the proposal's weaknesses: a tuple is an ad-hoc type that
> describes itself, while an anonymous enum isn't.

Yes, I understand the point about the type of such adhoc enum.
The only workaround I can see in this case(if we'd really want to have it 
in language) if adhoc enum type will be `(.Fit|.Fill)` i.e. textual 
representation if the declared type. As I understand this also could not be 
a solution.. I.e. for example `(Int,String,(.Fit|.Fill))->String`

 From other point of view, adding such type to typesystem will add some 
consistence : you can create a function that don't need definition of 
separate structure type(tuple will be used) and don't need separate enum 
type(ad-hoc enum will be used). I.e. all data the function needs to process 
could be described in function definition. Today we need to use ugly Bool 
flags in case we want to achieve the same target.

> Now if enum cases were equivalent if they had the same name (like how
> "Int" means the same thing no matter what tuple or generic type it is
> used in), we'd have a good foundation for a self-describing structural
> type. But this isn't how the existing named enum types work. Why would
> it be a good idea to make anonymous enum cases interchangeable by name?
> Properties on different types aren't interchangeable, even if they have
> the same type. In fact, no type member that I am aware of is
> interchangeable solely on the basis of name. An "ArtistAction.Draw" and
> "CowboyAction.Draw" might have the same name, but they mean completely
> different things.

I don't think they should be 'interchangeable by name', but just like 
tuples if you defined adhoc enum with exactly the same cases as ad-hoc enum 
in function parameters - then they are of the same type.

I.e. :

func foo(option: (.fit|.fill)) {..}

foo(.fit) // .fit is of type  (.fit|.fill) from definition

let e : (.fit|.fill) = .fit
foo(e) // e is of (.fit|.fill) type, equal to definition


func foo2(option: (.fit|.fill|.other)) {..}

foo2(.fit) // ok, here .fit is of (.fit|.fill|.other) type
foo2(e) --> Error, e is not of type (.fit|.fill|.other)

> Finally, I have to ask: if you are updating your anonymous enum in
> multiple places, how much effort have you actually saved over a one-line
> enum definition? In fact, tuples are a great example of this: best
> practices usually state that they are good for ad-hoc destructuring,
> such as retrieving multiple return values from a function or pattern
> matching across several values at once, but structs are better used for
> almost everything else, since they carry semantic meaning that tuples
> don't.

Just the same pros and cons for ad-hoc enums vs enum declaration as for 
tuples vs struct declaration. Yes can use it with care and you can use it 
in wrong way.

Btw, I feel like this could be very handy to return adhoc enum:

func something() -> (.one|.two|.three) {...}

> I hope that clarifies my thoughts on the matter.
> Best, Austin
>> On Jun 1, 2016, at 12:36 AM, Vladimir.S <svabox at gmail.com> wrote:
>> On 01.06.2016 9:55, Austin Zheng via swift-evolution wrote:
>>> Maybe it's overkill. My personal opinion is that breaking the
>>> symmetry of the language like this (are there any other types of
>>> function arguments that cannot be passed as either variable values
>>> or literals?) is too much a price to pay. Your library thinks it's
>>> being clever and vends its functions as taking anonymous enum flags,
>>> and now there are a bunch of things I can't do with those functions
>>> anymore.
>>> A regular enum can be declared in one line anyways:
>>> enum ScaleCropMode { case Fit, Fill }
>> Why do we have tuples? Struct could be defined by one line `struct
>> SomeValue { var x = 0, y = 0 }` ;-) I.e. from my point of view
>> developer should decide what he/she wants to use: ad-hoc enum or
>> defined enum type *exactly* as now he/she can decide to use the same
>> tuples in multiply functions instead of one defined struct type.
>> I replied regarding the variable on other message. (In short: I think
>> of the same principle as for tuples: you can declare variable `let e:
>> (.fill | .fit) = .fill` and use it)
>>> Austin
>>>> On May 31, 2016, at 11:44 PM, Charles Constant
>>>> <charles at charlesism.com <mailto:charles at charlesism.com>> wrote:
>>>>> It breaks the ability to pass in a variable containing the
>>>>> desired
>>>> value, rather than the literal value itself.
>>>> Maybe that's appropriate? If the caller is not passing in a
>>>> hardcoded enum case, then that enum is probably general enough
>>>> that it warrants a normal enum. But there are also situations
>>>> where the same function is called from several files in the same
>>>> code-base with different flags. Those are situations where it
>>>> feels like overkill to clutter up my codebase with separate enums,
>>>> only used by a single function.
>>>> On Tue, May 31, 2016 at 9:24 PM, Austin Zheng via swift-evolution
>>>> <swift-evolution at swift.org <mailto:swift-evolution at swift.org>>
>>>> wrote:
>>>> I admire the desire of this proposal to increase the readability
>>>> of code. I'm -1 to the proposal itself, though:
>>>> - It breaks the ability to pass in a variable containing the
>>>> desired value, rather than the literal value itself. (Unless you
>>>> actually want a not-so-anonymous enum type whose definition
>>>> happens to live in a function signature rather than somewhere
>>>> you'd usually expect a type definition to live.) - It breaks the
>>>> ability to store a reference to the function in a variable of
>>>> function type (ditto). - Almost every time I've wanted to use one
>>>> of these "anonymous enums" in my code, I've ended up needing to
>>>> use that same enum elsewhere. In my experience, 'lightweight
>>>> enums' don't end up saving much time compared to a full-fledged
>>>> one.
>>>> Like Brent said, I have to say no to any proposal that tries to
>>>> make enums synonyms for numerical values. What happens if you
>>>> rearrange your anonymous enum cases between library versions? Do
>>>> you somehow store an opaque case-to-UInt8 table somewhere for
>>>> every anonymous enum you define for resilience? What happens when
>>>> people start bringing back terrible C patterns, like doing
>>>> arithmetic or bitwise ops on the underlying case values? At least
>>>> you have to try pretty hard as it is to abuse Swift's enums.
>>>> Austin
>>>> On Tue, May 31, 2016 at 8:25 PM, Brent Royal-Gordon via
>>>> swift-evolution <swift-evolution at swift.org
>>>> <mailto:swift-evolution at swift.org>> wrote:
>>>>> And the obvious answer is you can have up to 255 of these babies
>>>>> for the anonymous enum type, and be able to pass numerical
>>>>> equivalents UInt8 with compile time substitution. That the
>>>>> ad-hoc enumeration is basically a syntactic shorthand for UInt8,
>>>>> with an enforced upper bound compile time check simplifies
>>>>> everything including switch statements.
>>>> If I wanted a language like that, I'd be writing C, not Swift.
>>>> -- Brent Royal-Gordon Architechies
>>>> _______________________________________________ swift-evolution
>>>> mailing list swift-evolution at swift.org
>>>> <mailto:swift-evolution at swift.org>
>>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>>> _______________________________________________ swift-evolution
>>>> mailing list swift-evolution at swift.org
>>>> <mailto:swift-evolution at swift.org>
>>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>> _______________________________________________ swift-evolution
>>> mailing list swift-evolution at swift.org
>>> https://lists.swift.org/mailman/listinfo/swift-evolution

More information about the swift-evolution mailing list