[swift-evolution] Ad hoc enums / options
Matthew Johnson
matthew at anandabits.com
Wed Jun 1 19:42:30 CDT 2016
Sent from my iPad
> On Jun 1, 2016, at 5:02 PM, Vladimir.S via swift-evolution <swift-evolution at swift.org> wrote:
>
> > in other words, we could consider allowing this:
> > func foo(bar: (.fit | .fill)) {
> > baz(bar: bar)
> > }
> > func baz(bar: (.fit | .fill | .florp) { ... }
> >
> > In other words, an ad hoc enum T can be used wherever an ad hoc enum U is
> > expected if T ⊆ U.
>
> Can't agree with this. Just because the same analogue with tuples : differently defined tuples are different types. Tuples with different order of types in declaration - are different types. So I expect here instance of (.fit | .fill) `bar` is not of the same type as (.fit | .fill | .florp)
They are not the same type but there is a structural subtype relationship between them. All values of type (.fit | .fill) are also values of type (.fit | .fill | .florp).
>
> But +1 to be able to 'convert' instance of (.fit | .fill) to instance of (.fit | .fill | .florp). For example(if we'll have init(caseName) and .caseName for enums):
>
> func foo(bar: (.fit | .fill)) {
> let bazbar = (.fit | .fill | .florp).init(caseName: bar.caseName)
> baz(bar: bazbar)
> }
> func baz(bar: (.fit | .fill | .florp) { ... }
>
>
>
>> On 02.06.2016 0:38, Tony Allevato wrote:
>> I find myself agreeing with the idea that ad hoc enums are to enums as
>> structs are to tuples. Based on that analogy, why should an ad hoc enum
>> *need* a name (autogenerated or otherwise) any more than a tuple needs a
>> name? Would those who dislike ad hoc enums argue that this also shouldn't
>> be allowed:
>>
>> func foo(bar: (x: Int, y: Int)) {}
>> let t: (x: Int, y: Int) = (x: 5, y: 5)
>>
>> If someone writes `(.fit | .fill)` (or whatever the hypothetical syntax
>> might be), that should just *be* the type the same way that `(x: Int, y:
>> Int)` is a type without a name, and that type can be used in argument
>> lists, variables, or whatever. There shouldn't be any worry about
>> declarations across multiple functions colliding or being incompatible any
>> more than we would worry about two functions declaring arguments of type
>> `(x: Int, y: Int)` would collide or be incompatible.
>>
>> One side of ad hoc enums that I'd like to see explored is that, by being
>> unnamed, they're basically anonymous finite sets and we could apply
>> well-defined subset relationships to them: in other words, we could
>> consider allowing this:
>>
>> func foo(bar: (.fit | .fill)) {
>> baz(bar: bar)
>> }
>> func baz(bar: (.fit | .fill | .florp) { ... }
>>
>> In other words, an ad hoc enum T can be used wherever an ad hoc enum U is
>> expected if T ⊆ U.
>>
>>
>> On Wed, Jun 1, 2016 at 1:43 PM L. Mihalkovic via swift-evolution
>> <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>
>>
>>
>> > On Jun 1, 2016, at 6:51 PM, Vladimir.S <svabox at gmail.com
>> <mailto:svabox at gmail.com>> wrote:
>> >
>> > Yes, I also can support the idea of autogenerated type name (like
>> Enum_fit_OR_fill) as long as it allows to do all the things we are
>> discussing here: declare (.fit|.fill) in function, use .fit on calling
>> side, use (.fit|.fill) to declare temporary variable of type compatible
>> with such function parameter etc.
>> >
>>
>> It all works because the compiler is just being a thoughless scribe
>> that just writes the standard enum we don't bother to write ourselves.
>> Because the heuristic is simple and straightforward then it is
>> predictible. The enum can be used with its long name be ause it is a
>> real enum. And writing the short form of it also works because the
>> compiler knowns uniquely what the long name is everytime it runs into
>> the short name.
>>
>>
>> > But how do you suggest to define a type of such function in
>> `typealias` for example? i.e. for func my(option: (.fit|.fill) {..}
>> >
>> > typealias MyFunc = ((.fit|.fill)) -> ()
>> > or as
>> >
>> > typealias MyFunc = (Enum_fit_OR_fill) -> ()
>> >
>>
>> Ideally there is no difference whatsoever, there is a single enum, it
>> is produced at the module level, and it has the long form name.
>>
>> There can be rules that would prevent us from doing that with
>> externally visible APIs, if the core team fuges that we should take the
>> time to write our enums manually and cleanly to make them visible to
>> the world, but it is not a necessary rule.
>>
>>
>> >
>> > But I still can't support the idea of limiting the usage of such
>> enums - i.e. "To deal with milti site definition, the compiler would
>> simply flag a error/warning, or be silent in the presence of a new
>> annotation:". I really think we need then introduce the same rule for
>> tuples - so no one can use the same tuple declaration in function -
>> they then should declare separate struct type or use @something for
>> such functions. Nobody wants such rule for tuples.
>> >
>>
>> Multi site thing is not a limitation... Is is a proposed rule to say
>> that we are able to be lazy twice without being penalized. Yhe compiler
>> does not like when we define the same thing twice, and thse short form
>> amount to doing what he does not let us do. But because this is about
>> concise and lazy, then the compiler can let us get away with it if we
>> use an annotation that lets it know that "it is not a mistake.. I
>> really dont want to write that enum myself, even though I am using the
>> same abbreviation twice". Otherwise, the compiler would let us know
>> that the second time could be a mistake because there is already
>> something with the same name...
>>
>> But again this is a separate idea from the core notion of a syntax
>> sugaring for writing real enums the lazy (clever) way
>>
>> >> On 01.06.2016 19:04, L. Mihalkovic wrote:
>> >> The only problem with this proposal is to consider them ad-hoc
>> enums... If we view them as having nothing ad-hoc about them and the
>> thing to be a simple sugaring exercise, then I think all the opositions
>> on grounds of breaking the language disapear. It still does not mean it
>> should be done if the core team does not like the idea of encouraging
>> laziness, or simply do not like what it makes them look like. No matter
>> what, this type of sugaring exercise has been clearly stated as out of
>> scope for 3.0
>> >>
>> >>>> On Jun 1, 2016, at 2:38 PM, Vladimir.S via swift-evolution
>> <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> >>>>
>> >>>> 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
>> >>>
>> >>> but
>> >>>
>> >>> 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
>> <mailto: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>
>> <mailto: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>
>> <mailto: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>
>> >>>>>>> <mailto: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>
>> >>>>>>> <mailto:swift-evolution at swift.org
>> <mailto:swift-evolution at swift.org>>
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>> >>>>>>>
>> >>>>>>>
>> >>>>>>>
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>> >>>>>>>
>> >>>>>>
>> >>>>>>
>> >>>>>>
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