[swift-evolution] [Pitch] Constrains for numeric types: Int<1...10> Double<0.0...1.0>

Wed May 18 11:34:29 CDT 2016

How about something like this instead? Seems a bit more general and perhaps more useful (though with numbers it's not as elegant)?

var alpha: Double where alpha >= 0.0 && alpha <= 0.0
var user: MNUser where user.isLoggedIn
var name: String where !name.isEmpty

Krystof

> On May 18, 2016, at 2:49 PM, Adrian Zubarev via swift-evolution <swift-evolution at swift.org> wrote:
> 
> As you guys brought my idea back to life and I’ve done some effort of digging into the Swifts generic future I can show you some fresh ideas.
> 
> `Refinement Types` could be really handy, but put them aside for a moment.
> 
> Actually we could achieve something like Int<minValue, maxValue> in Swift 3. Take a look at this section here: https://github.com/apple/swift/blob/master/docs/GenericsManifesto.md#generic-value-parameters <https://github.com/apple/swift/blob/master/docs/GenericsManifesto.md#generic-value-parameters>
> 
> There is a slightly problem with this, we cannot rewrite all types to be generic with default parameters. That said it would be nice if we could overload types somehow (Int vs. Int<Range>), but I’m not sure if this idea would suit the language like this.
> 
> Anyways we could then have:
> 
> struct Int<_ range: Range<Int>> { … }
> 
> Sadly this makes the type not safe at compile time but only signals the user that at runtime you can’t use any number our of its range.
> 
> -- 
> Adrian Zubarev
> Sent with Airmail
> 
> Am 18. Mai 2016 bei 14:13:09, Vladimir.S via swift-evolution (swift-evolution at swift.org <mailto:swift-evolution at swift.org>) schrieb:
> 
>> > I generally think it’s a cool idea and that it can be useful in minimizing
>> > partial functions by requiring that these cases are explicitly handled.
>> 
>> Support this opinion.
>> 
>> Other example where such feature could be useful: some public property in 
>> type that can accept values from some interval. For example we have class 
>> with transparency property that can be 0.0 ... 0.1:
>> 
>> class MyShape {
>> public var transparency: Double = 1.0 // 0.0 ... 0.1
>> }
>> 
>> yes, we can use willSet/didSet to check this every time. But if we have a 
>> number of such properties, we have a lot of repetitive and boilerplate code:
>> 
>> class C {
>> var transparancy : Double = 1.0 {
>> didSet { check(transparancy, 0.0...1.0) }
>> }
>> 
>> var prop1 : Int = 1 {
>> didSet { check(prop1, from: -10...10) }
>> }
>> 
>> var prop2 : Int = 1 {
>> didSet { check(prop2, from: 0...100) }
>> }
>> }
>> 
>> , and all will be worse if we also need didSet observers to do some 
>> 'useful' work here.
>> Proposed solution will looks like:
>> 
>> class C {
>> var transparancy : Double<0.0...1.0> = 1.0
>> 
>> var prop1 : Int<-10...10> = 1
>> 
>> var prop2 : Int<0...100> = 1
>> }
>> 
>> Probably the alternative could be some kind of `where` or `bounded` clause 
>> for numeric types and arguments:
>> 
>> class C {
>> var transparancy : Double = 1.0 where 0.0...1.0
>> //var transparancy : Double bounded 0.0...1.0 = 1.0
>> 
>> var prop1 : Int = 1 where -10...10
>> 
>> var prop2 : Int = 1 where 0.0...1.0
>> 
>> //var prop3 : Float bounded 0.0..<100.0 = 0.0
>> }
>> 
>> 
>> On 18.05.2016 11:30, David Rönnqvist via swift-evolution wrote:
>> > It reminds me of "Refinement Types" (see for example [this blog post][1] or
>> > [this paper][2]).
>> >
>> > I generally think it’s a cool idea and that it can be useful in minimizing
>> > partial functions by requiring that these cases are explicitly handled.
>> > For example, highlighting that the following `average` implementation
>> > divides by zero when the list is empty:
>> >
>> > func average(numbers: [Int]) -> Int {
>> > return sum(numbers) / numbers.count
>> > }
>> >
>> >
>> > and requiring that the empty list case is handled separately:
>> >
>> > func average(numbers: [Int]) -> Int {
>> > guard !numbers.isEmpty else { return 0 }
>> > return sum(numbers) / numbers.count
>> > }
>> >
>> >
>> > Regards,
>> > David
>> >
>> > [1]: http://goto.ucsd.edu/~rjhala/liquid/haskell/blog/blog/2013/01/01/refinement-types-101.lhs/
>> > <http://goto.ucsd.edu/%7Erjhala/liquid/haskell/blog/blog/2013/01/01/refinement-types-101.lhs/>
>> > [2]: http://goto.ucsd.edu/~nvazou/refinement_types_for_haskell.pdf
>> > <http://goto.ucsd.edu/%7Envazou/refinement_types_for_haskell.pdf>
>> >
>> >
>> >> On 11 May 2016, at 20:00, Adrian Zubarev via swift-evolution
>> >> <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> >>
>> >> Okay I’m fine with that for now. If you’d have to decide on some syntax
>> >> for such a future, how would it look like? I’m just curious.
>> >>
>> >> I tend to square brackets Double[0.0 … 1.0], because otherwise it might
>> >> look like a generic type, but I’m not sure if this type refinement could
>> >> be applied to other types as well so we actually would stick to the
>> >> generic type syntax here Float<-1.0 … 1.0>.
>> >>
>> >> --
>> >> Adrian Zubarev
>> >> Sent with Airmail
>> >>
>> >> Am 11. Mai 2016 bei 19:54:09, Matthew Johnson (matthew at anandabits.com
>> >> <mailto:matthew at anandabits.com>) schrieb:
>> >>
>> >>> This is called a refinement type. It would be cool to explore that
>> >>> direction in the future but it is definitely well out of scope for Swift 3.
>> >>>
>> >>> Sent from my iPad
>> >>>
>> >>> On May 11, 2016, at 12:45 PM, Adrian Zubarev via swift-evolution
>> >>> <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> >>>
>> >>>> Hello Swift community. I'd like to discuss with you if we need
>> >>>> something like this in Swift 3 or any future Swift version.
>> >>>>
>> >>>> As you may know there is no way to constrain a numeric type expect for
>> >>>> some scope internal assertion or precodintions which may produce a
>> >>>> runtime error if the input value is out of the defined bound.
>> >>>>
>> >>>> func foo(value: Int) {
>> >>>> assert(value > 0 && value <= 10)
>> >>>>
>> >>>> // passed
>> >>>> }
>> >>>>
>> >>>> How would it be if Swift would allow us to constraint numeric typs with
>> >>>> ranges/intervals?
>> >>>>
>> >>>> func newFoo(value: Int<1...10>) {
>> >>>> // no need for an assertion any more
>> >>>> }
>> >>>>
>> >>>> We could go even further and add more then one range/interval:
>> >>>>
>> >>>> func someFoo(value: Int<0...20, 40...60>) { /* do some work */ }
>> >>>>
>> >>>> Not only integers should have this ability but also floating point
>> >>>> types like Double and Float.
>> >>>>
>> >>>> Alternative form might look like this:
>> >>>>
>> >>>> Double[1.0...10.0]
>> >>>> Float[0.0...1.0, 10.0...100.0]
>> >>>>
>> >>>> One downside of half opened ranges/intervals is the left side of its
>> >>>> set. How do we exclude the left element?
>> >>>>
>> >>>> 1...10 means 1..<11 equals [1, 11)
>> >>>>
>> >>>> But how can we create something like (0.0, 1.0), do we need a strange
>> >>>> looking binary operator 0.0>..<1.0?
>> >>>>
>> >>>> What do you think? I'd love to hear any feedback to this.
>> >>>>
>> >>>> --
>> >>>> Adrian Zubarev
>> >>>> Sent with Airmail
>> >>>> _______________________________________________
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>> >>>> swift-evolution at swift.org <mailto:swift-evolution at swift.org>
>> >>>> https://lists.swift.org/mailman/listinfo/swift-evolution
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>> >
>> >
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