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

[David Rönnqvist]

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/
[2]: http://goto.ucsd.edu/~nvazou/refinement_types_for_haskell.pdf


> On 11 May 2016, at 20:00, Adrian Zubarev via swift-evolution <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
>>> _______________________________________________
>>> swift-evolution mailing list
>>> swift-evolution at swift.org <mailto:swift-evolution at swift.org>
>>> https://lists.swift.org/mailman/listinfo/swift-evolution <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 <https://lists.swift.org/mailman/listinfo/swift-evolution>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.swift.org/pipermail/swift-evolution/attachments/20160518/255b6286/attachment.html>