[swift-evolution] [Proposal] Random Unification

[Nate Cook]

The edge case is really the same (empty ranges), it’s about what we do with the edge case. If we include the methods on integer types, usage will look like this:

    let x = Int.random(in: 0..<5)     // 3
    let y = Int.random(in: 0..<0)     // runtime error

If we only have the collection methods, usage will look like this:

    let x = (0..<5).random()!         // 3
    let y = (0..<0).random()!         // runtime error

But my suspicion is that lots of people will write things like this:

    guard let x = (0..<5).random() 
        else { fatalError("not gonna happen") }

I’d rather have the numeric methods trap than add the optional unwrapping step to every one of these calls. For me, getting a random number and picking a random element of a collection are two different operations—where it’s common to work with empty collections, especially in generic code, trying to get a random value from an empty range is really a programming error. I think it’s okay for them to have slightly different semantics.

Nate


> On Oct 5, 2017, at 12:27 PM, Alejandro Alonso <aalonso128 at outlook.com> wrote:
> 
> Rather 0 ..< 0 my bad. I think if we include closedcountable, then there needs to be support for countable, but there are edge cases where users can input invalid ranges for countable. 
> 
> Enviado desde mi iPhone
> 
> El oct. 5, 2017, a la(s) 12:22, Alejandro Alonso via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> escribió:
> 
>> I agree with Ben here because users can still enter an invalid range with the static function. I.E. Int.random(in: 0 ... 0). 
>> I would really prefer excluding these static functions from numeric types.
>> 
>> - Alejandro
>> 
>> El oct. 5, 2017, a la(s) 12:03, Nate Cook via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> escribió:
>> 
>>>> On Oct 5, 2017, at 11:30 AM, Ben Cohen via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>>> 
>>>>> On Oct 4, 2017, at 9:12 PM, Chris Lattner via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>>>> 
>>>>>> ```
>>>>>> extension Int {
>>>>>>   static func random(in range: Countable{Closed}Range<Int>) -> Int
>>>>>> }
>>>>> 
>>>>> Nice.  Should these be initializers like:
>>>>> 
>>>>> extension Int {
>>>>>   init(randomIn: Countable{Closed}Range<Int>)
>>>>> }
>>>>> 
>>>> 
>>>> I don’t see much of a case for making it it random(in: SpecificCollection) instead of genericCollection.random().
>>> 
>>> I see a couple points in favor of these static methods (or initializers) on the numeric types:
>>> 
>>> 1) The collection method will need to return an optional to match the semantics of existing methods (like min()). If this is the only method available, every time someone needs a random value in the range 1...10, they’ll need to unwrap the result (with either force unwrapping, which people will complain about, or some kind of conditional binding, which is its own problem). Even if the semantics are the same (trapping on an empty range), the user experience of using a non-optional method will be better.
>>> 
>>> 2) Floating-point ranges won’t get the collection method, so either we’ll have inconsistent APIs (random FP value is non-optional, random integer is optional) or we’ll make the FP API optional just to match. Both of those seem bad.
>>> 
>>>> One possible reason is if you exclude half-open ranges, only having CountableClosedRange, then you don’t have to account for the possibility of an empty collection (via an optional or a trap) because they cannot be empty. But closed ranges aren’t the currency type – half-open ranges are. So it’d hit usability if you have to convert from one to t'other often.
>>>> 
>>>> Other possibility is discovery. But given the common use case is “random element from collection”, I don’t expect this to be an issue as it will quickly become common knowledge that this feature is available.
>>> 
>>> Agreed here—I don’t think discovery is really an issue between the two kinds. However, I don’t think the overlap in features (two ways to generate random integers) are a problem, especially as we’d have better alignment between integer and floating-point methods.
>>> 
>>> Nate
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