[swift-evolution] #available has a huge anti-pattern.

[Douglas Gregor]

> On Feb 3, 2016, at 1:02 PM, Félix Cloutier <felixcca at yahoo.ca> wrote:
> 
> Won't it be a concern with a cross-platform Swift?

#available is currently only implemented for Apple platforms. If someone wants to extend it to another platform, they need to do so in a manner that gives it consistent semantics.

	- Doug

> 
> Félix
> 
>> Le 3 févr. 2016 à 15:47:15, Douglas Gregor via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> a écrit :
>> 
>>> 
>>> On Feb 3, 2016, at 5:10 AM, Jean-Daniel Dupas via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>> 
>>> Using function availability has proven fragile in the past too. A function may be present but private on older system, and have a slightly different behavior or crash, and so should not be used.
>> 
>> This is a failing of the -respondsToSelector: idiom for checking availability. Swift’s #available feature checks the actual OS version, so it doesn’t suffer from this problem.
>> 
>> 	- Doug
>> 
>>> 
>>> 
>>>> Le 2 févr. 2016 à 11:03, James Campbell via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> a écrit :
>>>> 
>>>> Coming from a web background (before my iOS career) to me #avaliable has huge problem. It encourages fragility.
>>>> 
>>>> In my eyes we should encourage two types of detection: Features to make code more adaptable to different environments and language version detection: so we can understand the actual code.
>>>> 
>>>> See this example below:
>>>> 
>>>> func magic(object: Object)
>>>> {
>>>>   if(#avaliable(9.0, 10))
>>>>  {
>>>>   object.foo()
>>>>  }
>>>> }
>>>> 
>>>> Ideally for me I would love to check if the foo function exists like so:
>>>> 
>>>> func iOS9OnlyProtocolFunction(object: Object)
>>>> {
>>>>   if(#avaliable(Object.foo))
>>>>  {
>>>>     object.foo()
>>>>  }
>>>> else 
>>>> {
>>>>   object.baz()
>>>>  }
>>>> }
>>>> 
>>>> I think this encourages feature detection which results in less fragile code. What I would love to do is also to extend this to extensions so we could encourage polyfills.
>>>> 
>>>> extend object where not_avaliable(Object.foo) 
>>>> {
>>>>   func foo() 
>>>>  {
>>>>    //Polyfill for platforms which don't support the Object.foo method
>>>>  }
>>>> }
>>>> 
>>>> Not sure about compiler details but being able to polyfill the function results in much cleaner code for me. I love this approach from the web, so I created my own Objective-C Library to do this:
>>>> 
>>>> https://github.com/jcampbell05/Polly <https://github.com/jcampbell05/Polly>
>>>> ___________________________________
>>>> 
>>>> James⎥Lead Engineer
>>>> 
>>>> james at supmenow.com <mailto:james at supmenow.com>⎥supmenow.com <http://supmenow.com/>
>>>> Sup
>>>> 
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>>>> 
>>>> 
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>>>> 
>>>> London
>>>> 
>>>> 
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>>>> 
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>>> 
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