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

[Douglas Gregor]

> On Feb 3, 2016, at 5:10 AM, Jean-Daniel Dupas via swift-evolution <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>
>> ___________________________________
>> 
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>> 
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