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

[James Campbell]

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

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