[swift-dev] "Swift 4.1 or Swift 3.3"

[Chris Lattner]

On Jan 8, 2018, at 5:26 PM, Jordan Rose via swift-dev <swift-dev at swift.org> wrote:
>> On Jan 8, 2018, at 17:18, Chris Lattner <clattner at nondot.org <mailto:clattner at nondot.org>> wrote:
>>> On Jan 5, 2018, at 4:19 PM, Jordan Rose via swift-dev <swift-dev at swift.org <mailto:swift-dev at swift.org>> wrote:
>>> 
>>> Hi, all. Swift 4.1 is off on its own branch and going well, but we never quite came up with an answer for a particular problem developers might have: "am I running a Swift 4.1 compiler?”.
>> 
>> I agree, this is getting bad.  Ted mentioned that something like __has_feature in clang is probably the best way to go, so people could check the specific thing they care about, instead of a set of global version numbers.
>> 
>> Another thing that could help is something along the lines of:
>> 
>> 	#if swift_stdlib(>=5.0)
>> 
>> which would presumably be active in any language mode when the 5.0 standard library is available.  It would be even better to use a generalized availability system for this: the standard library version could be treated just like Foundation versions are handled, for example.
> 
> Yep, though there's a difference between compile-time availability (which is what `#if swift` and `@available(swift, …)` check) and run-time availability (which is what `@available(macOS, …)` and `#available` check). Thus far, availability of features in the standard library has been a compile-time check, but as soon as the standard library starts shipping with Apple's OS, it becomes a run-time check. The conclusion I draw is that there's a good chance we'll end up with different solutions for checking compiler versions vs. checking run-time versions.
> 
> (And then there's another thing people have asked for, which is statically checking SDK versions. 

I think your second paragraph answers the issue you observe in the first paragraph.  You’re right that we have #available and @available, but neither of them are the thing we need here.  We need a third thing that allows people to conditionally build (#ifdef-style) based on build time information.

I don’t see the OS or the standard library as being special here.  The same thing is true for source packages - a dependent package should be able to write conditional code to allow it to work with different versions of some other package it depends on.  I outlined this sort of stuff in this ancient doc:
https://github.com/apple/swift/blob/master/docs/proposals/archive/ProgramStructureAndCompilationModel.rst#sdks <https://github.com/apple/swift/blob/master/docs/proposals/archive/ProgramStructureAndCompilationModel.rst>


> Thus far they've been able to emulate that with `#if swift` because we've shipped a new Swift with every new Apple SDK, but it's still potentially an interesting feature.)


Sure, this is just a way of saying that people are already doing this - so we might as well make it nice and consistent.  Also, given that Swift is cross platform, it would be nice to have a solution that works in other environments too.

-Chris

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