On Jan 8, 2018, at 5:26 PM, Jordan Rose via swift-dev <swift-dev@swift.org> wrote: >> On Jan 8, 2018, at 17:18, Chris Lattner <clatt...@nondot.org >> <mailto:clatt...@nondot.org>> wrote: >>> On Jan 5, 2018, at 4:19 PM, Jordan Rose via swift-dev <swift-dev@swift.org >>> <mailto:swift-dev@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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