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

[David Hart]

I was really surprised when I saw that the release of 4.0 introduced this 3.2 version to mean “the 4.0 compiler running in 3.1 compatibility mode”. So of course, I’m even more surprised to see a new 3.3 version. I find it very counter-intuitive. It also means we will continue to have to increment all previous Swift language versions from now on whenever a new compiler is released:

Swift 3.4 = Swift 5 compiler in Swift 3 compatibility mode
Swift 4.2 = Swift 5 compiler in Swift 4 compatibility mode
Swift 3.5 = Swift 5.1 compiler in Swift 3 compatibility mode
Swift 4.3 = Swift 5.1 compiler in Swift 4 compatibility mode

I have the impression that what we really need is a different directive to test for the compiler version:

#if compiler(>=4.1)
// Swift 3.3
// Swift 4.1
#endif

> On 6 Jan 2018, at 01:19, Jordan Rose via swift-build-dev <swift-build-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?".
> 
> #if swift(>=3.2)
> // Swift 3.2 (4.0 in compatibility mode)
> // Swift 3.3 (4.1 in compatibility mode)
> // Swift 4.0
> // Swift 4.1
> #endif
> 
> #if swift(>=3.3)
> // Swift 3.3 (4.1 compatibily mode)
> // Swift 4.0
> // Swift 4.1
> // this one is probably not very useful
> #endif
> 
> #if swift(>=4.0)
> // Swift 4.0
> // Swift 4.1
> #endif
> 
> #if ???
> // Swift 3.3
> // Swift 4.1
> #endif
> 
> I don't think this is going to come up a lot, but given that we do have changes to the standard library and to the language, I can see people wanting it. Right now the only way to do it is the rather unwieldy:
> 
> #if swift(>=4.1) || (swift(>=3.3) && !swift(>=4.0))
> print("new")
> #else
> print("old")
> #endif
> 
> Do we need something better here, or do you think people will be okay with this? I'm realizing I don't really know how many people try to keep their libraries working across Swift versions and run into compatibility issues. 
> 
> (Strictly speaking this problem is already present with Swift 4.0.2 with 3.2.2 compatibility mode, but that's much less likely to come up.)
> 
> Jordan
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> swift-build-dev at swift.org
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