[swift-evolution] [swift-users] [Proposal] Linux specific Package.swift file

[Chris Lattner]

> On Nov 22, 2016, at 3:28 AM, rintaro ishizaki via swift-users <swift-users at swift.org> wrote:
> 
> Hi Said,
> 
> Since `Package.targets` is a mutable public property,
> https://github.com/apple/swift-package-manager/blob/master/Sources/PackageDescription/Package.swift#L60-L67 <https://github.com/apple/swift-package-manager/blob/master/Sources/PackageDescription/Package.swift#L60-L67>
> you can freely mutate it later, as documented in
> https://github.com/apple/swift-package-manager/blob/master/Documentation/Reference.md#customizing-builds <https://github.com/apple/swift-package-manager/blob/master/Documentation/Reference.md#customizing-builds>
Right.  If they were immutable, it seems that another (more general) solution to this would be to relax the restrictions around #if checks, enabling them to apply to elements in lists.  That would allow you to write:

let macOnlyTargets = [
#if !os(Linux)
    .Target(name: "SomeMacOSTarget")
#end
]

-Chris

> 
> 
> 2016-11-22 4:21 GMT+09:00 Said Sikira via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>>:
> Hello everyone,
> 
> Currently, we use Package.swift manifest file when defining our packages. Within the manifest file you define your targets, dependencies, excluded folders etc. This works fairly well while you’re using frameworks that exist on all supported platforms (ex. Foundation).
> 
> The main problem starts if you want to use code that exists only on macOS platform, for example, if you want to use Objective C runtime calls or UIKit. Then, you need to use bunch of #if os(Linux) to define what’s available on Linux and what’s available on macOS. While this approach can be non problematic for simple projects, it can introduce unnecessary complexity if you have a large target and dependency graph.
> 
> One way people tackle this problem is writing something like this:
> 
> #if os(Linux)
> let macOnlyTargets = []
> #else  
> let macOnlyTargets = [
>     .Target(name: "SomeMacOSTarget")
> ]
> #end
> This structure looks even worse if you need to define more complex behaviors. For example, look at the RxSwift Package.swift <https://github.com/ReactiveX/RxSwift/blob/master/Package.swift> file. In my case, I try to write #if os(Linux) as little as possible which leads me to writing my packages like:
> 
> #if os(Linux)
>     // Define full Linux package
>     let package = Package(...)
> #else
>     // Define full macOS package
>     let package = Package(..)
> #end
> Proposal
> 
> I propose that we support using different file for writing Linux package manifests. This file could be named:
> 
> PackageLinux.swift
> Package.Linux.swift
> Package.linux.swift
> Inside this file you would be able to define your package only for Linux in the same way you’re defining one in regular Package.swift. The defined behaviors of building a package would be:
> 
> If building on Linux and PackageLinux.swift is present, use that file when building the package.
> If building on Linux and PackageLinux.swift is not present, use regular Package.swift manifest.
> If building on macOS always use Package.swift manifest.
> Possible problems
> 
> This behavior would only introduce problems when SPM gains support for new platforms, but then again, you would need to use new #if os(somethingNew) checks.
> 
> Compatibility with current packages
> 
> This would be only a additive feature for Swift Package Manager and backwards compatibility will be maintained.
> 
> 
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