Use @main instead of top level code when initializing executable

[plemarquand]

Motivation:

Since @main is the general purpose way of marking the entry point to executables, use it over top level code when generating an executable template.

Modifications:

Update the template output when the specified --type is executable.

Result:

The template is now:

@main
struct MyProject {
  static func main() {
    print("Hello, world!")
  }
}

This patch reverts #6197 and partially reverts #6144

[MaxDesiatov]

This was changed to use top-level code instead of @main not so long ago in #6144, what's the reason for reverting?

[dschaefer2]

Yeah, this is a pretty major change and not very new user friendly. Maybe this is a separate template?

[dnadoba]

Top level code has various bugs that make it very easy to write code that miscompiles e.g.:

print(x)
var x: Set = ["Hello World"]

which crashes at runtime because x isn't initialized before we attempt to print it.

This isn't a great user experience. I think until this is sorted we should prefer an @main entry point.

[bitjammer]

I generally support this change. There should ideally be only one way to define a main entry point, and it should be an explicit form. If top-level code is truly problematic, then we should remove it from the language and be done with it. It would be nice if we could put @main on a bare function, since the function within the structure is static.

I would ask that you consider naming the file/structure something that reflects the target, package, or a noun that reflects what it is, rather than just Main.

[dschaefer2]

Another aspect of this template, if you are going to @main the main, then you should probably also create the module in a subdirectory of Sources.

I believe that's why the current template is like it is, this is a different use case. The original template is actually an executable package. There is no easy mechanism to add a second module, as you well know.

Which is a back to my point that this, where you're adding a module that's an executable is a different template.

[shicholas]

+1 for this change! Having the @main explicit here makes it simpler because it's the same thing everywhere. It's been confusing for me to know that main.Swift runs everything top to bottom instead of the @main tag.

[dnadoba]

Thanks! LGTM otherwise

[plemarquand]

At @dschaefer2's suggestion I've updated the PR to place the generated Source files for executables/tools into /Sources/PkgName instead of directly in Sources. This will remove the friction of having to move your sources in to a new folder when users want to add another target.

[FranzBusch]

Love this change! I think we should give the community a heads up in the forums though.

[plemarquand]

@FranzBusch Good call, I posted https://forums.swift.org/t/update-to-swift-package-init-executable-template/78813.

[heckj]

minor typo

[plemarquand]

@heckj Thanks!

[plemarquand]

@swift-ci test

[gavineadie]

I've updated the PR to place the generated Source files for executables/tools into /Sources/PkgName instead of directly in Sources.

Do you mean PkgName literally, or the name of the Package? If the former, I'd prefer it unabbreviated -- PackageName.

[plemarquand]

@gavineadie Sorry, I was unclear, PkgName refers to the name of the package being created. So swift package init --type executable --package-path ./Foo would create the source file in ./Sources/Foo/Foo.swift.