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$ swift package init --type executable --name Greeter
Creating executable package: Greeter
Creating Package.swift
Creating .gitignore
Creating Sources
Creating Sources/Greeter/Greeter.swift
Creating Tests/
Creating Tests/GreeterTests/
Creating Tests/GreeterTests/GreeterTests.swift

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// swift-tools-version: 6.3

import PackageDescription

let package = Package(
    name: "Greeter",
    targets: [
        .executableTarget(name: "Greeter"),
        .testTarget(name: "GreeterTests", dependencies: ["Greeter"]),
    ]
)

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$ swift build
Build complete! (13.63秒)

$ swift run
Build complete! (0.24秒)
Hello, world!

$ swift test
Build complete! (1.83秒)
◇ Test run started.
↳ Testing Library Version: 2084
↳ Target Platform: arm64e-apple-macos14.0
◇ Test example() started.
✔ Test example() passed after 0.001 seconds.
✔ Test run with 1 test in 0 suites passed after 0.001 seconds.

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// Sources/Greeter/Greeter.swift
@main
struct Greeter {
    static func main() {
        print("Hello, world!")
    }
}

// 运行 swift run 的结果：
// prints: Hello, world!

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@main
struct App {
    static func main() {
        print("程序从这里开始")
        // prints: 程序从这里开始
    }
}

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@main
struct AsyncApp {
    static func main() async throws {
        print("异步入口")
        // prints: 异步入口
    }
}

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// Sources/Greeter/main.swift
print("第一行")
print("第二行")
// prints: 第一行
// prints: 第二行

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// 假设运行：swift run Greeter Mia 42
let arguments = CommandLine.arguments
print(arguments)
// prints: [".../Greeter", "Mia", "42"]
print(arguments.dropFirst().joined(separator: ", "))
// prints: Mia, 42

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error: 'main' attribute cannot be used in a module that contains top-level code

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swift package init --type executable

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// swift-tools-version: 6.3

import PackageDescription

let package = Package(
    name: "tool",
    dependencies: [
        .package(url: "https://github.com/apple/swift-argument-parser.git", from: "1.5.0"),
    ],
    targets: [
        .executableTarget(
            name: "tool",
            dependencies: [
                .product(name: "ArgumentParser", package: "swift-argument-parser"),
            ]
        ),
    ]
)

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// Sources/tool/main.swift
import ArgumentParser

struct Tool: ParsableCommand {
    @Argument(help: "要说的话") var message: String = "你好"

    func run() {
        print("echo: \(message)")
    }
}

Tool.main()

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$ swift run tool 你好世界
Build complete! (1.35秒)
echo: 你好世界