Provide async command support

Adds a new `AsyncParsableCommand` protocol, which provides a
`static func main() async` entry point and can call through to the root
command's or a subcommand's asynchronous `run()` method. For this
asynchronous execution, the root command must conform to `AsyncParsableCommand`,
but its subcommands can be a mix of asynchronous and synchronous commands.

Due to an issue in Swift 5.5, you can only use `@main` on an
`AsyncParsableCommand` root command starting in Swift 5.6.
This change also includes a workaround for clients that are using Swift 5.5.
Declare a separate type that conforms to `AsyncMainProtocol` and add the `@main`
attribute to that type.

```
@main enum Main: AsyncMain {
    typealias Command = <#command#>
}
```

Author: natecook1000

Examples/count-lines/CountLines.swift

@@ -0,0 +1,90 @@
+//===----------------------------------------------------------*- swift -*-===//
+//
+// This source file is part of the Swift Argument Parser open source project
+//
+// Copyright (c) 2020 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+//
+//===----------------------------------------------------------------------===//
+
+#if os(macOS)
+
+import ArgumentParser
+import Foundation
+
+struct CountLines: AsyncParsableCommand {
+    @Argument(
+        help: "A file to count lines in. If omitted, counts the lines of stdin.",
+        completion: .file(), transform: URL.init(fileURLWithPath:))
+    var inputFile: URL?
+    
+    @Option(help: "Only count lines with this prefix.")
+    var prefix: String?
+    
+    @Flag(help: "Include extra information in the output.")
+    var verbose = false
+}
+
+extension CountLines {
+    var fileHandle: FileHandle {
+        get throws {
+            guard let inputFile = inputFile else {
+                return .standardInput
+            }
+            return try FileHandle(forReadingFrom: inputFile)
+        }
+    }
+    
+    func printCount(_ count: Int) {
+        guard verbose else {
+            print(count)
+            return
+        }
+        
+        if let filename = inputFile?.lastPathComponent {
+            print("Lines in '\(filename)'", terminator: "")
+        } else {
+            print("Lines from stdin", terminator: "")
+        }
+        
+        if let prefix = prefix {
+            print(", prefixed by '\(prefix)'", terminator: "")
+        }
+        
+        print(": \(count)")
+    }
+    
+    mutating func run() async throws {
+        let countAllLines = prefix == nil
+        
+        let lineCount = try await fileHandle.bytes.lines.reduce(0) { count, line in
+            if countAllLines || line.starts(with: prefix!) {
+                return count + 1
+            } else {
+                return count
+            }
+        }
+        
+        printCount(lineCount)
+    }
+}
+
+#if swift(>=5.6)
+    @main extension CountLines {}
+#else
+    @main struct AsyncMain: AsyncMainProtocol {
+        typealias Command = CountLines
+    }
+#endif
+
+#else
+
+@main enum Main {
+    static func main() {
+        print("Unsupported on this platform.")
+    }
+}
+
+#endif

Examples/math/main.swift renamed to Examples/math/Math.swift

@@ -11,6 +11,7 @@
 
 import ArgumentParser
 
+@main
 struct Math: ParsableCommand {
     // Customize your command's help and subcommands by implementing the
     // `configuration` property.
@@ -242,5 +243,3 @@ func customCompletion(_ s: [String]) -> [String] {
     ? ["aardvark", "aaaaalbert"]
     : ["hello", "helicopter", "heliotrope"]
 }
-
-Math.main()

Examples/repeat/main.swift renamed to Examples/repeat/Repeat.swift

@@ -11,6 +11,7 @@
 
 import ArgumentParser
 
+@main
 struct Repeat: ParsableCommand {
     @Option(help: "The number of times to repeat 'phrase'.")
     var count: Int?
@@ -33,5 +34,3 @@ struct Repeat: ParsableCommand {
         }
     }
 }
-
-Repeat.main()

Package.swift

@@ -1,4 +1,4 @@
-// swift-tools-version:5.2
+// swift-tools-version:5.5
 //===----------------------------------------------------------*- swift -*-===//
 //
 // This source file is part of the Swift Argument Parser open source project
@@ -12,8 +12,9 @@
 
 import PackageDescription
 
-var package = Package(
+let package = Package(
     name: "swift-argument-parser",
+    platforms: [.macOS(.v12)],
     products: [
         .library(
             name: "ArgumentParser",
@@ -23,48 +24,54 @@ var package = Package(
     targets: [
         .target(
             name: "ArgumentParser",
-            dependencies: ["ArgumentParserToolInfo"]),
+            dependencies: ["ArgumentParserToolInfo"],
+            exclude: ["CMakeLists.txt"]),
         .target(
             name: "ArgumentParserTestHelpers",
-            dependencies: ["ArgumentParser", "ArgumentParserToolInfo"]),
+            dependencies: ["ArgumentParser", "ArgumentParserToolInfo"],
+            exclude: ["CMakeLists.txt"]),
         .target(
             name: "ArgumentParserToolInfo",
-            dependencies: []),
+            dependencies: [],
+            exclude: ["CMakeLists.txt"]),
 
-        .target(
+        .executableTarget(
             name: "roll",
             dependencies: ["ArgumentParser"],
             path: "Examples/roll"),
-        .target(
+        .executableTarget(
             name: "math",
             dependencies: ["ArgumentParser"],
             path: "Examples/math"),
-        .target(
+        .executableTarget(
             name: "repeat",
             dependencies: ["ArgumentParser"],
             path: "Examples/repeat"),
+        .executableTarget(
+            name: "count-lines",
+            dependencies: ["ArgumentParser"],
+            path: "Examples/count-lines"),
 
-        .target(
+        .executableTarget(
             name: "changelog-authors",
             dependencies: ["ArgumentParser"],
             path: "Tools/changelog-authors"),
 
         .testTarget(
             name: "ArgumentParserEndToEndTests",
-            dependencies: ["ArgumentParser", "ArgumentParserTestHelpers"]),
+            dependencies: ["ArgumentParser", "ArgumentParserTestHelpers"],
+            exclude: ["CMakeLists.txt"]),
         .testTarget(
             name: "ArgumentParserUnitTests",
-            dependencies: ["ArgumentParser", "ArgumentParserTestHelpers"]),
+            dependencies: ["ArgumentParser", "ArgumentParserTestHelpers"],
+            exclude: ["CMakeLists.txt"]),
+        .testTarget(
+            name: "ArgumentParserPackageManagerTests",
+            dependencies: ["ArgumentParser", "ArgumentParserTestHelpers"],
+            exclude: ["CMakeLists.txt"]),
         .testTarget(
             name: "ArgumentParserExampleTests",
-            dependencies: ["ArgumentParserTestHelpers"]),
+            dependencies: ["ArgumentParserTestHelpers"],
+            resources: [.copy("CountLinesTest.txt")]),
     ]
 )
-
-#if swift(>=5.2)
-// Skip if < 5.2 to avoid issue with nested type synthesized 'CodingKeys'
-package.targets.append(
-    .testTarget(
-        name: "ArgumentParserPackageManagerTests",
-        dependencies: ["ArgumentParser", "ArgumentParserTestHelpers"]))
-#endif

[email protected]

@@ -13,6 +13,7 @@ add_library(ArgumentParser
   "Parsable Properties/Option.swift"
   "Parsable Properties/OptionGroup.swift"
 
+  "Parsable Types/AsyncParsableCommand.swift"
   "Parsable Types/CommandConfiguration.swift"
   "Parsable Types/EnumerableFlag.swift"
   "Parsable Types/ExpressibleByArgument.swift"

Sources/ArgumentParser/CMakeLists.txt

@@ -0,0 +1,54 @@
+//===----------------------------------------------------------*- swift -*-===//
+//
+// This source file is part of the Swift Argument Parser open source project
+//
+// Copyright (c) 2020 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+//
+//===----------------------------------------------------------------------===//
+
+/// A type that can be executed as part of a nested tree of commands.
+public protocol AsyncParsableCommand: ParsableCommand {
+  mutating func run() async throws
+}
+
+extension AsyncParsableCommand {
+  public static func main() async {
+    do {
+      var command = try parseAsRoot()
+      if var asyncCommand = command as? AsyncParsableCommand {
+        try await asyncCommand.run()
+      } else {
+        try command.run()
+      }
+    } catch {
+      exit(withError: error)
+    }
+  }
+}
+
+@available(
+  swift, deprecated: 5.6,
+  message: "Use @main directly on your root `AsyncParsableCommand` type.")
+public protocol AsyncMainProtocol {
+  associatedtype Command: ParsableCommand
+}
+
+@available(swift, deprecated: 5.6)
+extension AsyncMainProtocol {
+  public static func main() async {
+    do {
+      var command = try Command.parseAsRoot()
+      if var asyncCommand = command as? AsyncParsableCommand {
+        try await asyncCommand.run()
+      } else {
+        try command.run()
+      }
+    } catch {
+      Command.exit(withError: error)
+    }
+  }
+}
+

Sources/ArgumentParser/Parsable Types/AsyncParsableCommand.swift

@@ -0,0 +1,41 @@
+//===----------------------------------------------------------*- swift -*-===//
+//
+// This source file is part of the Swift Argument Parser open source project
+//
+// Copyright (c) 2020 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+//
+//===----------------------------------------------------------------------===//
+
+#if os(macOS)
+
+import XCTest
+import ArgumentParserTestHelpers
+
+final class CountLinesExampleTests: XCTestCase {
+  func testCountLines() throws {
+    let testFile = try XCTUnwrap(Bundle.module.url(forResource: "CountLinesTest", withExtension: "txt"))
+    try AssertExecuteCommand(command: "count-lines \(testFile.path)", expected: "20")
+    try AssertExecuteCommand(command: "count-lines \(testFile.path) --prefix al", expected: "4")
+  }
+  
+  func testCountLinesHelp() throws {
+    let helpText = """
+        USAGE: count-lines <input-file> [--prefix <prefix>] [--verbose]
+
+        ARGUMENTS:
+          <input-file>            A file to count lines in. If omitted, counts the
+                                  lines of stdin.
+
+        OPTIONS:
+          --prefix <prefix>       Only count lines with this prefix.
+          --verbose               Include extra information in the output.
+          -h, --help              Show help information.
+        """
+    try AssertExecuteCommand(command: "count-lines -h", expected: helpText)
+  }
+}
+
+#endif