Provides initial initializer parameter for Argument and Option property wrappers

This allows using `Optional` initial values in addition to non-`Optional`, e.g. environment variables.

Author: tarbayev

Sources/ArgumentParser/Parsable Properties/Argument.swift

@@ -335,9 +335,11 @@ extension Argument where Value: ExpressibleByArgument {
   /// - Parameters:
   ///   - help: Information about how to use this argument.
   ///   - completion: Kind of completion provided to the user for this option.
+  ///   - initial: An `Optional` initial value.
   public init(
     help: ArgumentHelp? = nil,
-    completion: CompletionKind? = nil
+    completion: CompletionKind? = nil,
+    initial: Value? = nil
   ) {
     self.init(_parsedValue: .init { key in
       let arg = ArgumentDefinition(
@@ -346,7 +348,7 @@ extension Argument where Value: ExpressibleByArgument {
         kind: .positional,
         help: help,
         parsingStrategy: .default,
-        initial: nil,
+        initial: initial,
         completion: completion)
 
       return ArgumentSet(arg)
@@ -407,10 +409,12 @@ extension Argument {
   ///   - completion: Kind of completion provided to the user for this option.
   ///   - transform: A closure that converts a string into this property's
   ///     element type or throws an error.
+  ///   - initial: An `Optional` initial value.
   public init(
     help: ArgumentHelp? = nil,
     completion: CompletionKind? = nil,
-    transform: @escaping (String) throws -> Value
+    transform: @escaping (String) throws -> Value,
+    initial: Value? = nil
   ) {
     self.init(_parsedValue: .init { key in
       let arg = ArgumentDefinition(
@@ -420,7 +424,7 @@ extension Argument {
         help: help,
         parsingStrategy: .default,
         transform: transform,
-        initial: nil,
+        initial: initial,
         completion: completion)
 
       return ArgumentSet(arg)

Sources/ArgumentParser/Parsable Properties/Option.swift

@@ -299,11 +299,13 @@ extension Option where Value: ExpressibleByArgument {
   ///   - parsingStrategy: The behavior to use when looking for this option's value.
   ///   - help: Information about how to use this option.
   ///   - completion: Kind of completion provided to the user for this option.
+  ///   - initial: An `Optional` initial value.
   public init(
     name: NameSpecification = .long,
     parsing parsingStrategy: SingleValueParsingStrategy = .next,
     help: ArgumentHelp? = nil,
-    completion: CompletionKind? = nil
+    completion: CompletionKind? = nil,
+    initial: Value? = nil
   ) {
     self.init(_parsedValue: .init { key in
       let arg = ArgumentDefinition(
@@ -312,7 +314,7 @@ extension Option where Value: ExpressibleByArgument {
         kind: .name(key: key, specification: name),
         help: help,
         parsingStrategy: parsingStrategy.base,
-        initial: nil,
+        initial: initial,
         completion: completion)
 
       return ArgumentSet(arg)
@@ -373,12 +375,15 @@ extension Option {
   ///   - parsingStrategy: The behavior to use when looking for this option's value.
   ///   - help: Information about how to use this option.
   ///   - completion: Kind of completion provided to the user for this option.
+  ///   - transform: A closure that converts a string into this property's type
+  ///   - initial: An `Optional` initial value.
   public init(
     name: NameSpecification = .long,
     parsing parsingStrategy: SingleValueParsingStrategy = .next,
     help: ArgumentHelp? = nil,
     completion: CompletionKind? = nil,
-    transform: @escaping (String) throws -> Value
+    transform: @escaping (String) throws -> Value,
+    initial: Value? = nil
   ) {
     self.init(_parsedValue: .init { key in
       let arg = ArgumentDefinition(
@@ -388,7 +393,7 @@ extension Option {
         help: help,
         parsingStrategy: parsingStrategy.base,
         transform: transform,
-        initial: nil,
+        initial: initial,
         completion: completion)
 
       return ArgumentSet(arg)

Tests/ArgumentParserEndToEndTests/DefaultsEndToEndTests.swift

@@ -385,7 +385,17 @@ fileprivate struct OptionPropertyInitArguments_Default: ParsableArguments {
   var transformedData: String = "test"
 }
 
-fileprivate struct OptionPropertyInitArguments_NoDefault_NoTransform: ParsableArguments {
+fileprivate struct RequiredOptionPropertyInitArguments_Initial: ParsableArguments {
+  @Option(initial: "test")
+  var data: String
+}
+
+fileprivate struct RequiredOptionPropertyInitArguments_Transform_Initial: ParsableArguments {
+  @Option(transform: exclaim, initial: "test")
+  var data: String
+}
+
+fileprivate struct RequiredOptionPropertyInitArguments_NoDefault_NoTransform: ParsableArguments {
   @Option()
   var data: String
 }
@@ -412,12 +422,37 @@ extension DefaultsEndToEndTests {
 
   /// Tests that *not* providing a default value still parses the argument correctly from the command-line.
   /// This test is almost certainly duplicated by others in the repository, but allows for quick use of test filtering during development on the initialization functionality.
-  func testParsing_OptionPropertyInit_NoDefault_NoTransform() throws {
-    AssertParse(OptionPropertyInitArguments_NoDefault_NoTransform.self, ["--data", "test"]) { arguments in
+  func testParsing_RequiredOptionPropertyInit_NoDefault_NoTransform() throws {
+    AssertParse(RequiredOptionPropertyInitArguments_NoDefault_NoTransform.self, ["--data", "test"]) { arguments in
       XCTAssertEqual(arguments.data, "test")
     }
   }
 
+  func testParsing_RequiredOptionPropertyInit_NoDefault_NoTransform_NoInput_Fails() throws {
+    XCTAssertThrowsError(try RequiredOptionPropertyInitArguments_NoDefault_NoTransform.parse([]))
+    XCTAssertThrowsError(try RequiredOptionPropertyInitArguments_NoDefault_NoTransform.parse(["--data"]))
+  }
+
+  func testParsing_RequiredOptionPropertyInitArguments_Initial_UsesInitialValue() {
+    AssertParse(RequiredOptionPropertyInitArguments_Initial.self, []) { arguments in
+      XCTAssertEqual(arguments.data, "test")
+    }
+  }
+
+  func testParsing_RequiredOptionPropertyInitArguments_Initial_IncompleteInput_Fails() throws {
+    XCTAssertThrowsError(try RequiredOptionPropertyInitArguments_Initial.parse(["--data"]))
+  }
+
+  func testParsing_RequiredOptionPropertyInitArguments_Transform_Initial_UsesInitialValue() {
+    AssertParse(RequiredOptionPropertyInitArguments_Transform_Initial.self, []) { arguments in
+      XCTAssertEqual(arguments.data, "test")
+    }
+  }
+
+  func testParsing_RequiredOptionPropertyInitArguments_Transform_Initial_IncompleteInput_Fails() throws {
+    XCTAssertThrowsError(try RequiredOptionPropertyInitArguments_Transform_Initial.parse(["--data"]))
+  }
+
   /// Tests that using default property initialization syntax on a property with a `transform` function provided parses the default value for the argument when nothing is provided from the command-line.
   func testParsing_OptionPropertyInit_Default_Transform_UseDefault() throws {
     AssertParse(OptionPropertyInitArguments_Default.self, []) { arguments in
@@ -439,6 +474,11 @@ extension DefaultsEndToEndTests {
       XCTAssertEqual(arguments.transformedData, "test!")
     }
   }
+
+  func testParsing_OptionPropertyInit_NoDefault_Transform_NoInput_Fails() throws {
+    XCTAssertThrowsError(try OptionPropertyInitArguments_NoDefault_Transform.parse([]))
+    XCTAssertThrowsError(try OptionPropertyInitArguments_NoDefault_Transform.parse(["--transformed-data"]))
+  }
 }
 
 
@@ -447,14 +487,24 @@ fileprivate struct ArgumentPropertyInitArguments_Default_NoTransform: ParsableAr
   var data: String = "test"
 }
 
+fileprivate struct ArgumentPropertyInitArguments_Initial_NoTransform: ParsableArguments {
+  @Argument(initial: "test")
+  var data: String
+}
+
 fileprivate struct ArgumentPropertyInitArguments_NoDefault_NoTransform: ParsableArguments {
   @Argument()
   var data: String
 }
 
 fileprivate struct ArgumentPropertyInitArguments_Default_Transform: ParsableArguments {
   @Argument(transform: exclaim)
-    var transformedData: String = "test"
+  var transformedData: String = "test"
+}
+
+fileprivate struct ArgumentPropertyInitArguments_Transform_Initial: ParsableArguments {
+  @Argument(transform: exclaim, initial: "test")
+  var transformedData: String
 }
 
 fileprivate struct ArgumentPropertyInitArguments_NoDefault_Transform: ParsableArguments {
@@ -470,6 +520,12 @@ extension DefaultsEndToEndTests {
     }
   }
 
+  func testParsing_ArgumentPropertyInit_Initial_NoTransform_UseDefault() throws {
+    AssertParse(ArgumentPropertyInitArguments_Initial_NoTransform.self, []) { arguments in
+      XCTAssertEqual(arguments.data, "test")
+    }
+  }
+
   /// Tests that using default property initialization syntax parses the command-line-provided value for the argument when provided.
   func testParsing_ArgumentPropertyInit_Default_NoTransform_OverrideDefault() throws {
     AssertParse(ArgumentPropertyInitArguments_Default_NoTransform.self, ["test2"]) { arguments in
@@ -492,6 +548,12 @@ extension DefaultsEndToEndTests {
     }
   }
 
+  func testParsing_ArgumentPropertyInit_Transform_Initial_UseDefault() throws {
+    AssertParse(ArgumentPropertyInitArguments_Transform_Initial.self, []) { arguments in
+      XCTAssertEqual(arguments.transformedData, "test")
+    }
+  }
+
   /// Tests that using default property initialization syntax on a property with a `transform` function provided parses and transforms the command-line-provided value for the argument when provided.
   func testParsing_ArgumentPropertyInit_Default_Transform_OverrideDefault() throws {
     AssertParse(ArgumentPropertyInitArguments_Default_Transform.self, ["test2"]) { arguments in