Closure.init()

Author: 0xLeif

.gitignore

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+.DS_Store
+/.build
+/Packages
+/*.xcodeproj
+xcuserdata/
+DerivedData/
+.swiftpm/xcode/package.xcworkspace/contents.xcworkspacedata

.swiftpm/xcode/package.xcworkspace/xcshareddata/IDEWorkspaceChecks.plist

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+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<plist version="1.0">
+<dict>
+	<key>IDEDidComputeMac32BitWarning</key>
+	<true/>
+</dict>
+</plist>

Package.swift

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+// swift-tools-version:5.5
+// The swift-tools-version declares the minimum version of Swift required to build this package.
+
+import PackageDescription
+
+let package = Package(
+  name: "Closure",
+  products: [
+    .library(
+      name: "Closure",
+      targets: ["Closure"]
+    ),
+  ],
+  targets: [
+    .target(
+      name: "Closure",
+      dependencies: []
+    ),
+    .testTarget(
+      name: "ClosureTests",
+      dependencies: ["Closure"]
+    ),
+  ]
+)

README.md

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+# Closure
+
+*Define and chain Closures with Inputs and Outputs*
+
+
+## Examples
+
+**No Scoped State**
+```swift
+let noStateCount = Closure<String, String> { text in
+  String(repeating: text, count: 4)
+}
+.then { string in
+  Int(string) ?? 0
+}
+
+
+XCTAssertEqual(noStateCount.method("5"), 5555)
+XCTAssertEqual(noStateCount.method("5"), 5555)
+XCTAssertEqual(noStateCount.method("5"), 5555)
+```
+
+**Scoped State**
+```swift
+let stateCount: Closure<String, Int> = Closure<String, String> {
+  var count = 1
+  
+  return { text in
+    defer {
+      count += 1
+    }
+    
+    return String(repeating: text, count: count)
+  }
+}
+.then { string in
+  Int(string) ?? 0
+}
+
+XCTAssertEqual(stateCount.method("5"), 5)
+XCTAssertEqual(stateCount.method("5"), 55)
+XCTAssertEqual(stateCount.method("5"), 555)
+```

Sources/Closure/Closure.swift

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+/// A Struct that defines a Closure with a given Input and a given Output
+public struct Closure<Input, Output> {
+  /// The Closure passed in during initialization
+  public let method: (Input) -> Output
+  
+  /// Initialize a Closure without any scoped state
+  public init(
+    _ closure: @escaping (Input) -> Output
+  ) {
+    method = closure
+  }
+  
+  /// Initialize a Closure with potential scoped state
+  public init(
+    _ closure: () -> ((Input) -> Output)
+  ) {
+    method = closure()
+  }
+}
+
+public extension Closure {
+  /// Return Void as the final Output
+  var toVoid: Closure<Input, Void> {
+    Closure<Input, Void> { input in
+      _ = method(input)
+      
+      return ()
+    }
+  }
+  
+  /// Chain another Closure with a NewOutput
+  func then<NewOutput>(
+    _ closure: @escaping (Output) -> NewOutput
+  ) -> Closure<Input, NewOutput> {
+    Closure<Input, NewOutput> { input in
+      closure(method(input))
+    }
+  }
+  
+  /// Run the Closure with Input and a Completion Handler
+  func run(
+    input: Input,
+    onCompletion onCompletionHandler: () -> Void = {}
+  ) -> Output {
+    defer {
+      onCompletionHandler()
+    }
+    return method(input)
+  }
+}

Tests/ClosureTests/ClosureTests.swift

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+import XCTest
+@testable import Closure
+
+final class ClosureTests: XCTestCase {
+  func testClosure() throws {
+    let noStateCount = Closure<String, String> { text in
+      String(repeating: text, count: 4)
+    }
+    .then { string in
+      Int(string) ?? 0
+    }
+    
+    
+    XCTAssertEqual(noStateCount.method("5"), 5555)
+    XCTAssertEqual(noStateCount.method("5"), 5555)
+    XCTAssertEqual(noStateCount.method("5"), 5555)
+  }
+  
+  func testStateClosure() throws {
+    let stateCount: Closure<String, Int> = Closure<String, String> {
+      var count = 1
+      
+      return { text in
+        defer {
+          count += 1
+        }
+        
+        return String(repeating: text, count: count)
+      }
+    }
+    .then { string in
+      Int(string) ?? 0
+    }
+    
+    XCTAssertEqual(stateCount.method("5"), 5)
+    XCTAssertEqual(stateCount.method("5"), 55)
+    XCTAssertEqual(stateCount.method("5"), 555)
+  }
+}