Design Patterns MCP Server

{ "id": "closures", "name": "Closures Pattern", "category": "Functional", "description": "Functions that capture and modify their environment, maintaining state between invocations", "when_to_use": "Encapsulation, callbacks, stateful operations, event handlers, builders", "benefits": "Encapsulation, flexibility, state management, composability", "drawbacks": "Memory leaks, debugging complexity, performance overhead", "use_cases": "Event handlers, builders, state machines, memoization, DSLs", "complexity": "Medium", "tags": ["functional", "kotlin", "closure", "stateful", "encapsulation"], "examples": { "kotlin": { "language": "kotlin", "code": "// Basic closure - capturing external variables\nfun createCounter(): () -> Int {\n var count = 0\n return { ++count } // Closure captures 'count'\n}\n\nval counter = createCounter()\nprintln(counter()) // 1\nprintln(counter()) // 2\nprintln(counter()) // 3\n\n// Multiple closures sharing state\nfun createStack(): Pair<() -> Unit, () -> Int?> {\n val stack = mutableListOf<Int>()\n \n val push = { value: Int -> stack.add(value) }\n val pop = { stack.removeLastOrNull() }\n \n return Pair(push, pop)\n}\n\nval (push, pop) = createStack()\npush(1)\npush(2)\npush(3)\nprintln(pop()) // 3\nprintln(pop()) // 2\n\n// Closure in event handling\nclass EventEmitter {\n private val listeners = mutableListOf<(String) -> Unit>()\n \n fun onEvent(handler: (String) -> Unit) {\n listeners.add(handler)\n }\n \n fun emit(event: String) {\n listeners.forEach { it(event) }\n }\n}\n\nval emitter = EventEmitter()\nvar eventCount = 0\n\nemitter.onEvent { event ->\n eventCount++\n println(\"Event $eventCount: $event\")\n}\n\nemitter.emit(\"click\")\nemitter.emit(\"hover\")\n\n// Closure in builder pattern\nclass HtmlBuilder {\n private val elements = mutableListOf<String>()\n \n fun div(block: HtmlBuilder.() -> Unit) {\n val childBuilder = HtmlBuilder()\n childBuilder.block() // 'this' is captured\n elements.add(\"<div>${childBuilder.build()}</div>\")\n }\n \n fun p(text: String) {\n elements.add(\"<p>$text</p>\")\n }\n \n fun build() = elements.joinToString(\"\")\n}\n\nfun html(block: HtmlBuilder.() -> Unit): String {\n val builder = HtmlBuilder()\n builder.block()\n return builder.build()\n}\n\nval page = html {\n div {\n p(\"Hello\")\n p(\"World\")\n }\n}\n\n// Closure with memoization\nfun <A, R> memoize(function: (A) -> R): (A) -> R {\n val cache = mutableMapOf<A, R>()\n return { input ->\n cache.getOrPut(input) { function(input) }\n }\n}\n\nval expensiveCalculation = memoize { n: Int ->\n println(\"Calculating fibonacci for $n\")\n fibonacci(n)\n}\n\nprintln(expensiveCalculation(10)) // Calculates\nprintln(expensiveCalculation(10)) // Uses cache\nprintln(expensiveCalculation(5)) // Calculates\n\n// Closure in state machine\nsealed class State\nobject Idle : State()\ndata class Running(val task: String) : State()\ndata class Error(val message: String) : State()\n\nfun createStateMachine(): (String) -> State {\n var currentState: State = Idle\n \n return { command ->\n currentState = when (command) {\n \"start\" -> when (currentState) {\n is Idle -> Running(\"default task\")\n else -> Error(\"Already running\")\n }\n \"stop\" -> when (currentState) {\n is Running -> Idle\n else -> Error(\"Not running\")\n }\n \"error\" -> Error(\"Simulated error\")\n else -> Error(\"Unknown command\")\n }\n currentState\n }\n}\n\nval stateMachine = createStateMachine()\nprintln(stateMachine(\"start\")) // Running(default task)\nprintln(stateMachine(\"start\")) // Error(Already running)\nprintln(stateMachine(\"stop\")) // Idle\n\n// Closure capturing 'this' reference\nclass Calculator {\n private var memory = 0.0\n \n fun add(value: Double) {\n memory += value\n }\n \n fun createOperation(): (Double) -> Double {\n return { input -> \n memory += input\n memory\n }\n }\n}\n\nval calc = Calculator()\ncalc.add(10.0)\nval operation = calc.createOperation()\nprintln(operation(5.0)) // 15.0\nprintln(operation(3.0)) // 18.0\n\n// Closure with cleanup\nfun withResource(block: (Resource) -> Unit) {\n val resource = Resource()\n try {\n block(resource)\n } finally {\n resource.close()\n }\n}\n\nclass Resource {\n private var isOpen = true\n \n fun use() {\n if (!isOpen) throw IllegalStateException(\"Resource closed\")\n println(\"Using resource\")\n }\n \n fun close() {\n if (isOpen) {\n println(\"Closing resource\")\n isOpen = false\n }\n }\n}\n\n// Usage\nwithResource { res ->\n res.use()\n // Resource automatically closed even if exception occurs\n}" } } }