Closure
A closure is a function or callable value together with access to bindings from the lexical environment in which it was created.
Learn Rust in a Month of Lunches
A closure is an anonymous Rust function-like value that can capture variables from its surrounding environment and be passed as an argument.
- How it is used
- Supplies customizable behavior to iterator methods and other functions without requiring a named function.
- How deep it goes
- Works through syntax, capture, the three closure traits, signatures, unique types, and returned closures; defers detailed closure-type internals.
- Examples
- Uses captured numbers, iterator transformations, compiler errors for missing arguments and distinct closure types, and functions that call closures.
- What you can do after
- Apply closures in iterator methods and function arguments, selecting `Fn`, `FnMut`, or `FnOnce` to fit captured-value use.
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Secrets of the JavaScript Ninja, Second Edition
A function together with the lexical environment in which it was created, allowing it to access variables from that environment after the outer function has finished executing.
- How it is used
- Keeps local state available to callbacks, object methods, and module interfaces after their enclosing functions return.
- How deep it goes
- Explains closures through worked examples, then traces retained lexical environments and execution contexts; qualifies the private-variable approximation.
- Examples
- Uses constructor-held counters, independently animated DOM elements, reassigned methods, and a click-counting module.
- What you can do after
- Build it from scratch: isolate callback state, approximate private variables, and expose module methods without exposing internal variables.
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Go in Practice, Second Edition
A closure is a function value that retains access to variables from its enclosing lexical scope.
- How it is used
- Uses closures for deferred panic recovery and for passing in-scope state to no-argument goroutine helpers.
- How deep it goes
- Explains deferred anonymous-function syntax and variable scope through working examples, without developing closure internals.
- Examples
- Shows deferred recovery, a variable changed before deferred execution, and a goroutine wrapper that traps panics.
- What you can do after
- Build it from scratch using deferred closures to recover panics, or pass captured state to a goroutine helper.
Rust in Action: Systems programming concepts and techniques
A closure is an anonymous callable value that can contain an expression body and capture values from its surrounding scope.
- How it is used
- Uses closures as inline callables for text processing and spawned threads.
- How deep it goes
- Explains syntax, captures, ownership moves, and compiler-generated closure structs; stops short of implementing concurrency libraries.
- Examples
- Compares addition as a named function and closure, shows a captured variable, and illustrates a thread-spawn borrow error.
- What you can do after
- Apply closures via `.map()` and `std::thread::spawn()`, using `move` when spawned work needs ownership of captured values.
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Tiny Python Projects: Learn coding and testing with puzzles and games
A closure is a function together with references to variables from its enclosing lexical scope.
- How it is used
- Uses closures to keep configuration local while transforming characters or filtering words.
- How deep it goes
- Explains nested scope, visibility, and captured variables through runnable examples, but does not descend into runtime implementation.
- Examples
- Demonstrates vowel replacement with new_char() and word-length filtering with word_len().
- What you can do after
- Apply it via Python nested functions to retain local values inside transformation and filtering callbacks.
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Go by Example
A closure is a function that retains access to variables in its surrounding lexical scope after the enclosing function returns.
- How it is used
- Supplies stateful parsers, function adapters, subtest bodies, and HTTP handlers without changing their eventual function signatures.
- How deep it goes
- Defines capture and lifetime, then implements pointer-capturing parsers, adapters, handlers, and memory-retention cautions.
- Examples
- Uses table-driven subtests, CLI flag value parsers, a default HTTP sender, and a URL-shortening handler.
- What you can do after
- Build it from scratch as parsers, adapters, and handlers that retain pointers or dependencies for later calls.
Functional Programming in JavaScript
A function together with the lexical environment in which it was created, enabling access to captured variables after the outer scope has finished executing.
- How it is used
- Uses retained lexical state to support higher-order functions, callbacks, event handling, and private-variable patterns.
- How deep it goes
- Defines lexical capture and demonstrates nested functions, name resolution, function factories, and closure-backed objects.
- Examples
- Uses nested functions, a zipCode accessor object, function factories, callback syntax, and an ambiguous loop-counter example.
- What you can do after
- Apply it via JavaScript functions that retain parameters and enclosing variables after their outer calls finish.
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100 Go Mistakes and How to Avoid Them
A closure is an anonymous function that retains references to variables from its surrounding lexical scope.
- How it is used
- Uses closures to configure servers and to reason about when deferred calls and goroutines observe variables.
- How deep it goes
- Works through captured variables and evaluation timing with concrete fixes, without descending into runtime internals.
- Examples
- Traces a port option, deferred status notifications, loop-launched goroutines, and deferred file closure.
- What you can do after
- Diagnose/debug with it when deferred work reads a stale argument or goroutines observe a changing loop variable.
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Python How-To
A closure is a function that retains access to variables from its enclosing lexical scope after that scope has finished executing.
- How it is used
- Explains how decorators return inner functions that retain access to the functions they decorate.
- How deep it goes
- Identifies the outer function, returned inner function, and bound variables, but defers a full explanation of scopes and nonlocal binding.
- Examples
- Uses a returned function called after outer(100), a monitoring decorator, and docstring and name checks on decorated functions.
- What you can do after
- Build it from scratch as a decorator wrapper that forwards arguments and results while preserving metadata with functools.wraps.
Idiomatic Rust
A closure is an anonymous function value that can accept parameters and capture values from its surrounding environment.
- How it is used
- Supplies caller-defined operations to higher-order functions and iterator transformations.
- How deep it goes
- Explains syntax, inferred parameters, capture, callable traits, and ownership errors through small examples.
- Examples
- Shows incrementing a value, consuming a captured string, and mapping integers to strings.
- What you can do after
- Apply it via Rust closures and iterator methods while recognizing when capture consumes a value or parsing drops errors.
Python Workout, Second Edition
A closure is a function that retains access to variables from an enclosing function scope after that enclosing function has returned.
- How it is used
- Uses returned inner functions to retain configuration or state for later calls, including password generation and counters.
- How deep it goes
- Explains nested-function creation, LEGB lookup, separate invocations, and nonlocal assignment, stopping short of deeper runtime internals.
- Examples
- Demonstrates multiplier functions, password generators, and a call-counting closure.
- What you can do after
- Build closures that capture arguments, generate configured functions, or update enclosing state with nonlocal.
Python Workout: 50 ten-minute exercises
A closure is a function that retains access to variables from its defining lexical environment after the enclosing function has returned.
- How it is used
- Uses nested functions to return configurable behavior that retains access to enclosing variables.
- How deep it goes
- Explains creation, LEGB lookup, separate instances, and nonlocal updates, without descending into runtime implementation details.
- Examples
- Uses configurable password generators, multiplication functions, and a call counter that preserves state across calls.
- What you can do after
- Apply closures in Python to package configuration or preserve mutable state across function calls.
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The Joy of JavaScript
A function together with the lexical environment in which it was created, allowing it to retain access to enclosed bindings.
- How it is used
- Supports JavaScript function composition, currying, higher-order functions, and controlled access to surrounding application state.
- How deep it goes
- Defines lexical retention and [[Scope]], illustrates nested access, and applies closures to currying without pursuing specification-level internals.
- Examples
- Uses nested scope logging and a deferred assemble function that captures keys before receiving an object.
- What you can do after
- Evaluate/understand closures well enough to create deferred functions, apply currying, and bound captured state for maintainable composition.
Julia as a Second Language
A closure is a function that captures external state from its enclosing scope rather than receiving that state as an argument.
- How it is used
- Adapts multi-input operations for single-input pipelines and provides a functional way to retain state.
- How deep it goes
- Defines captured state and works through named closures, a pipeline, and a stateful service; does not examine runtime implementation.
- Examples
- Uses delimiter-splitting and mapping functions, a string-transformation pipeline, and an encryption-backed password keeper.
- What you can do after
- Build it from scratch as returned Julia functions that capture configuration or shared state, then judge when objects are clearer.
Elixir in Action, Third Edition
A closure is a function value that retains access to variables from the lexical scope where it was created.
- How it is used
- Uses closures to pass callbacks to collection functions and data into concurrently spawned processes.
- How deep it goes
- Explains lexical capture, rebinding, reachability, process copying, and callback syntax, but stops short of broader closure implementation internals.
- Examples
- Demonstrates callback printing, compact captured lambdas, rebinding behavior, and five concurrent queries.
- What you can do after
- Apply closures to write compact callbacks and pass immutable data into spawned concurrent computations.
Modern C, Third Edition
A block closure is an anonymous function construct that can access variables from its surrounding context, subject to capture and modification rules.
- How it is used
- Uses an anonymous function construct to implement SWAP while avoiding shortcomings of compound expressions.
- How deep it goes
- Explains syntax, invocation, surrounding access, and modification restrictions.
- Examples
- Demonstrates block-closure syntax through an immediately invoked SWAP macro.
- What you can do after
- Build it from scratch as an immediately invoked anonymous operation inside a macro.
The Well-Grounded Rubyist, Fourth Edition
A closure is executable code together with the lexical variables captured from the environment in which it was created.
- How it is used
- Uses closures to preserve local state and context in callable Ruby objects.
- How deep it goes
- Explains creation context, scope resolution, preserved bindings, and independent counters through runnable examples.
- Examples
- Demonstrates captured variables, multiply_by, make_counter, and Symbol#to_proc mapping.
- What you can do after
- Build closures with Proc.new and use them to retain state across later calls.
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Refactoring to Rust
A closure is an anonymous function value that can be written inline and passed to or returned from other functions.
- How it is used
- Uses closures as inline function arguments, especially for transforming errors with map_err.
- How deep it goes
- Explains syntax, bodies, type inference, and error-mapping applications, but stops short of deeper closure mechanics.
- Examples
- Demonstrates addition closures and username validation that maps lowercase and uniqueness failures.
- What you can do after
- Apply it via map_err to replace small named error-conversion functions with concise inline transformations.
Clojure: The Essential Reference
A closure is a function that retains references to bindings from its defining lexical environment.
- How it is used
- Uses once-only closures to reduce retained memory in delays, futures, and macros that delegate work to wrapper functions.
- How deep it goes
- Explains closure lifetime, captured-reference clearing, compiler metadata, and feature differences, but defers ordinary fn usage to its separate contract.
- Examples
- Demonstrates fn* syntax and a future macro using ^:once metadata with future-call.
- What you can do after
- Evaluate closure lifetime and memory-retention tradeoffs, and recognize once-only function objects in macro implementations.
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Learn C++ by Example
A closure is a callable object that packages executable code with captured values from an enclosing scope.
- How it is used
- Uses closures as compact predicates for standard algorithms, ranges, filters, and composable views.
- How deep it goes
- Explains lambda syntax and value or reference capture, but defers fuller closure details.
- Examples
- Demonstrates non-negative predicates with all_of, ranges::all_of, filter views, and explicit capture-list variants.
- What you can do after
- Apply closures via standard algorithms and ranges to write local predicates and reusable view filters.
Related concepts
- functionkind of · A function is a callable program unit that performs an operation, may accept inputs, and can produce a result or side effect.
- moveconfigured by · Move semantics describe the transfer of a value or its resources to another binding or object rather than duplicating them.
- spawnused by · std::thread::spawn is a Rust standard-library function that starts a new thread to execute a supplied closure and returns a handle for that thread.
- module patternused by · The module pattern is a software design pattern that encapsulates implementation details and state while exposing a controlled public interface.
- FnMutimplements · FnMut is a Rust callable trait for values that can be invoked through mutable access and may mutate their captured environment.
- scopeuses · Scope is the region of a program in which names are visible and can be resolved according to the language’s lookup rules.
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