In
computer programming
Computer programming is the process of performing a particular computation (or more generally, accomplishing a specific computing result), usually by designing and building an executable computer program. Programming involves tasks such as ana ...
, the scope of a
name binding
In programming languages, name binding is the association of entities (data and/or code) with identifiers. An identifier bound to an object is said to reference that object. Machine languages have no built-in notion of identifiers, but name-objec ...
(an association of a name to an entity, such as a
variable
Variable may refer to:
* Variable (computer science), a symbolic name associated with a value and whose associated value may be changed
* Variable (mathematics), a symbol that represents a quantity in a mathematical expression, as used in many ...
) is the part of a
program
Program, programme, programmer, or programming may refer to:
Business and management
* Program management, the process of managing several related projects
* Time management
* Program, a part of planning
Arts and entertainment Audio
* Progra ...
where the name binding is valid; that is, where the name can be used to refer to the entity. In other parts of the program, the name may refer to a different entity (it may have a different binding), or to nothing at all (it may be unbound). Scope helps prevent
name collision
In computer programming, a name collision is the nomenclature problem that occurs when the same variable name is used for different things in two separate areas that are joined, merged, or otherwise go from occupying separate namespaces to sharin ...
s by allowing the same name to refer to different objects – as long as the names have separate scopes. The scope of a name binding is also known as the visibility of an entity, particularly in older or more technical literature—this is from the perspective of the referenced entity, not the referencing name.
The term "scope" is also used to refer to the set of ''all'' name bindings that are valid within a part of a program or at a given point in a program, which is more correctly referred to as ''context'' or ''environment''.
Strictly speaking and in practice for most
programming language
A programming language is a system of notation for writing computer programs. Most programming languages are text-based formal languages, but they may also be graphical. They are a kind of computer language.
The description of a programming ...
s, "part of a program" refers to a portion of
source code
In computing, source code, or simply code, is any collection of code, with or without comments, written using a human-readable programming language, usually as plain text. The source code of a program is specially designed to facilitate the wo ...
(area of text), and is known as lexical scope. In some languages, however, "part of a program" refers to a portion of
run time
Run(s) or RUN may refer to:
Places
* Run (island), one of the Banda Islands in Indonesia
* Run (stream), a stream in the Dutch province of North Brabant
People
* Run (rapper), Joseph Simmons, now known as "Reverend Run", from the hip-hop group ...
(time period during
execution
Capital punishment, also known as the death penalty, is the State (polity), state-sanctioned practice of deliberately killing a person as a punishment for an actual or supposed crime, usually following an authorized, rule-governed process to ...
), and is known as dynamic scope. Both of these terms are somewhat misleading—they misuse technical terms, as discussed in the
definition
A definition is a statement of the meaning of a term (a word, phrase, or other set of symbols). Definitions can be classified into two large categories: intensional definitions (which try to give the sense of a term), and extensional definitio ...
—but the distinction itself is accurate and precise, and these are the standard respective terms. Lexical scope is the main focus of this article, with dynamic scope understood by contrast with lexical scope.
In most cases, name resolution based on lexical scope is relatively straightforward to use and to implement, as in use one can read backwards in the source code to determine to which entity a name refers, and in
implementation
Implementation is the realization of an application, or execution of a plan, idea, model, design, specification, standard, algorithm, or policy.
Industry-specific definitions
Computer science
In computer science, an implementation is a realiza ...
one can maintain a list of names and contexts when
compiling
In computing, a compiler is a computer program that translates computer code written in one programming language (the ''source'' language) into another language (the ''target'' language). The name "compiler" is primarily used for programs that ...
or
interpreting
Interpreting is a translational activity in which one produces a first and final target-language output on the basis of a one-time exposure to an expression in a source language.
The most common two modes of interpreting are simultaneous inter ...
a program. Difficulties arise in
name masking
In programming languages, name resolution is the resolution of the tokens within program expressions to the intended program components.
Overview
Expressions in computer programs reference variables, data types, functions, classes, objects, libr ...
,
forward declaration
In computer programming, a forward declaration is a declaration of an identifier (denoting an entity such as a type, a variable, a constant, or a function) for which the programmer has not yet given a complete definition.
It is required for a com ...
s, and
hoisting, while considerably subtler ones arise with
non-local variable
In programming language theory, a non-local variable is a variable that is not defined in the local scope. While the term can refer to global variables, it is primarily used in the context of nested and anonymous functions where some variables can ...
s, particularly in
closures.
Definition
The strict definition of the (lexical) "scope" of a name (
identifier
An identifier is a name that identifies (that is, labels the identity of) either a unique object or a unique ''class'' of objects, where the "object" or class may be an idea, physical countable object (or class thereof), or physical noncountable ...
) is unambiguous: lexical scope is "the portion of source code in which a binding of a name with an entity applies". This is virtually unchanged from its 1960 definition in the specification of
ALGOL 60
ALGOL 60 (short for ''Algorithmic Language 1960'') is a member of the ALGOL family of computer programming languages. It followed on from ALGOL 58 which had introduced code blocks and the begin and end pairs for delimiting them, representing a k ...
. Representative language specifications follow:
; ALGOL 60 (1960)
["Report on the Algorithmic Language Algol 60", 2.7. Quantities, kinds and scopes]
: The following kinds of quantities are distinguished: simple variables, arrays, labels, switches, and procedures. The scope of a quantity is the set of statements and expressions in which the declaration of the identifier associated with that quantity is valid.
;
C (2007)
: An identifier can denote an object; a function; a tag or a member of a structure, union, or enumeration; a
typedef
typedef is a reserved keyword in the programming languages C, C++, and Objective-C. It is used to create an additional name (''alias'') for another data type, but does not create a new type, except in the obscure case of a qualified typedef of ...
name; a label name; a macro name; or a macro parameter. The same identifier can denote different entities at different points in the program.
..For each different entity that an identifier designates, the identifier is ''visible'' (i.e., can be used) only within a region of program text called its ''scope.''
;
Go (2013)
[The Go Programming Language Specification]
Declarations and scope
Version of Nov 13, 2013
: A declaration binds a non-blank identifier to a constant, type, variable, function, label, or package.
..The scope of a declared identifier is the extent of source text in which the identifier denotes the specified constant, type, variable, function, label, or package.
Most commonly "scope" refers to when a given name can refer to a given
variable
Variable may refer to:
* Variable (computer science), a symbolic name associated with a value and whose associated value may be changed
* Variable (mathematics), a symbol that represents a quantity in a mathematical expression, as used in many ...
—when a
declaration
Declaration may refer to:
Arts, entertainment, and media Literature
* ''Declaration'' (book), a self-published electronic pamphlet by Michael Hardt and Antonio Negri
* ''The Declaration'' (novel), a 2008 children's novel by Gemma Malley
Music ...
has effect—but can also apply to other entities, such as functions, types, classes,
labels
A label (as distinct from signage) is a piece of paper, plastic film, cloth, metal, or other material affixed to a container or product, on which is written or printed information or symbols about the product or item. Information printed dir ...
, constants, and enumerations.
Lexical scope vs. dynamic scope
A fundamental distinction in scope is what "part of a program" means. In languages with lexical scope (also called static scope), name resolution depends on the location in the source code and the ''lexical context'' (also called ''static context''), which is defined by where the named variable or function is defined. In contrast, in languages with dynamic scope the name resolution depends upon the
program state
In information technology and computer science, a system is described as stateful if it is designed to remember preceding events or user interactions; the remembered information is called the state of the system.
The set of states a system can oc ...
when the name is encountered which is determined by the ''execution context'' (also called ''runtime context'', ''calling context'' or ''dynamic context''). In practice, with lexical scope a name is resolved by searching the local lexical context, then if that fails, by searching the outer lexical context, and so on; whereas with dynamic scope, a name is resolved by searching the local execution context, then if that fails, by searching the outer execution context, and so on, progressing up the call stack.
Most modern languages use lexical scope for variables and functions, though dynamic scope is used in some languages, notably some dialects of Lisp, some "scripting" languages, and some
template language
A template processor (also known as a template engine or template parser) is software designed to combine templates with a data model to produce result documents.
The language that the templates are written in is known as a template language ...
s. Perl 5 offers both lexical and dynamic scope. Even in lexically scoped languages, scope for
closures can be confusing to the uninitiated, as these depend on the lexical context where the closure is defined, not where it is called.
Lexical resolution can be determined at
compile time
In computer science, compile time (or compile-time) describes the time window during which a computer program is compiled.
The term is used as an adjective to describe concepts related to the context of program compilation, as opposed to concept ...
, and is also known as ''early binding'', while dynamic resolution can in general only be determined at
run time
Run(s) or RUN may refer to:
Places
* Run (island), one of the Banda Islands in Indonesia
* Run (stream), a stream in the Dutch province of North Brabant
People
* Run (rapper), Joseph Simmons, now known as "Reverend Run", from the hip-hop group ...
, and thus is known as ''late binding''.
Related concepts
In
object-oriented programming
Object-oriented programming (OOP) is a programming paradigm based on the concept of "objects", which can contain data and code. The data is in the form of fields (often known as attributes or ''properties''), and the code is in the form of pr ...
,
dynamic dispatch
In computer science, dynamic dispatch is the process of selecting which implementation of a polymorphic operation (method or function) to call at run time. It is commonly employed in, and considered a prime characteristic of, object-oriented ...
selects an object
method
Method ( grc, μέθοδος, methodos) literally means a pursuit of knowledge, investigation, mode of prosecuting such inquiry, or system. In recent centuries it more often means a prescribed process for completing a task. It may refer to:
*Scien ...
at runtime, though whether the actual name binding is done at compile time or run time depends on the language. De facto dynamic scope is common in
macro languages, which do not directly do name resolution, but instead expand in place.
Some programming frameworks like
AngularJS
AngularJS is a discontinued free and open-source JavaScript-based web framework for developing single-page applications. It was maintained mainly by Google and a community of individuals and corporations. It aimed to simplify both the development ...
use the term "scope" to mean something entirely different than how it is used in this article. In those frameworks the scope is just an object of the programming language that they use (
JavaScript
JavaScript (), often abbreviated as JS, is a programming language that is one of the core technologies of the World Wide Web, alongside HTML and CSS. As of 2022, 98% of Website, websites use JavaScript on the Client (computing), client side ...
in case of AngularJS) that is used in certain ways by the framework to emulate dynamic scope in a language that uses lexical scope for its variables. Those
AngularJS scopes can themselves be in context or not in context (using the usual meaning of the term) in any given part of the program, following the usual rules of variable scope of the language like any other object, and using their own
inheritance
Inheritance is the practice of receiving private property, Title (property), titles, debts, entitlements, Privilege (law), privileges, rights, and Law of obligations, obligations upon the death of an individual. The rules of inheritance differ ...
and
transclusion
In computer science, transclusion is the inclusion of part or all of an electronic document into one or more other documents by reference via hypertext. Transclusion is usually performed when the referencing document is displayed, and is normal ...
rules. In the context of AngularJS, sometimes the term "$scope" (with a dollar sign) is used to avoid confusion, but using the dollar sign in variable names is often discouraged by the style guides.
Use
Scope is an important component of
name resolution, which is in turn fundamental to
language semantics. Name resolution (including scope) varies between programming languages, and within a programming language, varies by type of entity; the rules for scope are called scope rules (or scoping rules). Together with
namespaces
In computing, a namespace is a set of signs (''names'') that are used to identify and refer to objects of various kinds. A namespace ensures that all of a given set of objects have unique names so that they can be easily identified.
Namespaces ...
, scope rules are crucial in
modular programming
Modular programming is a software design technique that emphasizes separating the functionality of a program into independent, interchangeable modules, such that each contains everything necessary to execute only one aspect of the desired function ...
, so a change in one part of the program does not break an unrelated part.
Overview
When discussing scope, there are three basic concepts: ''scope,'' ''extent,'' and ''context.'' "Scope" and "context" in particular are frequently confused: scope is a property of a name binding, while context is a property of a part of a program, that is either a portion of source code (''lexical context'' or ''static context'') or a portion of
run time
Run(s) or RUN may refer to:
Places
* Run (island), one of the Banda Islands in Indonesia
* Run (stream), a stream in the Dutch province of North Brabant
People
* Run (rapper), Joseph Simmons, now known as "Reverend Run", from the hip-hop group ...
(''execution context,'' ''runtime context,'' ''calling context'' or ''dynamic context''). Execution context consists of lexical context (at the current execution point) plus additional runtime state such as the
call stack
In computer science, a call stack is a stack data structure that stores information about the active subroutines of a computer program. This kind of stack is also known as an execution stack, program stack, control stack, run-time stack, or ma ...
. Strictly speaking, during execution a program enters and exits various name bindings' scopes, and at a point in execution name bindings are "in context" or "not in context", hence name bindings "come into context" or "go out of context" as the program execution enters or exits the scope. However, in practice usage is much looser.
Scope is a source-code level concept, and a property of name bindings, particularly variable or function name bindings—names in the source code are
references
Reference is a relationship between objects in which one object designates, or acts as a means by which to connect to or link to, another object. The first object in this relation is said to ''refer to'' the second object. It is called a ''name'' ...
to entities in the program—and is part of the behavior of a compiler or interpreter of a language. As such, issues of scope are similar to
pointers
Pointer may refer to:
Places
* Pointer, Kentucky
* Pointers, New Jersey
* Pointers Airport, Wasco County, Oregon, United States
* The Pointers, a pair of rocks off Antarctica
People with the name
* Pointer (surname), a surname (including a l ...
, which are a type of reference used in programs more generally. Using the value of a variable when the name is in context but the variable is uninitialized is analogous to dereferencing (accessing the value of) a
wild pointer
Wild, wild, wilds or wild may refer to:
Common meanings
* Wild animal
* Wilderness, a wild natural environment
* Wildness, the quality of being wild or untamed
Art, media and entertainment Film and television
* ''Wild'' (2014 film), a 2014 Am ...
, as it is undefined. However, as variables are not destroyed until they go out of context, the analog of a
dangling pointer
Dangling pointers and wild pointers in computer programming are pointers that do not point to a valid object of the appropriate type. These are special cases of memory safety violations. More generally, dangling references and wild references are ...
does not exist.
For entities such as variables, scope is a subset of
lifetime (also known as
extent)—a name can only refer to a variable that exists (possibly with undefined value), but variables that exist are not necessarily visible: a variable may exist but be inaccessible (the value is stored but not referred to within a given context), or accessible but not via the given name, in which case it is not in context (the program is "out of the scope of the name"). In other cases "lifetime" is irrelevant—a label (named position in the source code) has lifetime identical with the program (for statically compiled languages), but may be in context or not at a given point in the program, and likewise for
static variable
In computer programming, a static variable is a variable that has been allocated "statically", meaning that its lifetime (or "extent") is the entire run of the program. This is in contrast to shorter-lived automatic variables, whose storage is ...
s—a
static global variable is in context for the entire program, while a
static local variable
In computer science, a local variable is a variable that is given ''local scope''. A local variable reference in the function or block in which it is declared overrides the same variable name in the larger scope. In programming languages with on ...
is only in context within a function or other local context, but both have lifetime of the entire run of the program.
Determining which entity a name refers to is known as
name resolution or
name binding
In programming languages, name binding is the association of entities (data and/or code) with identifiers. An identifier bound to an object is said to reference that object. Machine languages have no built-in notion of identifiers, but name-objec ...
(particularly in
object-oriented programming
Object-oriented programming (OOP) is a programming paradigm based on the concept of "objects", which can contain data and code. The data is in the form of fields (often known as attributes or ''properties''), and the code is in the form of pr ...
), and varies between languages. Given a name, the language (properly, the compiler or interpreter) checks all entities that are in context for matches; in case of ambiguity (two entities with the same name, such as a global and local variable with the same name), the name resolution rules are used to distinguish them. Most frequently, name resolution relies on an "inner-to-outer context" rule, such as the Python LEGB (Local, Enclosing, Global, Built-in) rule: names implicitly resolves to the narrowest relevant context. In some cases name resolution can be explicitly specified, such as by the
global
and
nonlocal
keywords in Python; in other cases the default rules cannot be overridden.
When two identical names are in context at the same time, referring to different entities, one says that ''
name masking
In programming languages, name resolution is the resolution of the tokens within program expressions to the intended program components.
Overview
Expressions in computer programs reference variables, data types, functions, classes, objects, libr ...
'' is occurring, where the higher-priority name (usually innermost) is "masking" the lower-priority name. At the level of variables, this is known as
variable shadowing
In computer programming, variable shadowing occurs when a variable declared within a certain scope (decision block, method, or inner class) has the same name as a variable declared in an outer scope. At the level of identifiers (names, rather than ...
. Due to the potential for
logic error
In computer programming, a logic error is a bug in a program that causes it to operate incorrectly, but not to terminate abnormally (or crash). A logic error produces unintended or undesired output or other behaviour, although it may not immedia ...
s from masking, some languages disallow or discourage masking, raising an error or warning at compile time or run time.
Various
programming language
A programming language is a system of notation for writing computer programs. Most programming languages are text-based formal languages, but they may also be graphical. They are a kind of computer language.
The description of a programming ...
s have various different scope rules for different kinds of declarations and names. Such scope rules have a large effect on
language semantics and, consequently, on the behavior and correctness of programs. In languages like
C++
C++ (pronounced "C plus plus") is a high-level general-purpose programming language created by Danish computer scientist Bjarne Stroustrup as an extension of the C programming language, or "C with Classes". The language has expanded significan ...
, accessing an unbound variable does not have well-defined semantics and may result in
undefined behavior
In computer programming, undefined behavior (UB) is the result of executing a program whose behavior is prescribed to be unpredictable, in the language specification to which the computer code adheres. This is different from unspecified behavior, ...
, similar to referring to a
dangling pointer
Dangling pointers and wild pointers in computer programming are pointers that do not point to a valid object of the appropriate type. These are special cases of memory safety violations. More generally, dangling references and wild references are ...
; and declarations or names used outside their scope will generate
syntax errors.
Scopes are frequently tied to other language constructs and determined implicitly, but many languages also offer constructs specifically for controlling scope.
Levels of scope
Scope can vary from as little as a single expression to as much as the entire program, with many possible gradations in between. The simplest scope rule is global scope—all entities are visible throughout the entire program. The most basic modular scope rule is two-level scope, with a global scope anywhere in the program, and local scope within a function. More sophisticated modular programming allows a separate module scope, where names are visible within the module (private to the module) but not visible outside it. Within a function, some languages, such as C, allow block scope to restrict scope to a subset of a function; others, notably functional languages, allow expression scope, to restrict scope to a single expression. Other scopes include file scope (notably in C) which behaves similarly to module scope, and block scope outside of functions (notably in Perl).
A subtle issue is exactly when a scope begins and ends. In some languages, such as C, a name's scope begins at the name declaration, and thus different names declared within a given block can have different scopes. This requires declaring functions before use, though not necessarily defining them, and requires
forward declaration
In computer programming, a forward declaration is a declaration of an identifier (denoting an entity such as a type, a variable, a constant, or a function) for which the programmer has not yet given a complete definition.
It is required for a com ...
in some cases, notably for mutual recursion. In other languages, such as Python, a name's scope begins at the start of the relevant block where the name is declared (such as the start of a function), regardless of where it is defined, so all names within a given block have the same scope. In JavaScript, the scope of a name declared with
let
or
const
begins at the name declaration, and the scope of a name declared with
var
begins at the start of the function where the name is declared, which is known as ''
variable hoisting
The syntax of JavaScript is the set of rules that define a correctly structured JavaScript program.
The examples below make use of the log function of the console object present in most browsers for standard text output.
The JavaScript stand ...
''. Behavior of names in context that have undefined value differs: in Python use of undefined names yields a runtime error, while in JavaScript undefined names declared with
var
are usable throughout the function because they are implicitly bound to the value
undefined
.
Expression scope
The scope of a name binding is an
expression
Expression may refer to:
Linguistics
* Expression (linguistics), a word, phrase, or sentence
* Fixed expression, a form of words with a specific meaning
* Idiom, a type of fixed expression
* Metaphorical expression, a particular word, phrase, o ...
, which is known as expression scope. Expression scope is available in many languages, especially
functional
Functional may refer to:
* Movements in architecture:
** Functionalism (architecture)
** Form follows function
* Functional group, combination of atoms within molecules
* Medical conditions without currently visible organic basis:
** Functional sy ...
languages which offer a feature called ''let-expressions'' allowing a declaration's scope to be a single expression. This is convenient if, for example, an intermediate value is needed for a computation. For example, in
Standard ML
Standard ML (SML) is a general-purpose, modular, functional programming language with compile-time type checking and type inference. It is popular among compiler writers and programming language researchers, as well as in the development of the ...
, if
f()
returns
12
, then
let val x = f() in x * x end
is an expression that evaluates to
144
, using a temporary variable named
x
to avoid calling
f()
twice. Some languages with block scope approximate this functionality by offering syntax for a block to be embedded into an expression; for example, the aforementioned Standard ML expression could be written in
Perl
Perl is a family of two high-level, general-purpose, interpreted, dynamic programming languages. "Perl" refers to Perl 5, but from 2000 to 2019 it also referred to its redesigned "sister language", Perl 6, before the latter's name was offici ...
as
do , or in
GNU C as
().
In Python, auxiliary variables in generator expressions and list comprehensions (in Python 3) have expression scope.
In C, variable names in a
function prototype
In computer programming, a function prototype or function interface is a declaration of a function that specifies the function’s name and type signature (arity, data types of parameters, and return type), but omits the function body. While a f ...
have expression scope, known in this context as function protocol scope. As the variable names in the prototype are not referred to (they may be different in the actual definition)—they are just dummies—these are often omitted, though they may be used for generating documentation, for instance.
Block scope
The scope of a name binding is a
block
Block or blocked may refer to:
Arts, entertainment and media Broadcasting
* Block programming, the result of a programming strategy in broadcasting
* W242BX, a radio station licensed to Greenville, South Carolina, United States known as ''96.3 ...
, which is known as block scope. Block scope is available in many, but not all, block-structured programming languages. This began with
ALGOL 60
ALGOL 60 (short for ''Algorithmic Language 1960'') is a member of the ALGOL family of computer programming languages. It followed on from ALGOL 58 which had introduced code blocks and the begin and end pairs for delimiting them, representing a k ...
, where "
ery declaration ... is valid only for that block.", and today is particularly associated with languages in the
Pascal
Pascal, Pascal's or PASCAL may refer to:
People and fictional characters
* Pascal (given name), including a list of people with the name
* Pascal (surname), including a list of people and fictional characters with the name
** Blaise Pascal, Fren ...
and
C families and traditions. Most often this block is contained within a function, thus restricting the scope to a part of a function, but in some cases, such as Perl, the block may not be within a function.
unsigned int sum_of_squares(const unsigned int N)
A representative example of the use of block scope is the C code shown here, where two variables are scoped to the loop: the loop variable
n, which is initialized once and incremented on each iteration of the loop, and the auxiliary variable
n_squared, which is initialized at each iteration. The purpose is to avoid adding variables to the function scope that are only relevant to a particular block—for example, this prevents errors where the generic loop variable
i has accidentally already been set to another value. In this example the expression
n * n
would generally not be assigned to an auxiliary variable, and the body of the loop would simply be written
ret += n * n
but in more complicated examples auxiliary variables are useful.
Blocks are primarily used for control flow, such as with if, while, and for loops, and in these cases block scope means the scope of variable depends on the structure of a function's flow of execution. However, languages with block scope typically also allow the use of "naked" blocks, whose sole purpose is to allow fine-grained control of variable scope. For example, an auxiliary variable may be defined in a block, then used (say, added to a variable with function scope) and discarded when the block ends, or a while loop might be enclosed in a block that initializes variables used inside the loop that should only be initialized once.
A subtlety of several programming languages, such as
Algol 68
ALGOL 68 (short for ''Algorithmic Language 1968'') is an imperative programming language that was conceived as a successor to the ALGOL 60 programming language, designed with the goal of a much wider scope of application and more rigorously de ...
and C (demonstrated in this example and standardized since
C99
C99 (previously known as C9X) is an informal name for ISO/IEC 9899:1999, a past version of the C programming language standard. It extends the previous version ( C90) with new features for the language and the standard library, and helps impl ...
), is that block-scope variables can be declared not only within the body of the block, but also within the control statement, if any. This is analogous to function parameters, which are declared in the function declaration (before the block of the function body starts), and in scope for the whole function body. This is primarily used in
for loop
In computer science a for-loop or for loop is a control flow statement for specifying iteration. Specifically, a for loop functions by running a section of code repeatedly until a certain condition has been satisfied.
For-loops have two part ...
s, which have an initialization statement separate from the loop condition, unlike while loops, and is a common idiom.
Block scope can be used for shadowing. In this example, inside the block the auxiliary variable could also have been called
n, shadowing the parameter name, but this is considered poor style due to the potential for errors. Furthermore, some descendants of C, such as Java and C#, despite having support for block scope (in that a local variable can be made to go out of context before the end of a function), do not allow one local variable to hide another. In such languages, the attempted declaration of the second
n would result in a syntax error, and one of the
n variables would have to be renamed.
If a block is used to set the value of a variable, block scope requires that the variable be declared outside of the block. This complicates the use of conditional statements with
single assignment
In computer programming, an assignment statement sets and/or re-sets the value stored in the storage location(s) denoted by a variable name; in other words, it copies a value into the variable. In most imperative programming languages, the assig ...
. For example, in Python, which does not use block scope, one may initialize a variable as such:
if c:
a = "foo"
else:
a = ""
where
a
is accessible after the
if
statement.
In Perl, which has block scope, this instead requires declaring the variable prior to the block:
my $a;
if (c) else
Often this is instead rewritten using multiple assignment, initializing the variable to a default value. In Python (where it is not necessary) this would be:
a = ""
if c:
a = "foo"
while in Perl this would be:
my $a = '';
if (c)
In case of a single variable assignment, an alternative is to use the
ternary operator
In mathematics, a ternary operation is an ''n''- ary operation with ''n'' = 3. A ternary operation on a set ''A'' takes any given three elements of ''A'' and combines them to form a single element of ''A''.
In computer science, a ternary operator ...
to avoid a block, but this is not in general possible for multiple variable assignments, and is difficult to read for complex logic.
This is a more significant issue in C, notably for string assignment, as string initialization can automatically allocate memory, while string assignment to an already initialized variable requires allocating memory, a string copy, and checking that these are successful.
Some languages allow the concept of block scope to be applied, to varying extents, outside of a function. For example, in the Perl snippet at right,
$counter
is a variable name with block scope (due to the use of the
my
keyword), while
increment_counter
is a function name with global scope. Each call to
increment_counter
will increase the value of
$counter
by one, and return the new value. Code outside of this block can call
increment_counter
, but cannot otherwise obtain or alter the value of
$counter
. This idiom allows one to define closures in Perl.
Function scope
When the scope of variables declared within a function does not extend beyond that function, this is known as function scope. Function scope is available in most programming languages which offer a way to create a ''
local variable
In computer science, a local variable is a Variable (programming), variable that is given ''local scope (programming), scope''. A local variable reference in the subroutine, function or block (programming), block in which it is declared overrides ...
'' in a function or
subroutine
In computer programming, a function or subroutine is a sequence of program instructions that performs a specific task, packaged as a unit. This unit can then be used in programs wherever that particular task should be performed.
Functions may ...
: a variable whose scope ends (that goes out of context) when the function returns. In most cases the lifetime of the variable is the duration of the function call—it is an
automatic variable
__NOTOC__
In computer programming, an automatic variable is a local variable which is allocated and deallocated automatically when program flow enters and leaves the variable's scope. The scope is the lexical context, particularly the function or b ...
, created when the function starts (or the variable is declared), destroyed when the function returns—while the scope of the variable is within the function, though the meaning of "within" depends on whether scope is lexical or dynamic. However, some languages, such as C, also provide for
static local variable
In computer science, a local variable is a variable that is given ''local scope''. A local variable reference in the function or block in which it is declared overrides the same variable name in the larger scope. In programming languages with on ...
s, where the lifetime of the variable is the entire lifetime of the program, but the variable is only in context when inside the function. In the case of static local variables, the variable is created when the program initializes, and destroyed only when the program terminates, as with a
static global variable, but is only in context within a function, like an automatic local variable.
Importantly, in lexical scope a variable with function scope has scope only within the ''lexical context'' of the function: it goes out of context when another function is called within the function, and comes back into context when the function returns—called functions have no access to the local variables of calling functions, and local variables are only in context within the body of the function in which they are declared. By contrast, in dynamic scope, the scope extends to the ''execution context'' of the function: local variables ''stay in context'' when another function is called, only going out of context when the defining function ends, and thus local variables are in context of the function in which they are defined ''and all called functions''. In languages with lexical scope and
nested function
In computer programming, a nested function (or nested procedure or subroutine) is a function which is defined within another function, the ''enclosing function''. Due to simple recursive scope rules, a nested function is itself invisible outside o ...
s, local variables are in context for nested functions, since these are within the same lexical context, but not for other functions that are not lexically nested. A local variable of an enclosing function is known as a
non-local variable
In programming language theory, a non-local variable is a variable that is not defined in the local scope. While the term can refer to global variables, it is primarily used in the context of nested and anonymous functions where some variables can ...
for the nested function. Function scope is also applicable to
anonymous function
In computer programming, an anonymous function (function literal, lambda abstraction, lambda function, lambda expression or block) is a function definition that is not bound to an identifier. Anonymous functions are often arguments being passed to ...
s.
def square(n):
return n * n
def sum_of_squares(n):
total = 0
i = 0
while i <= n:
total += square(i)
i += 1
return total
For example, in the snippet of Python code on the right, two functions are defined:
square
and
sum_of_squares
.
square
computes the square of a number;
sum_of_squares
computes the sum of all squares up to a number. (For example,
square(4)
is 4
2 =
16
, and
sum_of_squares(4)
is 0
2 + 1
2 + 2
2 + 3
2 + 4
2 =
30
.)
Each of these functions has a variable named
n that represents the argument to the function. These two
n variables are completely separate and unrelated, despite having the same name, because they are lexically scoped local variables with function scope: each one's scope is its own, lexically separate function and thus, they don't overlap. Therefore,
sum_of_squares
can call
square
without its own
n being altered. Similarly,
sum_of_squares
has variables named
total and
i; these variables, because of their limited scope, will not interfere with any variables named
total or
i that might belong to any other function. In other words, there is no risk of a ''name collision'' between these names and any unrelated names, even if they are identical.
No name masking is occurring: only one variable named
n is in context at any given time, as the scopes do not overlap. By contrast, were a similar fragment to be written in a language with dynamic scope, the
n in the calling function would remain in context in the called function—the scopes would overlap—and would be masked ("shadowed") by the new
n in the called function.
Function scope is significantly more complicated if functions are first-class objects and can be created locally to a function and then returned. In this case any variables in the nested function that are not local to it (unbound variables in the function definition, that resolve to variables in an enclosing context) create a
closure, as not only the function itself, but also its context (of variables) must be returned, and then potentially called in a different context. This requires significantly more support from the compiler, and can complicate program analysis.
File scope
The scope of a name binding is a file, which is known as file scope. File scope is largely particular to C (and C++), where scope of variables and functions declared at the top level of a file (not within any function) is for the entire file—or rather for C, from the declaration until the end of the source file, or more precisely
translation unit (internal linking). This can be seen as a form of module scope, where modules are identified with files, and in more modern languages is replaced by an explicit module scope. Due to the presence of include statements, which add variables and functions to the internal context and may themselves call further include statements, it can be difficult to determine what is in context in the body of a file.
In the C code snippet above, the function name
sum_of_squares
has file scope.
Module scope
The scope of a name binding is a module, which is known as module scope. Module scope is available in
modular programming languages where modules (which may span various files) are the basic unit of a complex program, as they allow information hiding and exposing a limited interface. Module scope was pioneered in the
Modula
The Modula programming language is a descendant of the Pascal language. It was developed in Switzerland, at ETH Zurich, in the mid-1970s by Niklaus Wirth, the same person who designed Pascal. The main innovation of Modula over Pascal is a module ...
family of languages, and Python (which was influenced by Modula) is a representative contemporary example.
In some
object-oriented programming
Object-oriented programming (OOP) is a programming paradigm based on the concept of "objects", which can contain data and code. The data is in the form of fields (often known as attributes or ''properties''), and the code is in the form of pr ...
languages that lack direct support for modules, such as C++, a similar structure is instead provided by the class hierarchy, where classes are the basic unit of the program, and a class can have private methods. This is properly understood in the context of
dynamic dispatch
In computer science, dynamic dispatch is the process of selecting which implementation of a polymorphic operation (method or function) to call at run time. It is commonly employed in, and considered a prime characteristic of, object-oriented ...
rather than name resolution and scope, though they often play analogous roles. In some cases both these facilities are available, such as in Python, which has both modules and classes, and code organization (as a module-level function or a conventionally private method) is a choice of the programmer.
Global scope
The scope of a name binding is an entire program, which is known as global scope. Variable names with global scope—called ''
global variables
In computer programming, a global variable is a variable with global scope, meaning that it is visible (hence accessible) throughout the program, unless shadowed. The set of all global variables is known as the ''global environment'' or ''global s ...
''—are frequently considered bad practice, at least in some languages, due to the possibility of name collisions and unintentional masking, together with poor modularity, and function scope or block scope are considered preferable. However, global scope is typically used (depending on the language) for various other sorts of names, such as names of functions, names of
classes and names of other
data type
In computer science and computer programming, a data type (or simply type) is a set of possible values and a set of allowed operations on it. A data type tells the compiler or interpreter how the programmer intends to use the data. Most progra ...
s. In these cases mechanisms such as
namespaces
In computing, a namespace is a set of signs (''names'') that are used to identify and refer to objects of various kinds. A namespace ensures that all of a given set of objects have unique names so that they can be easily identified.
Namespaces ...
are used to avoid collisions.
Lexical scope vs. dynamic scope
The use of local variables — of variable names with limited scope, that only exist within a specific function — helps avoid the risk of a name collision between two identically named variables. However, there are two very different approaches to answering this question: What does it mean to be "within" a function?
In lexical scope (or lexical scoping; also called static scope or static scoping), if a variable name's scope is a certain function, then its scope is the program text of the function definition: within that text, the variable name exists, and is bound to the variable's value, but outside that text, the variable name does not exist. By contrast, in dynamic scope (or dynamic scoping), if a variable name's scope is a certain function, then its scope is the time-period during which the function is executing: while the function is running, the variable name exists, and is bound to its value, but after the function returns, the variable name does not exist. This means that if function
f
invokes a separately defined function
g
, then under lexical scope, function
g
does ''not'' have access to
f
's local variables (assuming the text of
g
is not inside the text of
f
), while under dynamic scope, function
g
''does'' have access to
f
's local variables (since
g
is invoked during the invocation of
f
).
$ # bash language
$ x=1
$ function g()
$ function f()
$ f # does this print 1, or 3?
3
$ echo $x # does this print 1, or 2?
1
Consider, for example, the program on the right. The first line,
x=1, creates a global variable
x
and initializes it to
1
. The second line,
function g() , defines a function
g
that prints out ("echoes") the current value of
x
, and then sets
x
to
2
(overwriting the previous value). The third line,
function f() defines a function
f
that creates a local variable
x
(hiding the identically named global variable) and initializes it to
3
, and then calls
g
. The fourth line,
f, calls
f
. The fifth line,
echo $x, prints out the current value of
x
.
So, what exactly does this program print? It depends on the scope rules. If the language of this program is one that uses lexical scope, then
g
prints and modifies the global variable
x
(because
g
is defined outside
f
), so the program prints
1
and then
2
. By contrast, if this language uses dynamic scope, then
g
prints and modifies
f
's local variable
x
(because
g
is called from within
f
), so the program prints
3
and then
1
. (As it happens, the language of the program is
Bash
Bash or BASH may refer to:
Arts and entertainment
* ''Bash!'' (Rockapella album), 1992
* ''Bash!'' (Dave Bailey album), 1961
* '' Bash: Latter-Day Plays'', a dramatic triptych
* ''BASH!'' (role-playing game), a 2005 superhero game
* "Bash" ('' ...
, which uses dynamic scope; so the program prints
3
and then
1
. If the same code was run with
ksh93
KornShell (ksh) is a Unix shell which was developed by David Korn at Bell Labs in the early 1980s and announced at USENIX on July 14, 1983. The initial development was based on Bourne shell source code. Other early contributors were Bell L ...
which uses lexical scope, the results would be different.)
Lexical scope
With lexical scope, a name always refers to its lexical context. This is a property of the program text and is made independent of the runtime
call stack
In computer science, a call stack is a stack data structure that stores information about the active subroutines of a computer program. This kind of stack is also known as an execution stack, program stack, control stack, run-time stack, or ma ...
by the language implementation. Because this matching only requires analysis of the static program text, this type of scope is also called static scope. Lexical scope is standard in all
ALGOL
ALGOL (; short for "Algorithmic Language") is a family of imperative computer programming languages originally developed in 1958. ALGOL heavily influenced many other languages and was the standard method for algorithm description used by the ...
-based languages such as
Pascal
Pascal, Pascal's or PASCAL may refer to:
People and fictional characters
* Pascal (given name), including a list of people with the name
* Pascal (surname), including a list of people and fictional characters with the name
** Blaise Pascal, Fren ...
,
Modula-2
Modula-2 is a structured, procedural programming language developed between 1977 and 1985/8 by Niklaus Wirth at ETH Zurich. It was created as the language for the operating system and application software of the Lilith personal workstation. It w ...
and
Ada
Ada may refer to:
Places
Africa
* Ada Foah, a town in Ghana
* Ada (Ghana parliament constituency)
* Ada, Osun, a town in Nigeria
Asia
* Ada, Urmia, a village in West Azerbaijan Province, Iran
* Ada, Karaman, a village in Karaman Province, Tur ...
as well as in modern functional languages such as
ML and
Haskell
Haskell () is a general-purpose, statically-typed, purely functional programming language with type inference and lazy evaluation. Designed for teaching, research and industrial applications, Haskell has pioneered a number of programming lang ...
. It is also used in the
C language
C (''pronounced like the letter c'') is a general-purpose computer programming language. It was created in the 1970s by Dennis Ritchie, and remains very widely used and influential. By design, C's features cleanly reflect the capabilities o ...
and its syntactic and semantic relatives, although with different kinds of limitations. Static scope allows the programmer to reason about object references such as parameters, variables, constants, types, functions, etc. as simple name substitutions. This makes it much easier to make modular code and reason about it, since the local naming structure can be understood in isolation. In contrast, dynamic scope forces the programmer to anticipate all possible execution contexts in which the module's code may be invoked.
program A;
var I:integer;
K:char;
procedure B;
var K:real;
L:integer;
procedure C;
var M:real;
begin
(*scope A+B+C*)
end;
(*scope A+B*)
end;
(*scope A*)
end.
For example, Pascal is lexically scoped. Consider the Pascal program fragment at right. The variable
I
is visible at all points, because it is never hidden by another variable of the same name. The
char
variable
K
is visible only in the main program because it is hidden by the
real
variable
K
visible in procedure
B
and
C
only. Variable
L
is also visible only in procedure
B
and
C
but it does not hide any other variable. Variable
M
is only visible in procedure
C
and therefore not accessible either from procedure
B
or the main program. Also, procedure
C
is visible only in procedure
B
and can therefore not be called from the main program.
There could have been another procedure
C
declared in the program outside of procedure
B
. The place in the program where "
C
" is mentioned then determines which of the two procedures named
C
it represents, thus precisely analogous with the scope of variables.
Correct implementation of lexical scope in languages with
first-class nested function
In computer programming, a nested function (or nested procedure or subroutine) is a function which is defined within another function, the ''enclosing function''. Due to simple recursive scope rules, a nested function is itself invisible outside o ...
s is not trivial, as it requires each function value to carry with it a record of the values of the variables that it depends on (the pair of the function and this context is called a
closure). Depending on implementation and
computer architecture
In computer engineering, computer architecture is a description of the structure of a computer system made from component parts. It can sometimes be a high-level description that ignores details of the implementation. At a more detailed level, t ...
, variable
lookup
In computer science, a lookup table (LUT) is an array that replaces runtime computation with a simpler array indexing operation. The process is termed as "direct addressing" and LUTs differ from hash tables in a way that, to retrieve a value v wi ...
''may'' become slightly inefficient when very deeply lexically
nested
''Nested'' is the seventh studio album by Bronx-born singer, songwriter and pianist Laura Nyro, released in 1978 on Columbia Records.
Following on from her extensive tour to promote 1976's ''Smile'', which resulted in the 1977 live album ''Season ...
functions are used, although there are well-known techniques to mitigate this. Also, for nested functions that only refer to their own arguments and (immediately) local variables, all relative locations can be known at
compile time
In computer science, compile time (or compile-time) describes the time window during which a computer program is compiled.
The term is used as an adjective to describe concepts related to the context of program compilation, as opposed to concept ...
. No overhead at all is therefore incurred when using that type of nested function. The same applies to particular parts of a program where nested functions are not used, and, naturally, to programs written in a language where nested functions are not available (such as in the C language).
History
Lexical scope was first used in the early 1960s for the imperative language
ALGOL 60
ALGOL 60 (short for ''Algorithmic Language 1960'') is a member of the ALGOL family of computer programming languages. It followed on from ALGOL 58 which had introduced code blocks and the begin and end pairs for delimiting them, representing a k ...
and has been picked up in most other imperative languages since then.
[Borning A]
CSE 341 -- Lexical and Dynamic Scoping
University of Washington.
Languages like
Pascal
Pascal, Pascal's or PASCAL may refer to:
People and fictional characters
* Pascal (given name), including a list of people with the name
* Pascal (surname), including a list of people and fictional characters with the name
** Blaise Pascal, Fren ...
and
C have always had lexical scope, since they are both influenced by the ideas that went into
ALGOL 60
ALGOL 60 (short for ''Algorithmic Language 1960'') is a member of the ALGOL family of computer programming languages. It followed on from ALGOL 58 which had introduced code blocks and the begin and end pairs for delimiting them, representing a k ...
and
ALGOL 68
ALGOL 68 (short for ''Algorithmic Language 1968'') is an imperative programming language that was conceived as a successor to the ALGOL 60 programming language, designed with the goal of a much wider scope of application and more rigorously de ...
(although C did not include lexically
nested function
In computer programming, a nested function (or nested procedure or subroutine) is a function which is defined within another function, the ''enclosing function''. Due to simple recursive scope rules, a nested function is itself invisible outside o ...
s).
Perl
Perl is a family of two high-level, general-purpose, interpreted, dynamic programming languages. "Perl" refers to Perl 5, but from 2000 to 2019 it also referred to its redesigned "sister language", Perl 6, before the latter's name was offici ...
is a language with dynamic scope that added static scope afterwards.
The original
Lisp
A lisp is a speech impairment in which a person misarticulates sibilants (, , , , , , , ). These misarticulations often result in unclear speech.
Types
* A frontal lisp occurs when the tongue is placed anterior to the target. Interdental lisping ...
interpreter (1960) used dynamic scope. ''Deep binding'', which approximates static (lexical) scope, was introduced around 1962 in LISP 1.5 (via the
Funarg In computer science, the funarg problem ''(function argument problem)'' refers to the difficulty in implementing first-class functions (function (programming), functions as first-class objects) in programming language implementations so as to use st ...
device developed by
Steve Russell, working under
John McCarthy).
All early
Lisps used dynamic scope, when based on interpreters. In 1982, Guy L. Steele Jr. and the Common LISP Group publish ''An overview of Common LISP'', a short review of the history and the divergent implementations of Lisp up to that moment and a review of the features that a ''Common Lisp'' implementation should have. On page 102, we read:
Most LISP implementations are internally inconsistent in that by default the interpreter and compiler may assign different semantics to correct programs; this stems primarily from the fact that the interpreter assumes all variables to be dynamically scoped, while the compiler assumes all variables to be local unless forced to assume otherwise. This has been done for the sake of convenience and efficiency, but can lead to very subtle bugs. The definition of Common LISP avoids such anomalies by explicitly requiring the interpreter and compiler to impose identical semantics on correct programs.
Implementations of Common LISP were thus required to have
lexical scope
In computer programming, the scope of a name binding (an association of a name to an entity, such as a variable) is the part of a program where the name binding is valid; that is, where the name can be used to refer to the entity. In other parts o ...
. Again, from ''An overview of Common LISP'':
In addition, Common LISP offers the following facilities (most of which are borrowed from MacLisp, InterLisp or Lisp Machines Lisp): (...) Fully lexically scoped variables. The so-called "FUNARG problem" is completely solved, in both the downward and upward cases.
By the same year in which ''An overview of Common LISP'' was published (1982), initial designs (also by Guy L. Steele Jr.) of a compiled, lexically scoped Lisp, called ''
Scheme A scheme is a systematic plan for the implementation of a certain idea.
Scheme or schemer may refer to:
Arts and entertainment
* ''The Scheme'' (TV series), a BBC Scotland documentary series
* The Scheme (band), an English pop band
* ''The Schem ...
'' had been published and compiler implementations were being attempted. At that time, lexical scope in Lisp was commonly feared to be inefficient to implement. In ''A History of T'', Olin Shivers writes:
''All'' serious Lisps in production use at that time were dynamically scoped. No one who hadn't carefully read the Rabbit thesis (written by Guy Lewis Steele Jr. in 1978) believed lexical scope would fly; even the few people who ''had'' read it were taking a bit of a leap of faith that this was going to work in serious production use.
The term "lexical scope" dates at least to 1967, while the term "lexical scoping" dates at least to 1970, where it was used in
Project MAC
Computer Science and Artificial Intelligence Laboratory (CSAIL) is a research institute at the Massachusetts Institute of Technology (MIT) formed by the 2003 merger of the Laboratory for Computer Science (LCS) and the Artificial Intelligence Lab ...
to describe the scope rules of the Lisp dialect
MDL MDL may refer to:
Computing
* Fayyad & Irani's MDL method, a discretization method
* MDL (programming language), derived from LISP
* The extension for Valve's Source, Source 2 and ID Software's IDTech game engines proprietary model file format
* ...
(then known as "Muddle").
Dynamic scope
With dynamic scope, a name refers to execution context. In technical terms, this means that each name has a global
stack
Stack may refer to:
Places
* Stack Island, an island game reserve in Bass Strait, south-eastern Australia, in Tasmania’s Hunter Island Group
* Blue Stack Mountains, in Co. Donegal, Ireland
People
* Stack (surname) (including a list of people ...
of bindings. Introducing a local variable with name
x
pushes a binding onto the global
x
stack (which may have been empty), which is popped off when the
control flow
In computer science, control flow (or flow of control) is the order in which individual statements, instructions or function calls of an imperative program are executed or evaluated. The emphasis on explicit control flow distinguishes an ''imper ...
leaves the scope. Evaluating
x
in any context always yields the top binding. Note that this cannot be done at compile-time because the binding stack only exists at
run-time, which is why this type of scope is called ''dynamic'' scope.
Dynamic scope is uncommon in modern languages.
Generally, certain
block
Block or blocked may refer to:
Arts, entertainment and media Broadcasting
* Block programming, the result of a programming strategy in broadcasting
* W242BX, a radio station licensed to Greenville, South Carolina, United States known as ''96.3 ...
s are defined to create bindings whose lifetime is the execution time of the block; this adds some features of static scope to the dynamic scope process. However, since a section of code can be called from many different locations and situations, it can be difficult to determine at the outset what bindings will apply when a variable is used (or if one exists at all). This can be beneficial; application of the
principle of least knowledge suggests that code avoid depending on the ''reasons'' for (or circumstances of) a variable's value, but simply use the value according to the variable's definition. This narrow interpretation of shared data can provide a very flexible system for adapting the behavior of a function to the current state (or policy) of the system. However, this benefit relies on careful documentation of all variables used this way as well as on careful avoidance of assumptions about a variable's behavior, and does not provide any mechanism to detect interference between different parts of a program. Some languages, like
Perl
Perl is a family of two high-level, general-purpose, interpreted, dynamic programming languages. "Perl" refers to Perl 5, but from 2000 to 2019 it also referred to its redesigned "sister language", Perl 6, before the latter's name was offici ...
and
Common Lisp
Common Lisp (CL) is a dialect of the Lisp programming language, published in ANSI standard document ''ANSI INCITS 226-1994 (S20018)'' (formerly ''X3.226-1994 (R1999)''). The Common Lisp HyperSpec, a hyperlinked HTML version, has been derived fro ...
, allow the programmer to choose static or dynamic scope when defining or redefining a variable. Examples of languages that use dynamic scope include
Logo
A logo (abbreviation of logotype; ) is a graphic mark, emblem, or symbol used to aid and promote public identification and recognition. It may be of an abstract or figurative design or include the text of the name it represents as in a wordma ...
,
Emacs Lisp
Emacs Lisp is a dialect of the Lisp programming language used as a scripting language by Emacs (a text editor family most commonly associated with GNU Emacs and XEmacs). It is used for implementing most of the editing functionality built into Ema ...
,
LaTeX
Latex is an emulsion (stable dispersion) of polymer microparticles in water. Latexes are found in nature, but synthetic latexes are common as well.
In nature, latex is found as a milky fluid found in 10% of all flowering plants (angiosperms ...
and the shell languages
bash
Bash or BASH may refer to:
Arts and entertainment
* ''Bash!'' (Rockapella album), 1992
* ''Bash!'' (Dave Bailey album), 1961
* '' Bash: Latter-Day Plays'', a dramatic triptych
* ''BASH!'' (role-playing game), a 2005 superhero game
* "Bash" ('' ...
,
dash
The dash is a punctuation mark consisting of a long horizontal line. It is similar in appearance to the hyphen but is longer and sometimes higher from the baseline. The most common versions are the endash , generally longer than the hyphen b ...
, and
PowerShell
PowerShell is a task automation and configuration management program from Microsoft, consisting of a command-line shell (computing), shell and the associated scripting language. Initially a Windows component only, known as Windows PowerShell, it ...
.
Dynamic scope is fairly easy to implement. To find an name's value, the program could traverse the runtime stack, checking each activation record (each function's stack frame) for a value for the name. In practice, this is made more efficient via the use of an
association list
In computer programming and particularly in Lisp, an association list, often referred to as an alist, is a linked list in which each list element (or node) comprises a key and a value. The association list is said to ''associate'' the value with ...
, which is a stack of name/value pairs. Pairs are pushed onto this stack whenever declarations are made, and popped whenever variables go out of context. ''Shallow binding'' is an alternative strategy that is considerably faster, making use of a ''central reference table'', which associates each name with its own stack of meanings. This avoids a linear search during run-time to find a particular name, but care should be taken to properly maintain this table. Note that both of these strategies assume a last-in-first-out (
LIFO) ordering to bindings for any one variable; in practice all bindings are so ordered.
An even simpler implementation is the representation of dynamic variables with simple global variables. The local binding is performed by saving the original value in an anonymous location on the stack that is invisible to the program. When that binding scope terminates, the original value is restored from this location. In fact, dynamic scope originated in this manner. Early implementations of Lisp used this obvious strategy for implementing local variables, and the practice survives in some dialects which are still in use, such as GNU Emacs Lisp. Lexical scope was introduced into Lisp later. This is equivalent to the above shallow binding scheme, except that the central reference table is simply the global variable binding context, in which the current meaning of the variable is its global value. Maintaining global variables isn't complex. For instance, a symbol object can have a dedicated slot for its global value.
Dynamic scope provides an excellent abstraction for
thread-local storage
Thread-local storage (TLS) is a computer programming method that uses static or global memory local to a thread.
While the use of global variables is generally discouraged in modern programming, legacy operating systems such as UNIX are designed ...
, but if it is used that way it cannot be based on saving and restoring a global variable. A possible implementation strategy is for each variable to have a thread-local key. When the variable is accessed, the thread-local key is used to access the thread-local memory location (by code generated by the compiler, which knows which variables are dynamic and which are lexical). If the thread-local key does not exist for the calling thread, then the global location is used. When a variable is locally bound, the prior value is stored in a hidden location on the stack. The thread-local storage is created under the variable's key, and the new value is stored there. Further nested overrides of the variable within that thread simply save and restore this thread-local location. When the initial, outermost override's context terminates, the thread-local key is deleted, exposing the global version of the variable once again to that thread.
With
referential transparency
In computer science, referential transparency and referential opacity are properties of parts of computer programs. An expression is called ''referentially transparent'' if it can be replaced with its corresponding value (and vice-versa) withou ...
the dynamic scope is restricted to the argument stack of the current function only, and coincides with the lexical scope.
Macro expansion
In modern languages,
macro expansion
In computer programming, a macro (short for "macro instruction"; ) is a rule or pattern that specifies how a certain input should be Map (mathematics), mapped to a replacement output. Applying a macro to an input is known as macro expansion. Th ...
in a
preprocessor
In computer science, a preprocessor (or precompiler) is a program that processes its input data to produce output that is used as input in another program. The output is said to be a preprocessed form of the input data, which is often used by so ...
is a key example of de facto dynamic scope. The macro language itself only transforms the source code, without resolving names, but since the expansion is done in place, when the names in the expanded text are then resolved (notably free variables), they are resolved based on where they are expanded (loosely "called"), as if dynamic scope were occurring.
The
C preprocessor
The C preprocessor is the macro preprocessor for the C, Objective-C and C++ computer programming languages. The preprocessor provides the ability for the inclusion of header files, macro expansions, conditional compilation, and line control ...
, used for
macro expansion
In computer programming, a macro (short for "macro instruction"; ) is a rule or pattern that specifies how a certain input should be Map (mathematics), mapped to a replacement output. Applying a macro to an input is known as macro expansion. Th ...
, has de facto dynamic scope, as it does not do name resolution by itself and it is independent of where the macro is defined. For example, the macro:
#define ADD_A(x) x + a
will expand to add
a
to the passed variable, with this name only later resolved by the compiler based on where the macro
ADD_A
is "called" (properly, expanded). Properly, the C preprocessor only does
lexical analysis
In computer science, lexical analysis, lexing or tokenization is the process of converting a sequence of characters (such as in a computer program or web page) into a sequence of ''lexical tokens'' (strings with an assigned and thus identified m ...
, expanding the macro during the tokenization stage, but not parsing into a syntax tree or doing name resolution.
For example, in the following code, the name
a
in the macro is resolved (after expansion) to the local variable at the expansion site:
#define ADD_A(x) x + a
void add_one(int *x)
void add_two(int *x)
Qualified names
As we have seen, one of the key reasons for scope is that it helps prevent name collisions, by allowing identical names to refer to distinct things, with the restriction that the names must have separate scopes. Sometimes this restriction is inconvenient; when many different things need to be accessible throughout a program, they generally all need names with global scope, so different techniques are required to avoid name collisions.
To address this, many languages offer mechanisms for organizing global names. The details of these mechanisms, and the terms used, depend on the language; but the general idea is that a group of names can itself be given a name — a prefix — and, when necessary, an entity can be referred to by a ''qualified name'' consisting of the name plus the prefix. Normally such names will have, in a sense, two sets of scopes: a scope (usually the global scope) in which the qualified name is visible, and one or more narrower scopes in which the ''unqualified name'' (without the prefix) is visible as well. And normally these groups can themselves be organized into groups; that is, they can be ''nested''.
Although many languages support this concept, the details vary greatly. Some languages have mechanisms, such as ''namespaces'' in
C++
C++ (pronounced "C plus plus") is a high-level general-purpose programming language created by Danish computer scientist Bjarne Stroustrup as an extension of the C programming language, or "C with Classes". The language has expanded significan ...
and
C#, that serve almost exclusively to enable global names to be organized into groups. Other languages have mechanisms, such as ''packages'' in
Ada
Ada may refer to:
Places
Africa
* Ada Foah, a town in Ghana
* Ada (Ghana parliament constituency)
* Ada, Osun, a town in Nigeria
Asia
* Ada, Urmia, a village in West Azerbaijan Province, Iran
* Ada, Karaman, a village in Karaman Province, Tur ...
and ''structures'' in
Standard ML
Standard ML (SML) is a general-purpose, modular, functional programming language with compile-time type checking and type inference. It is popular among compiler writers and programming language researchers, as well as in the development of the ...
, that combine this with the additional purpose of allowing some names to be visible only to other members of their group. And object-oriented languages often allow classes or singleton objects to fulfill this purpose (whether or not they ''also'' have a mechanism for which this is the primary purpose). Furthermore, languages often meld these approaches; for example,
Perl
Perl is a family of two high-level, general-purpose, interpreted, dynamic programming languages. "Perl" refers to Perl 5, but from 2000 to 2019 it also referred to its redesigned "sister language", Perl 6, before the latter's name was offici ...
's packages are largely similar to C++'s namespaces, but optionally double as classes for object-oriented programming; and
Java
Java (; id, Jawa, ; jv, ꦗꦮ; su, ) is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea to the north. With a population of 151.6 million people, Java is the world's List ...
organizes its variables and functions into classes, but then organizes those classes into Ada-like packages.
By language
Scope rules for representative languages follow.
C
In C, scope is traditionally known as
linkage
Linkage may refer to:
* ''Linkage'' (album), by J-pop singer Mami Kawada, released in 2010
*Linkage (graph theory), the maximum min-degree of any of its subgraphs
*Linkage (horse), an American Thoroughbred racehorse
* Linkage (hierarchical cluster ...
or visibility, particularly for variables. C is a lexically scoped language with global scope (known as ''external linkage''), a form of module scope or file scope (known as ''internal linkage''), and local scope (within a function); within a function scopes can further be nested via block scope. However, standard C does not support nested functions.
The lifetime and visibility of a variable are determined by its
storage class
Storage may refer to:
Goods Containers
* Dry cask storage, for storing high-level radioactive waste
* Food storage
* Intermodal container, cargo shipping
* Storage tank
Facilities
* Garage (residential), a storage space normally used to store ...
. There are three types of lifetimes in C: static (program execution), automatic (block execution, allocated on the stack), and manual (allocated on the heap). Only static and automatic are supported for variables and handled by the compiler, while manually allocated memory must be tracked manually across different variables. There are three levels of visibility in C: external linkage (global), internal linkage (roughly file), and block scope (which includes functions); block scopes can be nested, and different levels of internal linkage is possible by use of includes. Internal linkage in C is visibility at the
translation unit level, namely a source file after being processed by the
C preprocessor
The C preprocessor is the macro preprocessor for the C, Objective-C and C++ computer programming languages. The preprocessor provides the ability for the inclusion of header files, macro expansions, conditional compilation, and line control ...
, notably including all relevant includes.
C programs are compiled as separate
object file
An object file is a computer file containing object code, that is, machine code output of an assembler or compiler. The object code is usually relocatable, and not usually directly executable. There are various formats for object files, and the ...
s, which are then linked into an executable or library via a
linker
Linker or linkers may refer to:
Computing
* Linker (computing), a computer program that takes one or more object files generated by a compiler or generated by an assembler and links them with libraries, generating an executable program or shar ...
. Thus name resolution is split across the compiler, which resolves names within a translation unit (more loosely, "compilation unit", but this is properly a different concept), and the linker, which resolves names across translation units; see
linkage
Linkage may refer to:
* ''Linkage'' (album), by J-pop singer Mami Kawada, released in 2010
*Linkage (graph theory), the maximum min-degree of any of its subgraphs
*Linkage (horse), an American Thoroughbred racehorse
* Linkage (hierarchical cluster ...
for further discussion.
In C, variables with block scope enter context when they are declared (not at the top of the block), go out of context if any (non-nested) function is called within the block, come back into context when the function returns, and go out of context at the end of the block. In the case of automatic local variables, they are also allocated on declaration and deallocated at the end of the block, while for static local variables, they are allocated at program initialization and deallocated at program termination.
The following program demonstrates a variable with block scope coming into context partway through the block, then exiting context (and in fact being deallocated) when the block ends:
#include
int main(void)
The program outputs:
m
m
b
m
There are other levels of scope in C. Variable names used in a function prototype have function prototype visibility, and exit context at the end of the function prototype. Since the name is not used, this is not useful for compilation, but may be useful for documentation. Label names for GOTO statement have function scope, while case label names for
switch statement
In computer programming languages, a switch statement is a type of selection control mechanism used to allow the value of a variable or expression to change the control flow of program execution via search and map.
Switch statements function some ...
s have block scope (the block of the switch).
C++
All the variables that we intend to use in a program must have been declared with its type specifier in an earlier
point in the code, like we did in the previous code at the beginning of the body of the function main when we
declared that a, b, and result were of type int.
A variable can be either of global or local scope. A global variable is a variable declared in the main body of the
source code, outside all functions, while a local variable is one declared within the body of a function or a block.
Modern versions
allow
Allow may refer to:
*River Allow, river in Ireland
*Allow, a low-carbon aluminium brand by Rusal
*"Allow", a directive in the website robots exclusion standard
The robots exclusion standard, also known as the robots exclusion protocol or sim ...
nested lexical scope.
Swift
Swift
Swift or SWIFT most commonly refers to:
* SWIFT, an international organization facilitating transactions between banks
** SWIFT code
* Swift (programming language)
* Swift (bird), a family of birds
It may also refer to:
Organizations
* SWIFT, ...
has a similar rule for scopes with C++, but contains different access modifiers.
Go
Go is lexically scoped using blocks.
[
]
Java
Java
Java (; id, Jawa, ; jv, ꦗꦮ; su, ) is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea to the north. With a population of 151.6 million people, Java is the world's List ...
is lexically scoped.
A Java class can contain three types of variables:
; Local variables: are defined inside a method, or a particular block. These variables are local to where they were defined and lower levels. For example, a loop inside a method can use that method's local variables, but not the other way around. The loop's variables (local to that loop) are destroyed as soon as the loop ends.
; Member variables: also called ''fields'' are variables declared within the class, outside of any method. By default, these variables are available for all methods within that class and also for all classes in the package.
; Parameters: are variables in method declarations.
In general, a set of brackets defines a particular scope, but variables at top level within a class can differ in their behavior depending on the modifier keywords used in their definition.
The following table shows the access to members permitted by each modifier.
JavaScript
JavaScript
JavaScript (), often abbreviated as JS, is a programming language that is one of the core technologies of the World Wide Web, alongside HTML and CSS. As of 2022, 98% of Website, websites use JavaScript on the Client (computing), client side ...
has simple ''scope rules'', but variable initialization and name resolution rules can cause problems, and the widespread use of closures for callbacks means the lexical context of a function when defined (which is used for name resolution) can be very different from the lexical context when it is called (which is irrelevant for name resolution). JavaScript objects have name resolution for properties, but this is a separate topic.
JavaScript has lexical scope nested at the function level, with the global context being the outermost context. This scope is used for both variables and for functions (meaning function declarations, as opposed to variables of function type In computer science and mathematical logic, a function type (or arrow type or exponential) is the type of a variable or parameter to which a function has or can be assigned, or an argument or result type of a higher-order function taking or returni ...
). Block scope with the let
and const
In some programming languages, const is a type qualifier (a keyword applied to a data type) that indicates that the data is read-only. While this can be used to declare constants, in the C family of languages differs from similar constructs i ...
keywords is standard since ECMAScript
ECMAScript (; ES) is a JavaScript standard intended to ensure the interoperability of web pages across different browsers. It is standardized by Ecma International in the documenECMA-262
ECMAScript is commonly used for client-side scripting o ...
6. Block scope can be produced by wrapping the entire block in a function and then executing it; this is known as the immediately-invoked function expression
An immediately invoked function expression (or IIFE, pronounced "iffy", IPA /ˈɪf.i/) is a programming language idiom which produces a lexical scope using function scoping. It was popular in JavaScript as a method to support modular programming b ...
(IIFE) pattern.
While JavaScript scope is simple—lexical, function-level—the associated initialization and name resolution rules are a cause of confusion. Firstly, assignment to a name not in scope defaults to creating a new global variable, not a local one. Secondly, to create a new local variable one must use the var
keyword; the variable is then created at the top of the function, with value undefined
and the variable is assigned its value when the assignment expression is reached:
:A variable with an ''Initialiser'' is assigned the value of its ''AssignmentExpression'' when the ''VariableStatement'' is executed, not when the variable is created.
This is known as ''variable hoisting
The syntax of JavaScript is the set of rules that define a correctly structured JavaScript program.
The examples below make use of the log function of the console object present in most browsers for standard text output.
The JavaScript stand ...
''—the declaration, but not the initialization, is hoisted to the top of the function. Thirdly, accessing variables before initialization yields undefined
, rather than a syntax error. Fourthly, for function declarations, the declaration and the initialization are both hoisted to the top of the function, unlike for variable initialization. For example, the following code produces a dialog with output undefined, as the local variable declaration is hoisted, shadowing the global variable, but the initialization is not, so the variable is undefined when used:
a = 1;
function f()
f();
Further, as functions are first-class objects in JavaScript and are frequently assigned as callbacks or returned from functions, when a function is executed, the name resolution depends on where it was originally defined (the lexical context of the definition), not the lexical context or execution context where it is called. The nested scopes of a particular function (from most global to most local) in JavaScript, particularly of a closure, used as a callback, are sometimes referred to as the scope chain, by analogy with the prototype chain of an object.
Closures can be produced in JavaScript by using nested functions, as functions are first-class objects. Returning a nested function from an enclosing function includes the local variables of the enclosing function as the (non-local) lexical context of the returned function, yielding a closure. For example:
function newCounter()
c = newCounter();
alert(c() + ' ' + c()); // outputs "1 2"
Closures are frequently used in JavaScript, due to being used for callbacks. Indeed, any hooking of a function in the local context as a callback or returning it from a function creates a closure if there are any unbound variables in the function body (with the context of the closure based on the nested scopes of the current lexical context, or "scope chain"); this may be accidental. When creating a callback based on parameters, the parameters must be stored in a closure, otherwise it will accidentally create a closure that refers to the variables in the enclosing context, which may change.
Name resolution of properties of JavaScript objects is based on inheritance in the prototype tree—a path to the root in the tree is called a ''prototype chain''—and is separate from name resolution of variables and functions.
Lisp
Lisp
A lisp is a speech impairment in which a person misarticulates sibilants (, , , , , , , ). These misarticulations often result in unclear speech.
Types
* A frontal lisp occurs when the tongue is placed anterior to the target. Interdental lisping ...
dialects have various rules for scope.
The original Lisp used dynamic scope; it was Scheme A scheme is a systematic plan for the implementation of a certain idea.
Scheme or schemer may refer to:
Arts and entertainment
* ''The Scheme'' (TV series), a BBC Scotland documentary series
* The Scheme (band), an English pop band
* ''The Schem ...
, inspired by ALGOL
ALGOL (; short for "Algorithmic Language") is a family of imperative computer programming languages originally developed in 1958. ALGOL heavily influenced many other languages and was the standard method for algorithm description used by the ...
, that introduced static (lexical) scope to the Lisp family.
Maclisp used dynamic scope by default in the interpreter and lexical scope by default in compiled code, though compiled code could access dynamic bindings by use of SPECIAL
declarations for particular variables. However, Maclisp treated lexical binding more as an optimization than one would expect in modern languages, and it did not come with the closure feature one might expect of lexical scope in modern Lisps. A separate operation, *FUNCTION
, was available to somewhat clumsily work around some of that issue.
Common Lisp
Common Lisp (CL) is a dialect of the Lisp programming language, published in ANSI standard document ''ANSI INCITS 226-1994 (S20018)'' (formerly ''X3.226-1994 (R1999)''). The Common Lisp HyperSpec, a hyperlinked HTML version, has been derived fro ...
adopted lexical scope from Scheme A scheme is a systematic plan for the implementation of a certain idea.
Scheme or schemer may refer to:
Arts and entertainment
* ''The Scheme'' (TV series), a BBC Scotland documentary series
* The Scheme (band), an English pop band
* ''The Schem ...
, as did Clojure
Clojure (, like ''closure'') is a dynamic and functional dialect of the Lisp programming language on the Java platform. Like other Lisp dialects, Clojure treats code as data and has a Lisp macro system. The current development process is comm ...
.
ISLISP
ISLISP (also capitalized as ISLisp) is a programming language in the Lisp family standardized by the International Organization for Standardization (ISO) and International Electrotechnical Commission (IEC) joint working group ISO/IEC JTC 1/SC 22/W ...
has lexical scope for ordinary variables. It also has dynamic variables, but they are in all cases explicitly marked; they must be defined by a defdynamic
special form, bound by a dynamic-let
special form, and accessed by an explicit dynamic
special form.
Some other dialects of Lisp, like Emacs Lisp
Emacs Lisp is a dialect of the Lisp programming language used as a scripting language by Emacs (a text editor family most commonly associated with GNU Emacs and XEmacs). It is used for implementing most of the editing functionality built into Ema ...
, still use dynamic scope by default. Emacs Lisp now has lexical scope available on a per-buffer basis.
Python
For variables, Python has function scope, module scope, and global scope. Names enter context at the start of a scope (function, module, or global scope), and exit context when a non-nested function is called or the scope ends. If a name is used prior to variable initialization, this raises a runtime exception. If a variable is simply accessed (not assigned to), name resolution follows the LEGB (Local, Enclosing, Global, Built-in) rule which resolves names to the narrowest relevant context. However, if a variable is assigned to, it defaults to declaring a variable whose scope starts at the start of the level (function, module, or global), not at the assignment. Both these rules can be overridden with a global
or nonlocal
(in Python 3) declaration prior to use, which allows accessing global variables even if there is a masking nonlocal variable, and assigning to global or nonlocal variables.
As a simple example, a function resolves a variable to the global scope:
>>> def f():
... print(x)
...
>>> x = "global"
>>> f()
global
Note that x
is defined before f
is called, so no error is raised, even though it is defined after its reference in the definition of f
. Lexically this is a forward reference
In computer programming, a forward declaration is a declaration of an identifier (denoting an entity such as a type, a variable, a constant, or a function) for which the programmer has not yet given a complete definition.
It is required for a co ...
, which is allowed in Python.
Here assignment creates a new local variable, which does not change the value of the global variable:
>>> def f():
... x = "f"
... print(x)
...
>>> x = "global"
>>> print(x)
global
>>> f()
f
>>> print(x)
global
Assignment to a variable within a function causes it to be declared local to the function, hence its scope is the entire function, and thus using it prior to this assignment raises an error. This differs from C, where the scope of the local variable start at its declaration. This code raises an error:
>>> def f():
... print(x)
... x = "f"
...
>>> x = "global"
>>> f()
Traceback (most recent call last):
File "", line 1, in
File "", line 2, in f
UnboundLocalError: local variable 'x' referenced before assignment
The default name resolution rules can be overridden with the global
or nonlocal
(in Python 3) keywords. In the below code, the global x
declaration in g
means that x
resolves to the global variable. It thus can be accessed (as it has already been defined), and assignment assigns to the global variable, rather than declaring a new local variable. Note that no global
declaration is needed in f
—since it does not assign to the variable, it defaults to resolving to the global variable.
>>> def f():
... print(x)
...
>>> def g():
... global x
... print(x)
... x = "g"
...
>>> x = "global"
>>> f()
global
>>> g()
global
>>> f()
g
global
can also be used for nested functions. In addition to allowing assignment to a global variable, as in an unnested function, this can also be used to access the global variable in the presence of a nonlocal variable:
>>> def f():
... def g():
... global x
... print(x)
... x = "f"
... g()
...
>>> x = "global"
>>> f()
global
For nested functions, there is also the nonlocal
declaration, for assigning to a nonlocal variable, similar to using global
in an unnested function:
>>> def f():
... def g():
... nonlocal x # Python 3 only
... x = "g"
... x = "f"
... g()
... print(x)
...
>>> x = "global"
>>> f()
g
>>> print(x)
global
R
R is a lexically scoped language, unlike other implementations of S where the values of free variables are determined by a set of global variables, while in R they are determined by the context in which the function was created. The scope contexts may be accessed using a variety of features (such as parent.frame()
) which can simulate the experience of dynamic scope should the programmer desire.
There is no block scope:
a <- 1
message(a)
## 2
Functions have access to scope they were created in:
a <- 1
f <- function()
f()
## 1
Variables created or modified within a function stay there:
a <- 1
f <- function()
f()
## 1
## 2
message(a)
## 1
Variables created or modified within a function stay there unless assignment to enclosing scope is explicitly requested:
a <- 1
f <- function()
f()
## 1
## 2
message(a)
## 2
Although R has lexical scope by default, function scopes can be changed:
a <- 1
f <- function()
my_env <- new.env()
my_env$a <- 2
f()
## 1
environment(f) <- my_env
f()
## 2
See also
* Closure (computer science)
In programming languages, a closure, also lexical closure or function closure, is a technique for implementing lexically scoped name binding in a language with first-class functions. Operationally, a closure is a record storing a function toge ...
* Global variable
In computer programming, a global variable is a variable with global scope, meaning that it is visible (hence accessible) throughout the program, unless shadowed. The set of all global variables is known as the ''global environment'' or ''global s ...
* Local variable
In computer science, a local variable is a Variable (programming), variable that is given ''local scope (programming), scope''. A local variable reference in the subroutine, function or block (programming), block in which it is declared overrides ...
* Let expression
In computer science, a "let" expression associates a function definition with a restricted scope.
The "let" expression may also be defined in mathematics, where it associates a Boolean condition with a restricted scope.
The "let" expression may b ...
* Non-local variable
In programming language theory, a non-local variable is a variable that is not defined in the local scope. While the term can refer to global variables, it is primarily used in the context of nested and anonymous functions where some variables can ...
* Name binding
In programming languages, name binding is the association of entities (data and/or code) with identifiers. An identifier bound to an object is said to reference that object. Machine languages have no built-in notion of identifiers, but name-objec ...
* Name resolution (programming languages)
In programming languages, name resolution is the resolution of the tokens within program expressions to the intended program components.
Overview
Expressions in computer programs reference variables, data types, functions, classes, objects, lib ...
* Variables (scope and extent)
* Information hiding
In computer science, information hiding is the principle of segregation of the ''design decisions'' in a computer program that are most likely to change, thus protecting other parts of the program from extensive modification if the design decisio ...
* Immediately-invoked function expression
An immediately invoked function expression (or IIFE, pronounced "iffy", IPA /ˈɪf.i/) is a programming language idiom which produces a lexical scope using function scoping. It was popular in JavaScript as a method to support modular programming b ...
s in Javascript
* Object lifetime
In object-oriented programming (OOP), the object lifetime (or life cycle) of an object is the time between an object's creation and its destruction. Rules for object lifetime vary significantly between languages, in some cases between implementa ...
Notes
References
*
*
"Lexical addressing"
*
** Chapter 3: Names, Scopes, and Bindings, pp. 111–174
** Section 13.4.1: Scripting Languages: Innovative Features: Names and Scopes, pp. 691–699
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