In
programming language theory
Programming language theory (PLT) is a branch of computer science that deals with the design, implementation, analysis, characterization, and classification of formal languages known as programming languages. Programming language theory is clos ...
, a non-local variable is a variable that is not defined in the local
scope
Scope or scopes may refer to:
People with the surname
* Jamie Scope (born 1986), English footballer
* John T. Scopes (1900–1970), central figure in the Scopes Trial regarding the teaching of evolution
Arts, media, and entertainment
* Cinema ...
. While the term can refer to
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 ...
s, it is primarily used in the context of
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 '' Sea ...
and
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 t ...
s where some variables can be in neither the
local nor the
global scope.
In
Lua they are called the ''upvalues'' of the function.
Programming in Lua (first edition)
''
Examples
Nested functions
In the Python 3 example that follows there is a nested function
inner
defined in the scope of another function
outer
. The variable
x
is local to
outer
, but non-local to
inner
(nor is it global):
def outer():
x = 1
def inner():
nonlocal x
x += 1
print(x)
return inner
In Javascript, the locality of a variable is determined by the closest
var
statement for this variable. In the following example,
x
is local to
outer
as it contains a
var x
statement, while
inner
doesn't. Therefore, x is non-local to
inner
:
function outer()
Anonymous functions
In the Haskell example that follows the variable
c
is non-local in the anonymous function
\x -> x + c
:
outer = let c = 1 in map (\x -> x + c) , 2, 3, 4, 5
The comma is a punctuation mark that appears in several variants in different languages. It has the same shape as an apostrophe or single closing quotation mark () in many typefaces, but it differs from them in being placed on the baseline o ...
Implementation issues
Non-local variables are the primary reason it is difficult to support nested, anonymous,
higher-order and thereby
first-class function
In computer science, a programming language is said to have first-class functions if it treats functions as first-class citizens. This means the language supports passing functions as arguments to other functions, returning them as the values from ...
s in a programming language.
If the nested function or functions are (mutually)
recursive, it becomes hard for the
compiler
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 tha ...
to know exactly where on 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 mac ...
the non-local variable was allocated, as the
frame pointer only points to the local variable of the nested function itself and there can be an arbitrary number of
activation records on the stack in between. This is generally solved using
access links or
display registers.
If the nested function is passed as an argument to a higher-order function a
closure needs to be built in order to locate the non-local variables. If the nested function is returned as a result from its outer function (or stored in a variable) the non-local variables will no longer be available on the stack. They need to be heap allocated instead, and their lifetime extends beyond the lifetime of the outer function that declared and allocated them. This generally requires garbage-collection.
Notes
{{reflist
References
* Aho, Lam, Sethi, and Ullman. "7.3 Access to Nonlocal Data on the Stack". ''
Compilers: Principles, Techniques, & Tools''. Second edition.
Programming language theory
Variable (computer science)