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

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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)
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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)

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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

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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)