Non-local variable: Difference between revisions

Revision as of 11:29, 16 September 2011

Revision as of 12:06, 16 September 2011

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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 function|nested]] and [[anonymous functionsfunction]]s where can some variable can be neither in the [[local scope|local]] nor in the [[global scope]].

== Example ==

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== SeeImplementation alsoissues ==

Non-local variables are the primary reason it is difficult to support nested, anonymous, [[higher-order function|higher-order]] and thereby [[first-class function]]s in a programming language.

* [[Closure (computer science)]]

* [[First-class function]]

If the nested function or functions are (mutually) [[recursive]], it becomes hard for the [[compiler]] to know exactly where on the [[stack]] 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 record]]s on the stack in between. This is generally solved using [[access link]]s or [[display register]]s.

If the nested function is passed as an argument to a higher-order function a [[closure (computer science)|closure]] needs to be build 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 extend beyond the lifetime of the outer function that declared and allocated them. This generally requires garbage-collection.

== References ==

* Aho, Lam, Sethi, and Ullman. "7.3 Access to Nonlocal Data on the Stack". ''[[Compilers: Principles, Techniques, & Tools]]''. Second edition.

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