Editing Callback (computer programming) (section)

{{Short description|A function passed as data to and to be called by another function}}
{{For|a discussion of callback with computer [[modem]]s|Callback (telecommunications)}}
{{More references|date=September 2015}}
[[File:Callback-notitle.svg|thumb|370px|A callback is often back on the level of the original caller.]]

In [[computer programming]], a '''callback''' is a [[Function (computer programming)|function]] that is stored as data (a [[Reference (computer science)|reference]]) and designed to be called by another function {{endash}} often ''back'' to the original [[Abstraction (computer science)|abstraction layer]].

A function that accepts a callback [[Parameter (computer programming)|parameter]] may be designed to call back before [[Return statement|returning]] to its caller which is known as ''[[Synchronization (computer science)|synchronous]]'' or ''blocking''. The function that accepts a callback may be designed to store the callback so that it can be called back after returning which is known as ''asynchronous'', ''[[Non-blocking algorithm|non-blocking]]'' or ''deferred''.

[[Programming languages]] support callbacks in different ways such as [[function pointers]], [[Lambda (programming)|lambda expressions]] and [[block (programming)|blocks]].

A callback can be likened to leaving instructions with a tailor for what to do when a suit is ready, such as calling a specific phone number or delivering it to a given address. These instructions represent a callback: a function provided in advance to be executed later, often by a different part of the system and not necessarily by the one that received it.

The term ''callback'' can be misleading, as it does not necessarily imply a return to the original caller, unlike a [[callback (telecommunications)|telephone callback]].

Mesa programming language formalised the callback mechanism used in Programming Languages. By passing a procedure as a parameter, Mesa essentially delegated the execution of that procedure to a later point in time when a specific event occurred, similar to how callbacks are implemented in modern programming languages.<ref>{{Cite journal |last=E. Sweet |first=Richard |title=The Mesa Programming Environment |url=https://dl.acm.org/doi/pdf/10.1145/17919.806843 |journal=ACM |issue=ACM 0-89791-165-2/85/006/0216}}</ref>