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Callback (computer programming)
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<p>In <a href="/facts/Computer_programming/QmSsFnu0">computer programming</a>, a callback is a <a href="/facts/Function_(computer_programming)/9vFJrg0A">function</a> that is stored as data (a <a href="/facts/Reference_(computer_science)/1vm2Bph1">reference</a>) and designed to be called by another function – often <i>back</i> to the original <a href="/facts/Abstraction_(computer_science)/hAnxmJle">abstraction layer</a>. </p><p>A function that accepts a callback <a href="/facts/Parameter_(computer_programming)/LcuLbbSq">parameter</a> may be designed to call back before <a href="/facts/Return_statement/lK7c71aa">returning</a> to its caller which is known as <i><a href="/facts/Synchronization_(computer_science)/n7ID76uM">synchronous</a></i> or <i>blocking</i>. 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 <i>asynchronous</i>, <i><a href="/facts/Non-blocking_algorithm/KhLDma0v">non-blocking</a></i> or <i>deferred</i>. </p><p><a href="/facts/Programming_languages/cWTYbgWa">Programming languages</a> support callbacks in different ways such as <a href="/facts/Function_pointers/PEhvvsQB">function pointers</a>, <a href="/facts/Lambda_(programming)/2vdyE4zL">lambda expressions</a> and <a href="/facts/Block_(programming)/E6yG3dnJ">blocks</a>. </p><p>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. </p><p>The term <i>callback</i> can be misleading, as it does not necessarily imply a return to the original caller, unlike a <a href="/facts/Callback_(telecommunications)/KPWus3Wa">telephone callback</a>. </p><p>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. </p>