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The routine <code>extend</code> referenced in the example above is a feature of a class in a [[graphical user interface]] (GUI) library to provide [[event-driven programming]] capabilities.
In other library classes, agents are seen to be used for different purposes. In a library supporting data structures, for example, a class modeling linear structures effects [[universal quantification]] with a function <code>for_all</code> of type <code>BOOLEAN</code> that accepts an agent, an instance of <code>FUNCTION</code>, as an argument. So, in the following example, <code>my_action</code> is executed only if all members of <code>my_list</code> contain the character '!':
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Since there is no standard way to make funcallable objects in Common Lisp, we fake it by defining a [[generic function]] called FUNCTOR-CALL. This can be specialized for any class whatsoever. The standard FUNCALL function is not generic; it only takes function objects.
It is this FUNCTOR-CALL generic function that gives us function objects, which are ''a computer programming construct allowing an object to be invoked or called as if it were an ordinary function, usually with the same syntax.'' We have ''almost'' the same syntax: FUNCTOR-CALL instead of FUNCALL. Some Lisps provide ''funcallable'' objects as a simple extension. Making objects callable using the same syntax as functions is a fairly trivial business. Making a function call operator work with different kinds of ''function things'', whether they be class objects or closures is no more complicated than making a + operator that works with different kinds of numbers, such as integers, reals or complex numbers.
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Scheme makes closures even simpler, and Scheme code tends to use such [[higher-order programming]] somewhat more idiomatically.
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* [http://www.newty.de/fpt/index.html The Function Pointer Tutorials] by Lars Haendel (2000/2001)
* Article "[https://web.archive.org/web/20041009232434/http://www.cuj.com/documents/s%3D8464/cujcexp0308sutter/ Generalized Function Pointers]" by [[Herb Sutter]]
* [
* [https://web.archive.org/web/20100330073950/http://www.amcgowan.ca/blog/computer-science/php-functors-function-objects-in-php/ PHP Functors - Function Objects in PHP]
* [https://web.archive.org/web/20041013202445/http://www.parashift.com/c++-faq-lite/pointers-to-members.html#faq-33.10 What the heck is a functionoid, and why would I use one?] (C++ FAQ)
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