Editing Lisp (programming language) (section)

==Language innovations==
[[Paul Graham (programmer)|Paul Graham]] identifies nine important aspects of Lisp that distinguished it from existing languages like [[Fortran]]:<ref>{{Cite web |title=What Made Lisp Different |date=May 2002 |url=http://paulgraham.com/diff.html |author=Paul Graham}}</ref>
* [[Conditional (computer programming)|Conditionals]] not limited to [[goto]]
* [[First-class function]]s
* [[Recursion]]
* Treating variables uniformly as [[Pointer (computer programming)|pointers]], leaving types to values
* [[Garbage collection (computer science)|Garbage collection]]
* Programs made entirely of [[Expression (computer science)|expressions]] with no [[Statement (computer science)|statements]]
* The [[Symbol (programming)|symbol]] data type, distinct from the [[String (computer science)|string]] data type
* Notation for code made of trees of symbols (using many [[parentheses]])
* Full language available at [[load time]], [[compile time]], and [[Runtime (program lifecycle phase)|run time]]

Lisp was the first language where the structure of program code is represented faithfully and directly in a standard data structure—a quality much later dubbed "[[homoiconicity]]". Thus, Lisp functions can be manipulated, altered or even created within a Lisp program without lower-level manipulations. This is generally considered one of the main advantages of the language with regard to its expressive power, and makes the language suitable for syntactic macros and [[meta-circular evaluator|meta-circular evaluation]].

A conditional using an ''[[if–then–else]]'' syntax was invented by McCarthy for a chess program written in [[Fortran]]. He proposed its inclusion in [[ALGOL]], but it was not made part of the [[Algol 58]] specification. For Lisp, McCarthy used the more general ''cond''-structure.<ref name="bUXap">{{cite web|url=http://www-formal.stanford.edu/jmc/history/lisp/node2.html|title=LISP prehistory - Summer 1956 through Summer 1958.|quote=I invented conditional expressions in connection with a set of chess legal move routines I wrote in FORTRAN for the IBM 704 at M.I.T. during 1957–58&nbsp;... A paper defining conditional expressions and proposing their use in Algol was sent to the Communications of the ACM but was arbitrarily demoted to a letter to the editor, because it was very short.}}</ref> [[Algol 60]] took up ''if–then–else'' and popularized it.

Lisp deeply influenced [[Alan Kay]], the leader of the research team that developed [[Smalltalk]] at [[Xerox PARC]]; and in turn Lisp was influenced by Smalltalk, with later dialects adopting object-oriented programming features (inheritance classes, encapsulating instances, message passing, etc.) in the 1970s. The [[Flavors (programming language)|Flavors]] object system introduced the concept of [[multiple inheritance]] and the [[mixin]]. The [[Common Lisp Object System]] provides multiple inheritance, multimethods with [[multiple dispatch]], and first-class [[generic functions]], yielding a flexible and powerful form of [[dynamic dispatch]]. It has served as the template for many subsequent Lisp (including [[Scheme (programming language)|Scheme]]) object systems, which are often implemented via a [[Metaobject#Metaobject Protocol|metaobject protocol]], a [[Reflective programming|reflective]] [[Meta-circular evaluator|meta-circular design]] in which the object system is defined in terms of itself: Lisp was only the second language after Smalltalk (and is still one of the very few languages) to possess such a metaobject system. Many years later, Alan Kay suggested that as a result of the confluence of these features, only Smalltalk and Lisp could be regarded as properly conceived object-oriented programming systems.<ref name="4I08d">{{cite web|date=2003-07-23|url=http://userpage.fu-berlin.de/~ram/pub/pub_jf47ht81Ht/doc_kay_oop_en|title=Meaning of 'Object-Oriented Programming' According to Dr. Alan Kay|quote=I didn't understand the monster LISP idea of tangible metalanguage then, but got kind of close with ideas about extensible languages&nbsp;... The second phase of this was to finally understand LISP and then using this understanding to make much nicer and smaller and more powerful and more late bound understructures&nbsp;... OOP to me means only messaging, local retention and protection and hiding of state-process, and extreme late-binding of all things. It can be done in Smalltalk and in LISP. There are possibly other systems in which this is possible, but I'm not aware of them.}}</ref>

Lisp introduced the concept of [[Garbage collection (computer science)|automatic garbage collection]], in which the system walks the [[Heap (memory management)|heap]] looking for unused memory. Progress in modern sophisticated garbage collection algorithms such as generational garbage collection was stimulated by its use in Lisp.<ref name="dpvwj">{{citation |last1=Lieberman |first1=Henry |last2=Hewitt |first2=Carl |title=A Real-Time Garbage Collector Based on the Lifetimes of Objects |url=http://web.media.mit.edu/~lieber/Lieberary/GC/Realtime/Realtime.html |journal=Communications of the ACM |volume=26 |issue=6 |date=June 1983 |pages=419–429 |doi=10.1145/358141.358147|hdl=1721.1/6335 |citeseerx=10.1.1.4.8633 |s2cid=14161480}}</ref>

[[Edsger W. Dijkstra]] in his 1972 [[Turing Award]] lecture said,
{{Blockquote|With a few very basic principles at its foundation, it [LISP] has shown a remarkable stability. Besides that, LISP has been the carrier for a considerable number of in a sense our most sophisticated computer applications. LISP has jokingly been described as "the most intelligent way to misuse a computer". I think that description a great compliment because it transmits the full flavour of liberation: it has assisted a number of our most gifted fellow humans in thinking previously impossible thoughts.<ref name="v8uNE">{{citation|url=http://www.cs.utexas.edu/~EWD/transcriptions/EWD03xx/EWD340.html|author=Edsger W. Dijkstra|year=1972|title=The Humble Programmer (EWD 340)}} (ACM Turing Award lecture).</ref>}}

Largely because of its resource requirements with respect to early computing hardware (including early microprocessors), Lisp did not become as popular outside of the [[AI]] community as [[Fortran]] and the [[ALGOL]]-descended [[C (programming language)|C]] language. Because of its suitability to complex and dynamic applications, Lisp enjoyed some resurgence of popular interest in the 2010s.<ref name="chI3V">{{cite web|url=https://www.linkedin.com/pulse/look-clojure-lisp-resurgence-todd-towles|title=A Look at Clojure and the Lisp Resurgence}}</ref>