Editing Declarative programming (section)

===Prolog===
[[Prolog]] (1972) stands for "PROgramming in LOGic." It was developed for natural language [[question answering]],<ref name="PrologHistory">{{cite web
  | url = http://alain.colmerauer.free.fr/alcol/ArchivesPublications/PrologHistory/19november92.pdf
  | title = Birth of Prolog
  | date = November 1992
  | access-date = 2022-05-25
  | archive-date = 2015-04-02
  | archive-url = https://web.archive.org/web/20150402111123/http://alain.colmerauer.free.fr/alcol/ArchivesPublications/PrologHistory/19november92.pdf
  | url-status = live
  }}</ref> using SL resolution<ref>{{cite journal |author1=Robert Kowalski |author2=Donald Kuehner |url=http://www.doc.ic.ac.uk/~rak/papers/sl.pdf |title=Linear Resolution with Selection Function |journal=Artificial Intelligence |issn=0004-3702 |volume=2 |issue=3–4 |date=Winter 1971 |pages=227–260 |doi=10.1016/0004-3702(71)90012-9 |access-date=2023-08-13 |archive-date=2015-09-23 |archive-url=https://web.archive.org/web/20150923215814/http://www.doc.ic.ac.uk/~rak/papers/sl.pdf |url-status=live }}</ref> both to deduce answers to queries and to parse and generate natural language sentences.

The building blocks of a Prolog program are ''facts'' and ''rules''. Here is a simple example:
<syntaxhighlight lang=prolog>
cat(tom).                        % tom is a cat
mouse(jerry).                    % jerry is a mouse

animal(X) :- cat(X).             % each cat is an animal
animal(X) :- mouse(X).           % each mouse is an animal

big(X)   :- cat(X).              % each cat is big
small(X) :- mouse(X).            % each mouse is small

eat(X,Y) :- mouse(X), cheese(Y). % each mouse eats each cheese
eat(X,Y) :- big(X),   small(Y).  % each big being eats each small being
</syntaxhighlight>

Given this program, the query <syntaxhighlight inline lang=prolog>eat(tom,jerry)</syntaxhighlight> succeeds, while <syntaxhighlight inline lang=prolog>eat(jerry,tom)</syntaxhighlight> fails. Moreover, the query <syntaxhighlight inline lang=prolog>eat(X,jerry)</syntaxhighlight> succeeds with the answer substitution <syntaxhighlight inline lang=prolog>X=tom</syntaxhighlight>.

Prolog executes programs top-down, using [[SLD resolution]] to [[backward chaining |reason backwards]], reducing goals to subgoals. In this example, it uses the last rule of the program to reduce the goal of answering the query <syntaxhighlight inline lang=prolog>eat(X,jerry)</syntaxhighlight> to the subgoals of first finding an X such that <syntaxhighlight inline lang=prolog>big(X)</syntaxhighlight> holds and then of showing that <syntaxhighlight inline lang=prolog>small(jerry)</syntaxhighlight> holds. It repeatedly uses rules to further reduce subgoals to other subgoals, until it eventually succeeds in [[Unification (computer science)#Application: unification in logic programming |unifying]] all subgoals with facts in the program. This backward reasoning, goal-reduction strategy treats rules in logic programs as procedures, and makes Prolog both a declarative and [[procedural programming#Logic programming |procedural programming]] language.<ref>Robert Kowalski [http://www.doc.ic.ac.uk/~rak/papers/IFIP%2074.pdf Predicate Logic as a Programming Language] {{Webarchive|url=https://web.archive.org/web/20160207012437/http://www.doc.ic.ac.uk/~rak/papers/IFIP%2074.pdf |date=2016-02-07 }} Memo 70, Department of Artificial Intelligence, University of Edinburgh.  1973. Also in Proceedings IFIP Congress, Stockholm, North Holland Publishing Co., 1974, pp. 569-574.</ref>

The broad range of Prolog applications is highlighted in the Year of Prolog Book,<ref name="Prolog Book">{{cite book |last1=Warren |first1=D.S. |editor-last1=Warren |editor-first1=D.S. |editor-last2=Dahl |editor-first2=V. |editor-last3=Eiter |editor-first3=T. |editor-last4=Hermenegildo |editor-first4=M.V. |editor-last5=Kowalski |editor-first5=R. |editor-last6=Rossi |editor-first6=F. |chapter=Introduction to Prolog |title=Prolog: The Next 50 Years |series=Lecture Notes in Computer Science() |date=2023 |volume=13900 |publisher=Springer, Cham. |doi=10.1007/978-3-031-35254-6_1 |pages=3–19|isbn=978-3-031-35253-9 }}</ref> celebrating the 50 year anniversary of Prolog.