Editing Prolog (section)

=== Loops and recursion ===
Iterative algorithms can be implemented by means of recursive predicates.<ref>{{cite book |last=Carlsson |first=Mats |url=https://books.google.com/books?id=dZimAwAAQBAJ&q=prolog%20failure%20driven%20loop%20%22iteration%22&pg=PA148 |title=SICStus Prolog User's Manual 4.3: Core reference documentation |date=27 May 2014 |publisher=BoD – Books on Demand |isbn=978-3-7357-3744-1 |via=Google Books}}</ref>

Consider the <code>parent_child/2</code> predicate defined in the family relation program above. The following Prolog program defines the ''ancestor'' relation:<syntaxhighlight lang="prolog">
ancestor(X, Y) :- parent_child(X, Y).
ancestor(X, Y) :- parent_child(X, Z), ancestor(Z, Y).
</syntaxhighlight>It expresses that X is an ancestor of Y if X is parent of Y or X is parent of an ancestor of Y.  It is recursive because it is defined in terms of itself (there is a call to predicate <code>ancestor/2</code> in the body of the second clause).