Editing Logic programming (section)

===Constraint logic programming===
{{Main|Constraint logic programming}}
[[Constraint logic programming]] (CLP) combines Horn clause logic programming with [[constraint solving]]. It extends Horn clauses by allowing some predicates, declared as constraint predicates, to occur as literals in the body of a clause. Constraint predicates are not defined by the facts and rules in the program, but are predefined by some domain-specific model-theoretic structure or theory.

Procedurally, subgoals whose predicates are defined by the program are solved by goal-reduction, as in ordinary logic programming, but constraints are simplified and checked for satisfiability by a domain-specific constraint-solver, which implements the semantics of the constraint predicates. An initial problem is solved by reducing it to a satisfiable conjunction of constraints.

Interestingly, the first version of Prolog already included a constraint predicate dif(term1, term2), from Philippe Roussel's 1972 PhD thesis, which succeeds if both of its arguments are different terms, but which is delayed if either of the terms contains a variable.<ref name=":02"/>

The following constraint logic program represents a toy temporal database of <code>john's</code> history as a teacher:
<syntaxhighlight lang="prolog">
teaches(john, hardware, T) :- 1990 ≤ T, T < 1999.
teaches(john, software, T) :- 1999 ≤ T, T < 2005.
teaches(john, logic, T) :- 2005 ≤ T, T ≤ 2012.
rank(john, instructor, T) :- 1990 ≤ T, T < 2010.
rank(john, professor, T) :- 2010 ≤ T, T < 2014.
</syntaxhighlight>
Here <code>≤</code> and <code><</code> are constraint predicates, with their usual intended semantics. The following goal clause queries the database to find out when <code>john</code> both taught <code>logic</code> and was a <code>professor</code>:
<syntaxhighlight lang="prolog">
?- teaches(john, logic, T), rank(john, professor, T).
</syntaxhighlight>
The solution 
<code>
2010 ≤ T, T ≤ 2012
</code>
results from simplifying the constraints
<code>
2005 ≤ T, T ≤ 2012, 2010 ≤ T, T < 2014.
</code>

Constraint logic programming has been used to solve problems in such fields as [[civil engineering]], [[mechanical engineering]], [[digital circuit]] verification, [[automated timetabling]], [[air traffic control]], and finance. It is closely related to [[abductive logic programming]].