<artificial intelligence, programming, language> A
declarative, relational style of programming based on
first-order logic. The original logic programming language
was Prolog. The concept is based on Horn clauses.
The programmer writes a "database" of "facts", e.g.
wet(water).
("water is wet") and "rules", e.g.
mortal(X) :- human(X).
("X is mortal is implied by X is human"). Facts and rules are
collectively known as "clauses".
The user supplies a "goal" which the system attempts to
prove using "resolution" or "backward chaining". This
involves matching the current goal against each fact or the
left hand side of each rule using "unification". If the
goal matches a fact, the goal succeeds; if it matches a rule
then the process recurses, taking each sub-goal on the right
hand side of the rule as the current goal. If all sub-goals
succeed then the rule succeeds.
Each time a possible clause is chosen, a "choice point" is
created on a stack. If subsequent resolution fails then
control eventually returns to the choice point and subsequent
clauses are tried. This is known as "backtracking".
Clauses may contain logic variables which take on any value
necessary to make the fact or the left hand side of the rule
match a goal. Unification binds these variables to the
corresponding subterms of the goal. Such bindings are
associated with the choice point at which the clause was
chosen and are undone when backtracking reaches that choice
point.
The user is informed of the success or failure of his first
goal and if it succeeds and contains variables he is told what
values of those variables caused it to succeed. He can then
ask for alternative solutions.
