The.Algorithm.Design.Manual.Springer-Verlag.1998

War Story: What's Past is Prolog
Next: War Story: Text Compression Up: Breaking Problems Down Previous: War Story: Evolution of
War Story: What's Past is Prolog
``But our heuristic works very, very well in practice.'' He was simultaneously boasting and crying for
help.
Unification is the basic computational mechanism in logic programming languages like Prolog. A Prolog
program consists of a set of rules, where each rule has a head and an associated action whenever the rule
head matches or unifies with the current computation.
An execution of a Prolog program starts by specifying a goal, say p(a,X,Y), where a is a constant and X
and Y are variables. The system then systematically matches the head of the goal with the head of each of
the rules that can be unified with the goal. Unification means binding the variables with the constants, if
it is possible to match them. For the nonsense program below, p(X,Y,a) unifies with either of the first two
rules, since X and Y can be bound to match the extra characters. The goal p(X,X,a) would only match the
first rule, since the variable bound to the first and second positions must be the same.
p(a,a,a) := h(a);
p(b,a,a) := h(a) * h(b);
p(c,b,b) := h(b) + h(c);
p(d,b,b) := h(d) + h(b);
file:///E|/BOOK/BOOK2/NODE51.HTM (1 of 4) [19/1/2003 1:28:54]