Prolog A Tutorial Introduction - Courses

Resolution: replace edge(a,SomeNode) by nothing (since we unified with a fact) and apply the substitution
above to get the new query:
edge(b,Node2)
3. There is only one atom in the query.
Unify
edge(b,Node2) and edge(b,d),
giving the substitution
Node2 = d.
Resolution: replace edge(b,Node2) by nothing (since we unified with a fact). Since the resulting query
is empty, we are done.
At this point the substitution can be determined. At this point it is a good idea to pause to observe what
has happened with the parameters of the atoms involved in the computation. In our original query we had
one constant and one variable. If you consider carefully, you may see that the constant parameter has been
passed to each successive atom as a value, while the variable X has been passed a reference. Notice that at no
point has there been anything like a traditional computation – just the unification to match up arguments
(pass parameters?) and the resolution to reform the query.
Backtracking
We are actually not finished with the example. You may have noticed that there is another solution: we could
redo the computation above and get the substitution X = b or X = c or X = d. The way Prolog does this
is, when a query is finally reduced to the empty query, Prolog backtracks to the most recent unification to
determine whether there is another fact or rule with which unification can succeed. If there is, an additional
solution may be found. Backtracking continues until all possible answers are determined.
Problem 3
1. Consider the problem of backtracking in response to the query ‘?- tedge(a,X)’. Assume the graph
of Example 3.3. Write out the steps of the unification/resolution above and clearly mark any place
where more than one rule or fact must be checked for unification (remember that each one may lead
to another solution). Then continue the unification/resolution process via backtracking to determine
all possible answers to the query.
2. (Extra Credit!) Your family tree — Probably the most common first example of a logic program is
that involving a family tree. Create a set of facts that define for your immediate family the relations:
male = { The males in your family. }
female = { The females in your family. }
parent_of = { The pairs (x,y) where x is the parent of y
in your family. }
[Don’t go overboard if you have a large family — you don’t want to be typing data till next week!]
Note that the predicates male and female are unary, involving only one argument. You need one
entry for each person in your family. Next define the following predicates based on the previous three
predicates.
father, mother, son, daughter, grandfather,
aunt, uncle, cousin, ancestor
CSCI 208 10 Prolog Tutorial