Turbo Prolog

Finally, consider execution of the goal
long_novel (X).
When Turbo Prolog tries to fulfill a goal, it investigates whether or not there exists a
matching fact or left side of a rule. In this case, the match is with the left side of a rule
long_novel( X )
long_novel(Title):written_by(_,book(Title,Length)),Length>300.
since the free variable X can be unified with any other term and, in particular, another
free variable. Next, Turbo Prolog makes the first clause on the right side of this rule the
current sub-goal, and unification is achieved with the first fact for written-1Jyas follows:
written_by( Name ,book(Title ,Length))
written_by(fleming,book("DR NOli, 210))
in which Length has become bound to 210.
Now the second clause on the right side of the longJJovel rule becomes the current
sub-goal
Length > 300
Before unification is attempted, the bound variable Length is replaced with the value to
which it is bound, 210. Since
210 > 300
is a legal comparison of two integer values, the comparison is made-and, of course,
returns false. Turbo Prolog now attempts a different unification of
written_by(Name,book(Title,Length))
(see the next section) and binds Title to "MOBY DICK" and Length to 600. Now
Length > 300
unifies with Length replaced by 600 (the value to which it is bound) and indeed succeeds,
so that longJJovel also succeeds with Title bound to "MOBY DICK". Turbo
Prolog displays
X = "MOBY DICK"
1 Solution
Summary of Turbo Prolog's Unification Algorithm
• A free variable can be unified with any term. The variable is then bound to the other
term.
• A constant (an integer, for example) can unify with itself or with a free variable.
• A compound term can unify with another compound term, provided they both
involve the same functor and have the same number of arguments. Further, all the
subterms must unify pairwise. (Lists are treated as a special kind of compound term).
Tutorial III: Turbo Prolog's Relentless Search for Solutions 53