More on Prolog
Simple program
dog (fido).
--fact
spotted (fido).
--fact
dalmation (X) :- dog(X), spotted(X).
--rule
?- dalmation (fido).
-- query
▫ answers Yes.
leopard (leo).
--new fact
spotted (leo).
--new fact
?-dalmation (leo).
--new query
-answers Yes.
• Want to know all spotted? That is, “For what X is X
spotted?”
• Query
?-spotted(X).
• Answer
X=fido.
X=leo.
No.
F8
F8
Basic Elements of Prolog
• Prolog statements constructed from terms
• Term – constant, variable or structure
▫ constant
 either atom (begin with lowercase) or an integer
▫ variable (begin with uppercase)
 not bound by declarations to types
 binding of value (thus type) to a var is an instantiation
and occurs during the resolution process
 instantiations last only as long as it takes to establish a
goal
 NOT like vars in imperative languages
▫ structure – represent propositions of predicate
calculus
 functor (parm list)
atom
list of atoms, vars, or other structures
 specify facts and rules (difference indicated by 2 modes)
 specify a predicate when a query
 responses: yes – proved goal was true
no – proved false or unable to prove true
Inferencing Process (Resolution)
• Queries are called goals
▫ when goal is compound proposition, each of structures
is a subgoal
▫ To prove goal is true, process must use chain of
inference rules that connect goal to 1 or more facts.
Example: Q is a goal
Q must be fact or there must be a seq of propositions
p1, p2, … pn such that
p1 => p2, p2 => p3, … pn => Q
and
p1 is a fact.
2 Kinds of resolutions
• bottom-up (forward chaining)
▫ When you have a large # possibly correct answers
▫ begin with facts and rules and attempt to find a Q that
matches that to the goal
• top-down (backward chaining)
▫ small set of candidate answers
▫ begin with goal and work backwards to set of facts
▫ implementations that use this use a depth-first search
and backtracking
Simple Arithmetic
• supported (orig. arith ops were functors)
• all variables except LHS variable must be
instantiated.
▫ A is B/17 + C
▫ illegal: sum is sum + number
 because RHS sum is instantiated
 is is not exactly like := in C++
• most Prolog programmers don’t need them
speed(ford, 100).
speed(chvy, 105).
speed(dodge, 95).
time(ford, 20).
time(chevy, 21).
time(dodge, 24).
distance(X,Y) :- speed(X, S),
time(X, T),
Y is S* T.
?-distance(chevy, Chevy_Distance),
write(Chevy_Distance), nl.
response 2205
yes.
How did it work?
indicates
subgoal to
be matched
(1)
(2)
(2)
(3)
(3)
(4)
(4)
(5)
call
depth
action
1 Call:
distance(chevy, _0)?
2 Call:
speed (chevy, _5)?
2 Exit:
speed(chevy, 105)
2 Call:
time(chevy, _6)?
2 Exit:
time(chevy, 21)
2 Call:
_0 is 105 * 21 ?
2 Exit:
2205 is 105 * 21
1 Exit:
distance (chevy, 2205)
Chevy_Distance = 2205
_0 internal var
actions – call, fail, exit, redo
( success)
call
fail
exit
redo
fail
call
redo
exit
Study Question - Traces
likes (jake, chocolate).
likes(jake, apricots).
likes(darcie, licorice).
likes(darcie, apricots).
trace
trace
likes(jake, X), likes(darcie, X).
likes(X, apricots).