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PROLOG Edward S. Blurock
PROLOG Edward S. Blurock
Rules John likes all people Could list all people - - PDF document
Rules John likes all people Could list all people - - PDF document
PROLOG Edward S. Blurock Rules John likes all people Could list all people likes(john,alfred). likes(john,bertrand). likes(john,charles). likes(john,david). . . . Rule more Compact John likes any object provided it is a person PROLOG
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PROLOG Edward S. Blurock
Formulating Rules
John likes anyone who likes wine John likes any something if it likes wine John likes X if X likes wine
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PROLOG Edward S. Blurock
Rule Syntax
likes(john,X) :- likes(X,wine). head body rule delimitor
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PROLOG Edward S. Blurock
Variable Scope
likes(john,X) :- likes(X,wine), likes(X,food). X’s Within Scope of Rule instantiated here checked here returned here
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PROLOG Edward S. Blurock
Royal Parents
Predicate The parents of X are Y and Z Y is the mother Z is the father Database male(albert). male(edward). female(alice). female(victoria). parents(edward,victoria,albert). parents(alice,victoria,albert).
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PROLOG Edward S. Blurock
Sisters
X is a sister of Y if: X is female X has mother M and father F Y has mother M and father F Rule sisters of(X,Y) :- female(X), parents(X,M,F), parents(Y,M,F).
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PROLOG Edward S. Blurock
Scope
sister_of(X,Y) :- female(X), parents(X,M,F), parents(Y,M,F). Scope
- nly within body
- f rule
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PROLOG Edward S. Blurock
Sisters Question
sister_of(X,Y) :- female(X), parents(X,M,F), parents(Y,M,F).
Rule Question
Y = edward X = alice
female(alice), parents(alice,M,F), parents(edward,M,F).
New Goal
sister_of(X,Y) sister_of(alice,edward).
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PROLOG Edward S. Blurock
Sisters Question
female(alice), parents(alice,M,F), parents(edward,M,F).
New Goal
Database
yes next goal M = victoria, F = albert parents(alice,victoria,albert) next goal
parents(alice,M,F)
found yes, goal satisfied
parents(edward,victoria,albert) female(alice)
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PROLOG Edward S. Blurock
Who is the Sister?
sister_of(X,Y) :- female(X), parents(X,M,F), parents(Y,M,F).
Rule Question
Y = X (toplevel) X = alice
female(alice), parents(alice,M,F), parents(X,M,F).
New Goal
sister_of(X,Y) sister_of(alice,X).
Different X’s uninstantiated variable
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PROLOG Edward S. Blurock
Who is the Sister?
female(alice), parents(alice,M,F), parents(X,M,F).
New Goal
Database
yes next goal M = victoria, F = albert parents(alice,victoria,albert) next goal
parents(alice,M,F)
X = edward
parents(X,victoria,albert) female(alice)
parents(edward,victoria,albert) 1 2 3
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PROLOG Edward S. Blurock
Stealing
The Rule A person may steal something if the person is a thief and he likes the thing Prolog Rule may steal(P,T) :- thief(P),likes(P,T).
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PROLOG Edward S. Blurock
Example
[sun:examples!7] cat thief.pro thief(john). likes(mary,food). likes(mary,wine). likes(john,X) :- likes(X,wine). may_steal(X,Y) :- thief(X),likes(X,Y). /software/prolog/sicstus/bin/sicstus SICStus 2.1 #8:MET DST 1995 | ?- [’thief.pro’]. {consulting thief.pro...} {thief.pro consulted,0 msec 1136 bytes} yes | ?- may_steal(john,X). X = mary ? ; no | ?- halt. [sun:examples!9]
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PROLOG Edward S. Blurock
Trace and Debug
/software/prolog/sicstus/bin/sicstus SICStus 2.1 #8: DST 1995 | ?- debug. {The debugger will first leap -- showing spypoints (debug)} yes {debug} | ?- trace. {The debugger will first creep -- showing everything (trace)} {trace}
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PROLOG Edward S. Blurock
Function Trace
| ?- may_steal(john,X). + 1 1 Call: may_steal(john,_67) ? + 2 2 Call: thief(john) ? + 2 2 Exit: thief(john) ? + 3 2 Call: likes(john,_67) ? + 4 3 Call: likes(_67,wine) ? + 4 3 Exit: likes(mary,wine) ? + 3 2 Exit: likes(john,mary) ? + 1 1 Exit: may_steal(john,mary) ? X = mary ? ;
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PROLOG Edward S. Blurock