Prolog Notes Links to tutorials on course webpage /home/hwang/cs430/prolog
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Prolog Notes
Links to tutorials on course webpage
Example programs on csserver in
/home/hwang/cs430/prolog
Monday, April 7
CS 430 Artificial Intelligence
1
Prolog
PROgramming LOGic language, uses firstorder logic back-chaining to drive computations
Will use GNU Prolog (gprolog) for this course,
most other Prolog implementations have similar
syntax. Links available on course webpage
Identifier syntax convention is opposite FOL
Variables start with uppercase letters
Constants and predicate names start with
lowercase
Comments: % to end of line or /* */ pairs
Monday, April 7
CS 430 Artificial Intelligence
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Basic Syntax - Terms
Atoms: symbols, single-quoted sequence, builtin operators
Numbers: the usual
Variables: the usual, one special "anonymous"
variable underscore ( _ )
Structures: predicates and nested predicates,
names can be overloaded with different arity
Terms cannot have space between
predicate and '('
Monday, April 7
CS 430 Artificial Intelligence
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Basic Syntax - Built-Ins
Operators
:­
left implication ()
=
unification
,
conjunction
\=
non-unification
;
disjunction
is
expression
evaluation &
assignment
Predicates
exit the interpreter
halt
write display argument
nl
Monday, April 7
display newline
CS 430 Artificial Intelligence
4
Interpreter
Read-execute-write loop against a KB.
Prompt is: ?­
Given a query sentence, responds "yes" if can
be proved and "no" if it cannot.
All interaction is via query sentences, so
"command" to exit interpreter is:
?­ halt.
(note the period at the end of the sentence)
Monday, April 7
CS 430 Artificial Intelligence
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Consulting
Reading in a file of facts and rules is called
consulting.
Query to do this: [<filename>]. E.g.,
?­ [criminal].
Note: no quotation marks around the file name.
Also, will assume extension of ".pl"
To read from keyboard, consult the user with
[user]. Ctrl-D indicates end of input.
Note: there are no retained sentences from
previous consults.
Monday, April 7
CS 430 Artificial Intelligence
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Knowledge Bases
KB is a set of sentences.
<head> :- <term1>, <term2>, ..., <termn>.
means <term1> ∧ <term2> ∧...∧ <termn> <head>
sibling(X, Y) :- parent(P, X), parent(P, Y), X \= Y.
If no variables, sentence is called a fact
female(mary).
parent(mary, george).
Monday, April 7
CS 430 Artificial Intelligence
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Knowledge Bases
Sentences with variables are called rules.
Equivalent of a function.
May consist only of a head.
vertical(line(point(X,Y)), line(point(X,Z))).
Often, rules are "recursive"
Base case must be listed first.
Recursive case is a rule of the same name that
uses the rule on the right-hand side.
Monday, April 7
CS 430 Artificial Intelligence
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Structures
Structures are compound terms used to group
data. Examples:
owns(john, 'Wuthering Heights').
owns(john, book('Wuthering Heights', bronte)).
owns(john, book('Wuthering Heights',
author(bronte, emily))).
owns(john, book('Jane Eyre',
author(bronte, charlotte))).
Write a query that asks: "Does John own any
books by any author whose last name is
Bronte?"
Monday, April 7
CS 430 Artificial Intelligence
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Lists
Lists are enclosed in [ ] and separated by
commas. Empty list is [ ]. May be nested. E.g.,
[a, [b,c], d]
Built-in list functions like member(Elem,List)
and append(List1,List2,Result)
Manipulated by matching head (first element)
and tail (rest of elements) using [H|T] notation
Write function rules to answer queries:
"What is the last element of a list?"
"What is the length of a list?"
Monday, April 7
CS 430 Artificial Intelligence
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Arithmetic
Operators
X =:= Y
X =\= Y
X =< Y
<, >, >=
+, -, *, /
//
mod
X and Y stand for the same number
X and Y stand for different numbers
X less than or equal to
usual meanings
usual meanings
integer quotient
integer remainder
Write function rules for Minimum. Write
Population and Area facts, and a function to
compute Density
Monday, April 7
CS 430 Artificial Intelligence
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Debugging
Can turn on the debugger for tracing using
Default is single-stepping using command 'c'
(for creep). Command 'a' to abort run.
Turn off tracing using
?­ trace.
?­ notrace.
See gprolog manual for setting breakpoints,
etc.
Monday, April 7
CS 430 Artificial Intelligence
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Backtracking
When a goal fails (or user requests alternative
answers), Prolog backtracks up proof tree to
consider other possibilities for unification
Sometimes don't want this to happen
Know there's only one match
Know if get to a particular place in proof tree, query
will fail
Only want one result
Control backtracking with cut (!) operator
Monday, April 7
CS 430 Artificial Intelligence
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Cut (!)
Cut (!) operator is a built-in term
Effect of cut is:
Any variables that are bound to values at this point
cannot take on other values
No other versions of predicates called before the
cut will be considered
No other subsequent version of the predicate at the
head of the current rule will be considered
The cut always succeeds.
Monday, April 7
CS 430 Artificial Intelligence
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Prolog Problems
Compute letter grades
Compute sum of 1 to N
Use cut for efficiency
Use cut to produce one answer; prevents infinite
recursion when there is a request for backtracking.
Display instructions for Tower of Hanoi
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