prolog详解

Artificial Intelligence = to perceive, reason and act
– LISP/Prolog concerned mostly with reasoning – Symbolic manipulation of (model of) world
– an atom does not have a value – only one atom with any given name (no local scope, only global)
Named atoms start with lower-case letter lower– may contain letters (UPPER and lower case), numbers & underscores
Syntax of Prolog Facts
Consist of functor…
– A name of the predicate
parent mark, alex
…and arguments Arguments in (parentheses)…
– Right up against the functor
Some Predicates to Start With
?- help. ?- help(help). ?- apropos(help). ?- consult(‘family_1.pl’). ?- [‘family_1.pl’]. ?- edit(‘family_1.pl’). ?- [user]. ?- halt. % general help % help on … % help available ... % load a file % ditto % edit/reload file % enter facts interactively % exit Prolog
Exercise
Write a set of facts about your family using the second representation above.
– your parents’ family if you don’t have one
My family:
parents(mark, bob, isabel). parents(di, ralph, esther). parents(alex, mark, di). parents(zachary, mark, di).
Symbolic Programming
Most languages work with numbers & text
– Also more complicated combinations of same
Prolog/LISP work with symbols
– Also work with numbers & text – Not as suitable for numeric/textual programs
Symbols represent things in the world
– Language makes it easy to use them
Prolog
PROgrammation en LOGique
– PROgramming in LOGic
Logic model
– Program = axioms – Execution gives theorems
“no” means “don’t know of any”
?- parent(Who, isabel). no
Variable Names
Prolog doesn’t care about the names ?- parent(bob, X). X = mark yes use meaningful variable names ?- parent(bob, Child). X usually not good Child = mark yes ?- parent(alex, Anyone). no
– If there’s no such, Prolog just says no
Show Prolog’s Response…
parent(mark, alex). parent(di, alex). parent(bob, mark). parent(isabel, mark). parent(ralph, di). parent(esther, di). parent(franklin, bob). ?- parent(Who, bob).
Restricted logic – can’t say as much
Logic Model
Proof (vs. Command) (vs. Command) Process
– predicate specification – predicate application assertions questions atoms, lists & terms
Introduction to Prolog
Facts, Questions & Rules Atoms & Variables
LISP/Prolog Data
LISP/Prolog developed for AI
– LISP in late 1950s – Prolog in mid-late 1970s mid-
Asking Who Questions
Can also ask who is parent of whom
?- parent(Who, mark). Who = bob yes
Prolog pauses after saying who Who is
– Press Enter or space to signal you’re ready – (systems differ in what keys to press)
Program File
A program may consist of just facts
– family_1.pl
Want to start Prolog, assert the facts, and then start asking questions
– We want to consult the file – Windows – double-click file to start Prolog and doublefile is consulted automatically – Linux – need to use consult/1 predicate
% parent(Parent, Child) parent(mark, alex). % Mark is Alex’s parent % parents(Child, Father, Mother) parents(mark, bob, isabel). % Mark’s dad & mom are Bob And Isabel % father(Child, Parent) father(alex, mark). % Alex’s father is Mark
?- parent(ralph, Whom).
?- parent(alex, Whom).
Variables & No
“Closed world assumption”
– if it’s not in the database, it isn’t true – everything relevant to the question is known ?- parent(garvie, franklin). no
% sign starts a comment Comment continues to end of line
% Mark is Alex’s parent % Di is Alex’s parent % Bob is Mark’s parent
Prolog Atoms
mark, di, alex & bob are not variables Atoms are themselves and nothing else
Show Prolog’s Response…
parent(mark, alex). parent(di, alex). parent(bob, mark). parent(isabel, mark). parent(ralph, di). parent(esther, di). parent(franklin, bob). ?- parent(mark, alex). ?- parent(ralph, esther). ?- parent(alex, mark). ?- parent(ralph, di).
Words that start with Capital Letters are variables
– More accurately “unknowns”
If you ask a question with variables, Prolog says what values they could have to make what you wrote true
Arity of parenthood relationship is 2
– The predicate is parent/2 – May be other predicates with same name
Multiple Representations
Choose based on how info to be used
Asking Whom Questions