ICS 141 Klefstad

Logic Programming Languages

Outline

Introduction

declarative
- specify what not how
a logic program declares
- facts
- rules of inference
the language processor
- given a problem
- infer a solution

EG prime number test

isPrime(X) :- X>1, 1<J<X, not X mod J /= 0.

EG sorting

sort(X, Y) :- permutation(X, Y), isOrdered(Y).

Examples of nonprocedurality in Languages

Setl

based on set theory
predicate calculus

{x in X| odd(x)}
-- subset of X which are odd
exists x in X|Pred(x)
-- true if there exists an element in X which satisfies Pred
[x:x in [2..100]|not exists y in [2..x-1]|x mod y=0]
-- the tuple of all primes less than 100

YACC

a parser generator

E : E '+' E ;
E : E '*' E ;
E : Identifier ;

Lex

a lexical analyzer generator

alpha [a-z]
numeric [0-9]
%%
{alpha}({alpha}|{numeric})*  { return Identifier;}

Snobol4

a string-oriented language which uses pattern-matching

ws = ' '
subject = 'I' | 'We'
verb = 'see' | 'hit' | 'grab'
object = 'him' | 'her' | 'you'
sentence = subject ws verb ws object ws '.'
input = We see you.
input sentence :S(found)F(notfound)

Prolog

the most successful logic programming language
a Prolog program declares relationships believed to be true
the relationships may be specific facts or rules of inference

Terms

a Prolog program is a list of sentences
sentences are either facts or rules of inference
sentences are made up of terms

a term is either
- a constant
  - an atom: father, mother, bart
  - an integer: 10, 1000
- a variable: X, Y, Age
- a structure: father(homer, bart)
  - father is the functor
  - (homer, bart) is the component list
- more complex example
```
tvShow(theSimpsons, on(fox, date(sunday), time(8,00)))
```

Sentences

a sentence of one of the following forms:
- term. % term is true
- term :- term1, term2. % term is implied by term1 and term2
- term :- term1; term2. % term is implied by term1 or term2

facts

mother(marge,bart).
father(homer,bart).
mother(marge,lisa).
father(homer,lisa).
mother(marge,maggie).
father(homer,maggie).

a rule of inference

siblings(X,Y):-mother(M,X),mother(M,Y),father(D,X),father(D,Y).

What about the following rules of inference
- sister(X,Y):-
- brother(X,Y):-
- grandparent(X,Y):-
- ancestor(X,Y):-

Goal Statement

the question
```
?- mother(marge, Y).
Y = bart
```
semicoln asks for another solution
```
; 
Y = lisa
; 
Y = maggie
; 
no
```

Arithmetic

expressions are structure terms
they are not normally evaluated

is will cause evaluation

sumToN(L,L,L).
sumToN(L,U,R) :- L<=U, NewU is U - 1, sumToN(L,NewU,NewR), R is U+NewR.

Print integers from Lower to Upper

print(U,U) :- write(U), nl.
print(L,U) :- L < U, write(L), nl, M is L+1, print(M, U).

Lists

Prolog also supports lists
[H|T] may unify H with first element and T with rest of list
[H|T] may also construct a new list with H added on to the list T

simpson([maggie, bart, lisa, homer, marge]).
member(X,[X|T]).
member(X, [H|T]) :- X /== H, member(X, T).

Append two lists

append

append([],L,L).
append([H|T],L,[H|NewT]) :- append(T,L,NewT).

Running prolog

how to run prolog

% cprolog  (usually in /usr/public/bin)
?- consult(mydefs).
  mydefs consulted 816 bytes 0.06 sec.
?- father(Y, bart).
  Y = homer
?- ^D
  [ Prolog execution halted ]