Backward-chaining rule interpreter

A backward chaining interpreter for if-then rules.

Source: PROLOG programming for artificial intelligence, 3rd Edition, Harlow, 2001, ISBN 0-201-40375-7.

Program source code: backward-chaining_rule_interpreter.pl

%  Figure 15.6   A backward chaining interpreter for if-then rules.
 
 
% A simple backward chaining rule interpreter
 
:-  op( 800, fx, if).
:-  op( 700, xfx, then).
:-  op( 300, xfy, or).
:-  op( 200, xfy, and).
 
is_true( P)  :-
   fact( P).
 
is_true( P)  :-
   if Condition then P,        % A relevant rule   
   is_true( Condition).        % whose condition is true 
 
is_true( P1 and P2)  :-
   is_true( P1),
   is_true( P2).
 
is_true( P1 or P2)  :-
   is_true( P1)
   ;
   is_true( P2).