A small interpreter for pattern-directed programs.

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

Program source code: pattern_interpreter.pl

%  Figure 23.13  A small interpreter for pattern-directed programs.
 
 
%   A small interpreter for pattern-directed programs
%   The system's database is manipulated through assert/retract
 
 
:-  op( 800, xfx, --->).
 
% run: execute production rules of the form
%      Condition ---> Action until action `stop' is triggered
 
run  :-
  Condition ---> Action,         % A production rule
  test( Condition),              % Precondition satisfied?
  execute( Action).
 
% test( [ Condition1, Condition2, ...])  if all conditions true
 
test( []).                       % Empty condition
 
test( [First|Rest])  :-          % Test conjunctive condition
  call( First),
  test( Rest).
 
% execute( [ Action1, Action2, ...]): execute list of actions
 
execute( [ stop])  :-  !.         % Stop execution
 
execute( [])  :-                 % Empty action (execution cycle completed)
  run.                           % Continue with next execution cycle
 
execute( [First | Rest])  :-
  call( First),
  execute( Rest).
 
replace( A, B)  :-               % Replace A with B in database
  retract( A), !,                % Retract once only
  assert( B).