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From: schaefer@ogcvax.UUCP (Barton E. Schaefer)
Newsgroups: comp.lang.prolog
Subject: Re: self-reproducing programs
Message-ID: <1272@ogcvax.UUCP>
Date: Tue, 5-May-87 13:45:19 EDT
Article-I.D.: ogcvax.1272
Posted: Tue May  5 13:45:19 1987
Date-Received: Sat, 9-May-87 01:34:12 EDT
References: <430@ivax.doc.ic.ac.uk> <brl-smok.5829>
Reply-To: schaefer@ogcvax.UUCP (Barton E. Schaefer)
Organization: Oregon Graduate Center, Beaverton, OR
Lines: 62

In article <brl-smok.5829> broome@brl.arpa (Paul Broome (CTAB) <broome>) writes:
>Here's a frivolous little program that removes itself.
>
>suicide :-
>	 abolish(suicide,0).

How about this one?  The following predicate, invoked as
	?- everyclause(everyclause(_,_),X).
will remove itself from the database, binding X to a list of all its clauses in
the process, then put itself back again.

/* `everyclause(X,Y)' returns in Y a list of all clauses whose heads have the
    same functor and arity as X.
   `everyclause' works by  calling the primitive `clauses', retracting the
    clause found, recurring to find the next clause, and then re-asserting the
    retracted clause.  It cannot be used to test directly that a list contains
    every clause; its second argument must be a variable.
 */

/* Since `clause' fails with an error if called with a variable in its first
    argument, `everyclause' fails if it would have to make such a call.
 */
everyclause(X,Y) :-
    var(X), !,
    fail.

/* The useful part of `everyclause'.
 */
everyclause(X,[]) :-
    not clause(X,_), !.
everyclause(X,[Y|Z]) :-
    var(Y), !,
    functor(X,F,N),
    X =.. [F|N1],
    newvars(N1,N2),
    A =.. [F|N2],
    clause(A,C),
    Y = (A :- C),
    functor(B,F,N),
    clause(B,D),
    retract((B :- D)),
    ((everyclause(X,Z), !,
    asserta((B :- D)));
    (asserta((B :- D)), !)).

newvars(X,X) :- atomic(X).
newvars(X,Y) :- var(X).
newvars((X1,Y1),(X2,Y2)) :- !,
    newvars(X1,X2),
    newvars(Y1,Y2).
newvars([H1|R1],[H2|R2]) :- !,
    newvars(H1,H2),
    newvars(R1,R2).
newvars(X,Y) :-
    X =.. [F|A],
    newvars(A,B),
    Y =.. [F|B].
-- 
Bart Schaefer						Dept. of CS&E
CSNET:	schaefer@Oregon-Grad				Oregon Graduate Center
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