add coinduction.yap code.

changes-6.0.html

@@ -17,6 +17,7 @@
 
 <h2>Yap-6.0.7:</h2>
 <ul>
+<li> NEW: coinduction.yap. </li>
 <li> FIXED: remove leftover files including two lib*.a (obs from
   Bernd Gutmann).</li>
 <li> FIXED: Make clean should result in recompiling all *.o (obs from

library/Makefile.in

@@ -37,6 +37,7 @@ PROGRAMS= \
 	$(srcdir)/charsio.yap \
 	$(srcdir)/cleanup.yap \
 	$(srcdir)/clp/clpfd.pl \
+	$(srcdir)/coinduction.yap \
 	$(srcdir)/dbqueues.yap \
 	$(srcdir)/dbusage.yap \
 	$(srcdir)/dgraphs.yap \

library/coinduction.yap

@@ -0,0 +1,189 @@
+/*************************************************************************
+*									 *
+*	 YAP Prolog 							 *
+*									 *
+*	Yap Prolog was developed at NCCUP - Universidade do Porto	 *
+*									 *
+* Copyright L.Damas, V.S.Costa and Universidade do Porto 1985-1997	 *
+*									 *
+**************************************************************************
+*									 *
+* File:		atts.yap						 *
+* Last rev:	8/2/88							 *
+* mods:									 *
+* comments:	attribute support for Prolog				 *
+*									 *
+*************************************************************************/
+
+% :- yap_flag(unknown,error).
+% :- style_check(all).
+
+%
+% Code originally written by Arvin Bansal and Vitor Santos Costa
+% Includes nice extensions from Jan Wielemaker (from the SWI version).
+%
+
+:- module(coinduction,
+          [ (coinductive)/1,
+            op(1150, fx, (coinductive))
+          ]).
+
+:- use_module(library(error)).
+
+/** <module> Co-Logic Programming
+
+This simple module implements the   directive coinductive/1 as described
+in "Co-Logic Programming: Extending Logic  Programming with Coinduction"
+by Luke Somin et al. The idea behind coinduction is that a goal succeeds
+if it unifies to a parent goal.  This enables some interesting programs,
+notably on infinite trees (cyclic terms).
+
+    ==
+    :- use_module(library(coinduction)).
+
+    :- coinductive stream/1. 
+    stream([H|T]) :- i(H), stream(T).
+
+    % inductive
+    i(0).
+    i(s(N)) :- i(N).
+
+    ?- X=[s(s(A))|X], stream(X).
+     X=[s(s(A))|X], stream(X).
+     A = 0,
+     X = [s(s(0)),**]
+    ==
+
+This predicate is  true  for  any   cyclic  list  containing  only  1-s,
+regardless of the cycle-length.
+
+@bug    Programs mixing normal predicates and coinductive predicates must
+        be _stratified_.  The theory does not apply to normal Prolog calling
+        coinductive predicates, calling normal Prolog predicates, etc.
+
+        Stratification is not checked or enforced in any other way and thus
+        left as a responsibility to the user.
+@see    "Co-Logic Programming: Extending Logic  Programming with Coinduction"
+        by Luke Somin et al.
+*/
+
+:- meta_predicate coinductive(0).
+
+:- dynamic coinductive/3.
+
+
+%-----------------------------------------------------
+
+expand_coinductive_declaration(Spec, Clauses) :-
+        prolog_load_context(module, Module),
+        phrase(expand_specs(Spec, Module), Clauses).
+
+expand_specs(Var, _) -->
+        { var(Var), !,
+          instantiation_error(Var)
+        }.
+expand_specs(M:Spec, _) --> !,
+        expand_specs(Spec, M).
+expand_specs((A,B), Module) --> !,
+        expand_specs(A, Module),
+        expand_specs(B, Module).
+expand_specs(Head, Module) -->
+        { valid_pi(Head, Name, Arity),
+          functor(GenHead, Name, Arity)
+        },
+        [ coinduction:coinductive_declaration(GenHead, Module) ].
+
+coinductive(Spec) :-
+	var(Spec),
+	!,
+	throw(error(instantiation_error,coinductive(Spec))).
+coinductive(Module:Spec) :-
+        coinductive_declaration(Spec, Module, coinductive(Module:Spec)).
+coinductive(Spec) :-
+        prolog_load_context(module, Module),
+        coinductive_declaration(Spec, Module, coinductive(Spec)).
+	
+coinductive_declaration(Spec, _M, G) :-
+	var(Spec),
+	!,
+	throw(error(instantiation_error,G)).
+coinductive_declaration((A,B), M, G) :- !,
+	coinductive_declaration(A, M, G),
+	coinductive_declaration(B, M, G).
+coinductive_declaration(M:Spec, _, G) :- !,
+	coinductive_declaration(Spec, M, G).
+coinductive_declaration(Spec, M, _G) :-
+	valid_pi(Spec, F, N),
+	functor(S,F,N), 
+	atomic_concat(['__coinductive__',F,'/',N],NF),
+	functor(NS,NF,N), 
+	match_args(N,S,NS),
+	atomic_concat(['__stack_',M,':',F,'/',N],SF), 
+	nb_setval(SF, _),
+	assert((M:S :-
+	       b_getval(SF,L),
+		coinduction:in_stack(S, L, End),
+		(
+		 nonvar(End)
+		->
+		 true
+		;
+		 End = [S|_],
+		 M:NS)
+	       )
+	      ),
+	assert(coinduction:coinductive(S,M,NS)).
+  
+valid_pi(Name/Arity, Name, Arity) :-
+        must_be(atom, Name),
+        must_be(integer, Arity).
+
+match_args(0,_,_) :- !.
+match_args(I,S1,S2) :-
+	arg(I,S1,A),
+	arg(I,S2,A),
+	I1 is I-1,
+	match_args(I1,S1,S2).
+
+%-----------------------------------------------------
+
+co_term_expansion((M:H :- B), _, (M:NH :- B)) :- !,
+	co_term_expansion((H :- B), M, (NH :- B)).
+co_term_expansion((H :- B), M, (NH :- B)) :- !,
+	coinductive(H, M, NH), !.
+co_term_expansion(H, M, NH) :-
+	coinductive(H, M, NH), !.
+
+user:term_expansion(M:Cl,M:NCl ) :- !,
+	co_term_expansion(Cl, M, NCl).
+
+user:term_expansion(G, NG) :-
+        prolog_load_context(module, Module),
+	co_term_expansion(G, Module, NG).
+
+
+%-----------------------------------------------------
+
+in_stack(_, V, V) :- var(V), !.
+in_stack(G, [G|_], [G|_]).
+in_stack(G, [_|T], End) :- in_stack(G, T, End).
+
+writeG_val(G_var) :- 
+  b_getval(G_var, G_val),
+  write(G_var), write(' ==> '), write(G_val), nl.
+
+%-----------------------------------------------------
+
+/**************************************
+
+  Some examples from Coinductive Logic Programming and its Applications by Gopal Gupta et al, ICLP 97
+
+:- coinductive stream/1. 
+stream([H|T]) :- i(H), stream(T).
+
+% inductive
+i(0).
+i(s(N)) :- i(N).
+
+**************************************/
+