/* Part of SWI-Prolog Author: Jan Wielemaker E-mail: J.Wielemaker@vu.nl WWW: http://www.swi-prolog.org Copyright (c) 2010-2011, VU University Amsterdam All rights reserved. Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met: 1. Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer. 2. Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution. THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. */ :- module(coinduction, [ (coinductive)/1, op(1150, fx, (coinductive)) ]). :- use_module(library(error)). /** Co-Logic Programming This simple module implements the directive coinductive/1 as described in "Co-Logic Programming: Extending Logic Programming with Coinduction" by Luke Simon 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 p/1. p([1|T]) :- p(T). == 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 Simon et al. */ :- multifile system:term_expansion/2, coinductive_declaration/2. % Head, Module %! head(+Term, -QHead) is semidet. % % Must be first to allow reloading! head(Var, _) :- var(Var), !, fail. head((H:-_B), Head) :- !, head(H, Head). head(H, Head) :- ( H = _:_ -> Head = H ; prolog_load_context(module, M), Head = M:H ). %! coinductive(:Spec) % % The declaration :- coinductive name/arity, ... defines % predicates as _coinductive_. The predicate definition is wrapped % such that goals unify with their ancestors. This directive must % preceed all clauses of the predicate. coinductive(Spec) :- throw(error(context_error(nodirective, coinductive(Spec)), _)). 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) ]. valid_pi(Name/Arity, Name, Arity) :- must_be(atom, Name), must_be(integer, Arity). %! wrap_coinductive(+Head, +Term, -Clauses) is det. % % Create a wrapper. The first clause deal with the case where we % already created the wrapper. The second creates the wrapper and % the first clause. wrap_coinductive(Pred, Term, Clause) :- current_predicate(_, Pred), !, rename_clause(Term, 'coinductive ', Clause). wrap_coinductive(Pred, Term, [Wrapper_1,Wrapper_2,FirstClause]) :- Pred = M:Head, functor(Head, Name, Arity), length(Args, Arity), GenHead =.. [Name|Args], atom_concat('coinductive ', Name, WrappedName), WrappedHead =.. [WrappedName|Args], Wrapper_1 = (GenHead :- prolog_current_frame(F), prolog_frame_attribute(F, parent, FP), prolog_frame_attribute(FP, parent_goal, M:GenHead)), Wrapper_2 = (GenHead :- WrappedHead, coinduction:no_lco), rename_clause(Term, 'coinductive ', FirstClause). :- public no_lco/0. no_lco. % true, but do not optimize away %! rename_clause(+Clause, +Prefix, -Renamed) is det. % % Rename a clause by prefixing its old name wit h Prefix. rename_clause((Head :- Body), Prefix, (NewHead :- Body)) :- !, rename_clause(Head, Prefix, NewHead). rename_clause(M:Head, Prefix, M:NewHead) :- rename_clause(Head, Prefix, NewHead). rename_clause(Head, Prefix, NewHead) :- Head =.. [Name|Args], atom_concat(Prefix, Name, WrapName), NewHead =.. [WrapName|Args]. /******************************* * EXPANSION HOOKS * *******************************/ system:term_expansion((:- coinductive(Spec)), Clauses) :- expand_coinductive_declaration(Spec, Clauses). system:term_expansion(Term, Wrapper) :- head(Term, Module:Head), coinductive_declaration(Head, Module), wrap_coinductive(Module:Head, Term, Wrapper).