swi-prolog-nox 7.6.4+dfsg-1build1 / usr / lib / swi-prolog / library / coinduction.pl

/*  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.

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    modification, are permitted provided that the following conditions
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       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
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    FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
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*/

:- 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 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).