View source with raw comments or as raw

    1/*  Part of SWI-Prolog
    2
    3    Author:        Jan Wielemaker
    4    E-mail:        J.Wielemaker@vu.nl
    5    WWW:           http://www.swi-prolog.org
    6    Copyright (c)  2010-2011, VU University Amsterdam
    7    All rights reserved.
    8
    9    Redistribution and use in source and binary forms, with or without
   10    modification, are permitted provided that the following conditions
   11    are met:
   12
   13    1. Redistributions of source code must retain the above copyright
   14       notice, this list of conditions and the following disclaimer.
   15
   16    2. Redistributions in binary form must reproduce the above copyright
   17       notice, this list of conditions and the following disclaimer in
   18       the documentation and/or other materials provided with the
   19       distribution.
   20
   21    THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
   22    "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
   23    LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
   24    FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
   25    COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
   26    INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
   27    BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
   28    LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
   29    CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
   30    LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
   31    ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
   32    POSSIBILITY OF SUCH DAMAGE.
   33*/
   34
   35:- module(coinduction,
   36          [ (coinductive)/1,
   37            op(1150, fx, (coinductive))
   38          ]).   39:- 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.

See also: -
"Co-Logic Programming: Extending Logic Programming with Coinduction" by Luke Simon et al. */
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.

   70:- multifile
   71    system:term_expansion/2,
   72    coinductive_declaration/2.      % Head, Module

head(+Term, -QHead) is semidet: Must be first to allow reloading!

   78head(Var, _) :-
   79    var(Var), !, fail.
   80head((H:-_B), Head) :-
   81    !,
   82    head(H, Head).
   83head(H, Head) :-
   84    (   H = _:_
   85    ->  Head = H
   86    ;   prolog_load_context(module, M),
   87        Head = M:H
   88    ).

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.

   97coinductive(Spec) :-
   98    throw(error(context_error(nodirective, coinductive(Spec)), _)).
   99
  100expand_coinductive_declaration(Spec, Clauses) :-
  101    prolog_load_context(module, Module),
  102    phrase(expand_specs(Spec, Module), Clauses).
  103
  104expand_specs(Var, _) -->
  105    { var(Var),
  106      !,
  107      instantiation_error(Var)
  108    }.
  109expand_specs(M:Spec, _) -->
  110    !,
  111    expand_specs(Spec, M).
  112expand_specs((A,B), Module) -->
  113    !,
  114    expand_specs(A, Module),
  115    expand_specs(B, Module).
  116expand_specs(Head, Module) -->
  117    { valid_pi(Head, Name, Arity),
  118      functor(GenHead, Name, Arity)
  119    },
  120    [ coinduction:coinductive_declaration(GenHead, Module) ].
  121
  122
  123valid_pi(Name/Arity, Name, Arity) :-
  124    must_be(atom, Name),
  125    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.

  134wrap_coinductive(Pred, Term, Clause) :-
  135    current_predicate(_, Pred),
  136    !,
  137    rename_clause(Term, 'coinductive ', Clause).
  138wrap_coinductive(Pred, Term, [Wrapper_1,Wrapper_2,FirstClause]) :-
  139    Pred = M:Head,
  140    functor(Head, Name, Arity),
  141    length(Args, Arity),
  142    GenHead =.. [Name|Args],
  143    atom_concat('coinductive ', Name, WrappedName),
  144    WrappedHead =.. [WrappedName|Args],
  145    Wrapper_1 = (GenHead :-
  146                    prolog_current_frame(F),
  147                    prolog_frame_attribute(F, parent, FP),
  148                    prolog_frame_attribute(FP, parent_goal, M:GenHead)),
  149    Wrapper_2 = (GenHead :- WrappedHead, coinduction:no_lco),
  150    rename_clause(Term, 'coinductive ', FirstClause).
  151
  152:- public no_lco/0.  153
  154no_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.

  160rename_clause((Head :- Body), Prefix, (NewHead :- Body)) :-
  161    !,
  162    rename_clause(Head, Prefix, NewHead).
  163rename_clause(M:Head, Prefix, M:NewHead) :-
  164    rename_clause(Head, Prefix, NewHead).
  165rename_clause(Head, Prefix, NewHead) :-
  166    Head =.. [Name|Args],
  167    atom_concat(Prefix, Name, WrapName),
  168    NewHead =.. [WrapName|Args].
  169
  170
  171                 /*******************************
  172                 *        EXPANSION HOOKS       *
  173                 *******************************/
  174
  175system:term_expansion((:- coinductive(Spec)), Clauses) :-
  176    expand_coinductive_declaration(Spec, Clauses).
  177system:term_expansion(Term, Wrapper) :-
  178    head(Term, Module:Head),
  179    coinductive_declaration(Head, Module),
  180    wrap_coinductive(Module:Head, Term, Wrapper)