attvar.pl

/*  Part of SWI-Prolog

    Author:        Jan Wielemaker
    E-mail:        J.Wielemaker@vu.nl
    WWW:           http://www.swi-prolog.org
    Copyright (c)  2004-2016, University of Amsterdam
                              VU University Amsterdam
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*/

:- module('$attvar',
          [ '$wakeup'/1,                % +Wakeup list
            freeze/2,                   % +Var, :Goal
            frozen/2,                   % @Var, -Goal
            call_residue_vars/2,        % :Goal, -Vars
            copy_term/3                 % +Term, -Copy, -Residue
          ]).

/** <module> Attributed variable handling

Attributed  variable  and  coroutining  support    based  on  attributed
variables. This module is complemented with C-defined predicates defined
in pl-attvar.c
*/

%!  '$wakeup'(+List)
%
%   Called from the kernel if assignments have been made to
%   attributed variables.

'$wakeup'([]).
'$wakeup'(wakeup(Attribute, Value, Rest)) :-
    call_all_attr_uhooks(Attribute, Value),
    '$wakeup'(Rest).

call_all_attr_uhooks([], _).
call_all_attr_uhooks(att(Module, AttVal, Rest), Value) :-
    uhook(Module, AttVal, Value),
    call_all_attr_uhooks(Rest, Value).


%!  uhook(+AttributeName, +AttributeValue, +Value)
%
%   Run the unify hook for attributed named AttributeName after
%   assigning an attvar with attribute AttributeValue the value
%   Value.
%
%   This predicate deals with reserved attribute names to avoid
%   the meta-call overhead.

uhook(freeze, Goal, Y) :-
    !,
    (   attvar(Y)
    ->  (   get_attr(Y, freeze, G2)
        ->  put_attr(Y, freeze, '$and'(G2, Goal))
        ;   put_attr(Y, freeze, Goal)
        )
    ;   unfreeze(Goal)
    ).
uhook(Module, AttVal, Value) :-
    Module:attr_unify_hook(AttVal, Value).


%!  unfreeze(+ConjunctionOrGoal)
%
%   Handle  unfreezing  of  conjunctions.  As  meta-calling  control
%   structures is slower than meta-interpreting them   we do this in
%   Prolog. Another advantage is that   having unfreeze/1 in between
%   makes the stacktrace and profiling   easier  to intepret. Please
%   note that we cannot use a direct conjunction as this would break
%   freeze(X, (a, !, b)).

unfreeze('$and'(A,B)) :-
    !,
    unfreeze(A),
    unfreeze(B).
unfreeze(Goal) :-
    Goal.

%!  freeze(@Var, :Goal)
%
%   Suspend execution of Goal until Var is unbound.

:- meta_predicate
    freeze(?, 0).

freeze(Var, Goal) :-
    '$freeze'(Var, Goal),
    !.        % Succeeds if delayed
freeze(_, Goal) :-
    Goal.

%!  frozen(@Var, -Goals)
%
%   Unify Goals with the goals frozen on Var or true if no
%   goals are grozen on Var.

frozen(Var, Goals) :-
    get_attr(Var, freeze, Goals0),
    !,
    make_conjunction(Goals0, Goals).
frozen(_, true).

make_conjunction('$and'(A0, B0), (A, B)) :-
    !,
    make_conjunction(A0, A),
    make_conjunction(B0, B).
make_conjunction(G, G).


                 /*******************************
                 *             PORTRAY          *
                 *******************************/

%!  portray_attvar(@Var)
%
%   Called from write_term/3 using the option attributes(portray) or
%   when the prolog flag write_attributes   equals portray. Its task
%   is the write the attributes in a human readable format.

:- public
    portray_attvar/1.

portray_attvar(Var) :-
    write('{'),
    get_attrs(Var, Attr),
    portray_attrs(Attr, Var),
    write('}').

portray_attrs([], _).
portray_attrs(att(Name, Value, Rest), Var) :-
    portray_attr(Name, Value, Var),
    (   Rest == []
    ->  true
    ;   write(', '),
        portray_attrs(Rest, Var)
    ).

portray_attr(freeze, Goal, Var) :-
    !,
    format('freeze(~w, ~W)', [ Var, Goal,
                               [ portray(true),
                                 quoted(true),
                                 attributes(ignore)
                               ]
                             ]).
portray_attr(Name, Value, Var) :-
    G = Name:attr_portray_hook(Value, Var),
    (   '$c_current_predicate'(_, G),
        G
    ->  true
    ;   format('~w = ...', [Name])
    ).


                 /*******************************
                 *          CALL RESIDUE        *
                 *******************************/

%!  call_residue_vars(:Goal, -Vars)
%
%   If Goal is  true,  Vars  is   the  set  of  residual  attributed
%   variables created by Goal. Goal  is   called  as in call/1. This
%   predicate  is  for  debugging  constraint   programs.  Assume  a
%   constraint program that creates  conflicting   constraints  on a
%   variable that is not part of the   result  variables of Goal. If
%   the solver is powerful enough it   will  detect the conflict and
%   fail. If the solver is too  weak   however  it  will succeed and
%   residual attributed variables holding the conflicting constraint
%   form a witness of this problem.

:- meta_predicate
    call_residue_vars(0, -).

call_residue_vars(Goal, Vars) :-
    prolog_current_choice(Chp),
    setup_call_cleanup(
        '$call_residue_vars_start',
        run_crv(Goal, Chp, Vars, Det),
        '$call_residue_vars_end'),
    (   Det == true
    ->  !
    ;   true
    ).
call_residue_vars(_, _) :-
    fail.

run_crv(Goal, Chp, Vars, Det) :-
    call(Goal),
    deterministic(Det),
    '$attvars_after_choicepoint'(Chp, Vars).

%!  copy_term(+Term, -Copy, -Gs) is det.
%
%   Creates a regular term Copy  as  a   copy  of  Term (without any
%   attributes), and a list Gs of goals that when executed reinstate
%   all attributes onto Copy. The nonterminal attribute_goals//1, as
%   defined in the modules the  attributes   stem  from,  is used to
%   convert attributes to lists of goals.

copy_term(Term, Copy, Gs) :-
    term_attvars(Term, Vs),
    (   Vs == []
    ->  Gs = [],
        copy_term(Term, Copy)
    ;   findall(Term-Gs,
                ( phrase(attvars_residuals(Vs), Gs),
                  delete_attributes(Term)
                ),
                [Copy-Gs])
    ).

attvars_residuals([]) --> [].
attvars_residuals([V|Vs]) -->
    (   { get_attrs(V, As) }
    ->  attvar_residuals(As, V)
    ;   []
    ),
    attvars_residuals(Vs).

attvar_residuals([], _) --> [].
attvar_residuals(att(Module,Value,As), V) -->
    (   { nonvar(V) }
    ->  % a previous projection predicate could have instantiated
        % this variable, for example, to avoid redundant goals
        []
    ;   (   { Module == freeze }
        ->  frozen_residuals(Value, V)
        ;   { current_predicate(Module:attribute_goals//1),
              phrase(Module:attribute_goals(V), Goals)
            }
        ->  list(Goals)
        ;   [put_attr(V, Module, Value)]
        )
    ),
    attvar_residuals(As, V).

list([])     --> [].
list([L|Ls]) --> [L], list(Ls).

delete_attributes(Term) :-
    term_attvars(Term, Vs),
    delete_attributes_(Vs).

delete_attributes_([]).
delete_attributes_([V|Vs]) :-
    del_attrs(V),
    delete_attributes_(Vs).


%!  frozen_residuals(+FreezeAttr, +Var)// is det.
%
%   Instantiate  a  freeze  goal  for  each    member  of  the  $and
%   conjunction. Note that we cannot  map   this  into a conjunction
%   because  freeze(X,  a),  freeze(X,  !)  would  create  freeze(X,
%   (a,!)),  which  is  fundamentally  different.  We  could  create
%   freeze(X,  (call(a),  call(!)))  or  preform  a  more  eleborate
%   analysis to validate the semantics are not changed.

frozen_residuals('$and'(X,Y), V) -->
    !,
    frozen_residuals(X, V),
    frozen_residuals(Y, V).
frozen_residuals(X, V) -->
    [ freeze(V, X) ].