/* $Id$
Part of SWI-Prolog
Author: Tom Schrijvers, K.U.Leuven
E-mail: Tom.Schrijvers@cs.kuleuven.ac.be
WWW: http://www.swi-prolog.org
Copyright (C): 2003-2004, K.U.Leuven
This program is free software; you can redistribute it and/or
modify it under the terms of the GNU General Public License
as published by the Free Software Foundation; either version 2
of the License, or (at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU Lesser General Public
License along with this library; if not, write to the Free Software
Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
As a special exception, if you link this library with other files,
compiled with a Free Software compiler, to produce an executable, this
library does not by itself cause the resulting executable to be covered
by the GNU General Public License. This exception does not however
invalidate any other reasons why the executable file might be covered by
the GNU General Public License.
*/
:- module(when,
[ when/2 % +Condition, :Goal
]).
:- set_prolog_flag(generate_debug_info, false).
:- meta_predicate
when(+, 0),
suspend_list(+, 0),
trigger(+, 0),
trigger_disj(+, 0),
trigger_conj(+, +, 0).
/** <module> Conditional coroutining
This library implements the when/2 constraint, delaying a goal until its
arguments are sufficiently instantiated. For example, the following
delayes the execution of =:=/2 until the expression is instantiated.
==
...
when(ground(Expr), 0 =:= Expr),
==
@author Tom Schrijvers (initial implementation)
@author Jan Wielemaker
*/
%% when(+Condition, :Goal)
%
% Execute Goal when Condition is satisfied. I.e., Goal is executed
% as by call/1 if Condition is true when when/2 is called.
% Otherwise Goal is _delayed_ until Condition becomes true.
% Condition is one of the following:
%
% * nonvar(X)
% * ground(X)
% * ?=(X,Y)
% * (Cond1,Cond2)
% * (Cond2;Cond2)
%
% For example (note the order =a= and =b= are written):
%
% ==
% ?- when(nonvar(X), writeln(a)), writeln(b), X = x.
% b
% a
% X = x
% ==
when(Condition, Goal) :-
'$eval_when_condition'(Condition, Optimised),
trigger_first(Optimised, Goal).
%% '$eval_when_condition'(+Condition, -Optimised)
%
% C-building block defined in pl-attvar.c. It pre-processes the
% when-condition, checks it for errors (instantiation errors,
% domain-errors and cyclic terms) and simplifies it. Notably, it
% removes already satisfied conditions from Condition, unifying
% Optimised to =true= if there is no need to suspend. Nested
% disjunctions are reported as or(List).
trigger_first(true, Goal) :- !,
call(Goal).
trigger_first(nonvar(X), Goal) :- !,
'$suspend'(X, when, trigger_nonvar(X, Goal)).
trigger_first(Cond, Goal) :-
trigger(Cond, Goal).
trigger(nonvar(X),Goal) :-
trigger_nonvar(X,Goal).
trigger(ground(X),Goal) :-
trigger_ground(X,Goal).
trigger(?=(X,Y),Goal) :-
trigger_determined(X,Y,Goal).
trigger((G1,G2),Goal) :-
trigger_conj(G1,G2,Goal).
trigger(or(GL),Goal) :-
trigger_disj(GL, check_disj(_DisjID,GL,Goal)).
trigger_nonvar(X, Goal) :-
( nonvar(X)
-> call(Goal)
; '$suspend'(X, when, trigger_nonvar(X, Goal))
).
trigger_ground(X, Goal) :-
term_variables(X, Vs),
( Vs = [H]
-> '$suspend'(H, when, trigger_ground(H, Goal))
; Vs = [H|_]
-> T =.. [f|Vs],
'$suspend'(H, when, trigger_ground(T, Goal))
; call(Goal)
).
trigger_determined(X, Y, Goal) :-
unifiable(X, Y, Unifier), !,
( Unifier == []
-> call(Goal)
; put_attr(Det, when, det(trigger_determined(X,Y,Goal))),
suspend_list(Unifier, wake_det(Det))
).
trigger_determined(_, _, Goal) :-
call(Goal).
wake_det(Det) :-
( var(Det) ->
get_attr(Det,when,Attr),
del_attr(Det,when),
Det = (-),
Attr = det(Goal),
call(Goal)
;
true
).
trigger_conj(G1,G2,Goal) :-
trigger(G1, trigger(G2,Goal)).
trigger_disj([],_).
trigger_disj([H|T], G) :-
trigger(H, G),
trigger_disj(T, G).
%% check_disj(DisjVar, Disj, Goal)
%
% If there is a disjunctive condition, we share a variable between
% the disjunctions. If the goal is fired due to one of the
% conditions, the shared variable is boud to (-). Note that this
% implies that the attributed variable is left in place. The
% predicate when_goal//1 skips such goals on behalfe of
% copy_term/3.
check_disj(Disj,_,Goal) :-
( Disj == (-)
-> true
; Disj = (-),
call(Goal)
).
suspend_list([],_Goal).
suspend_list([V=W|Unifier],Goal) :-
'$suspend'(V, when, Goal),
( var(W)
-> '$suspend'(W, when, Goal)
; true
),
suspend_list(Unifier,Goal).
attr_unify_hook(call(Goal), Other) :-
( get_attr(Other, when, call(GOTher))
-> del_attr(Other, when),
Goal, GOTher
; Goal
).
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
attribute_goals(V) -->
{ get_attr(V, when, Attr) },
when_goals(Attr).
when_goals(det(trigger_determined(X, Y, G))) --> !,
( { disj_goal(G, Disj, DG) }
-> disj_or(Disj, DG)
; { G = when:trigger(C, Goal) }
-> [ when((?=(X,Y),C), Goal) ]
; [ when(?=(X,Y), G) ]
).
when_goals(call(Conj)) -->
when_conj_goals(Conj).
when_conj_goals((A,B)) --> !,
when_conj_goals(A),
when_conj_goals(B).
when_conj_goals(when:G) -->
when_goal(G).
when_goal(trigger_nonvar(X, G)) -->
( { disj_goal(G, Disj, DG) }
-> disj_or(Disj, DG)
; { G = when:trigger(C, Goal) }
-> [ when((nonvar(X),C), Goal) ]
; [ when(nonvar(X),G) ]
).
when_goal(trigger_ground(X, G)) -->
( { disj_goal(G, Disj, DG) }
-> disj_or(Disj, DG)
; { G = when:trigger(C, Goal) }
-> [ when((ground(X),C), Goal) ]
; [ when(ground(X),G) ]
).
when_goal(wake_det(_)) -->
[].
disj_goal(when:check_disj(X, _, _), [], -) :- X == (-).
disj_goal(when:check_disj(-, Or, DG), Or, DG).
disj_or([], _) --> [].
disj_or(List, DG) -->
{ or_list(List, Or) },
[when(Or, DG)].
or_list([H], H) :- !.
or_list([H|T], (H;OT)) :-
or_list(T, OT).