463 lines
13 KiB
Prolog
463 lines
13 KiB
Prolog
/*************************************************************************
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* *
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* YAP Prolog *
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* *
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* Yap Prolog was developed at NCCUP - Universidade do Porto *
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* *
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* Copyright L.Damas, V.S.Costa and Universidade do Porto 1985-1997 *
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* *
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**************************************************************************
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* *
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* File: corout.pl *
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* Last rev: *
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* mods: *
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* comments: Coroutines implementation *
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* *
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*************************************************************************/
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:- module('$coroutining',[
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op(1150, fx, block)
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%dif/2,
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%when/2,
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%block/1,
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%wait/1,
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%frozen/2
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]).
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attr_unify_hook(DelayList, _) :-
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wake_delays(DelayList).
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wake_delays([]).
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wake_delays(Delay.List) :-
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wake_delay(Delay),
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wake_delays(List).
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%
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% Interface to attributed variables.
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%
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wake_delay(redo_dif(Done, X, Y)) :-
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redo_dif(Done, X, Y).
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wake_delay(redo_freeze(Done, V, Goal)) :-
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redo_freeze(Done, V, Goal).
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wake_delay(redo_eq(Done, X, Y, Goal)) :-
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redo_eq(Done, X, Y, Goal, G).
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wake_delay(redo_ground(Done, X, Goal)) :-
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redo_ground(Done, X, Goal).
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attribute_goals(Var) -->
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{ get_attr(Var, '$coroutining', Delays) },
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attgoal_for_delays(Delays, Var).
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attgoal_for_delays([], V) --> [].
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attgoal_for_delays([G|AllAtts], V) -->
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attgoal_for_delay(G, V),
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attgoal_for_delays(AllAtts, V).
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attgoal_for_delay(redo_dif(Done, X, Y), V) --> { var(Done), first_att(dif(X,Y), V) }, !, [prolog:dif(X,Y)].
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attgoal_for_delay(redo_freeze(Done, V, Goal), V) --> { var(Done) }, !, [prolog:freeze(V,Goal)].
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attgoal_for_delay(redo_eq(Done, X, Y, Goal), V) --> { var(Done), first_att(Goal, V) }, !, [prolog:when(X=Y,Goal)].
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attgoal_for_delay(redo_ground(Done, X, Goal), V) --> { var(Done) }, !, [prolog:when(ground(X),Goal)].
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attgoal_for_delay(_, V) --> [].
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%
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% operators defined in this module:
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%
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prolog:freeze(V, G) :-
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var(V), !,
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freeze_goal(V,G).
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prolog:freeze(_, G) :-
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'$execute'(G).
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freeze_goal(V,VG) :-
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var(VG), !,
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'$current_module'(M),
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internal_freeze(V, redo_freeze(_Done,V,M:VG)).
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freeze_goal(V,M:G) :- !,
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internal_freeze(V, redo_freeze(_Done,V,M:G)).
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freeze_goal(V,G) :-
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'$current_module'(M),
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internal_freeze(V, redo_freeze(_Done,V,M:G)).
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%
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%
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% Dif is tricky because we need to wake up on the two variables being
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% bound together, or on any variable of the term being bound to
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% another. Also, the day YAP fully supports infinite rational trees,
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% dif should work for them too. Hence, term comparison should not be
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% implemented in Prolog.
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%
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% This is the way dif works. The '$can_unify' predicate does not know
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% anything about dif semantics, it just compares two terms for
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% equaility and is based on compare. If it succeeds without generating
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% a list of variables, the terms are equal and dif fails. If it fails,
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% dif succeeds.
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%
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% If it succeeds but it creates a list of variables, dif creates
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% suspension records for all these variables on the '$redo_dif'(V,
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% X, Y) goal. V is a flag that says whether dif has completed or not,
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% X and Y are the original goals. Whenever one of these variables is
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% bound, it calls '$redo_dif' again. '$redo_dif' will then check whether V
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% was bound. If it was, dif has succeeded and redo_dif just
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% exits. Otherwise, '$redo_dif' will call dif again to see what happened.
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%
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% Dif needs two extensions from the suspension engine:
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%
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% First, it needs
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% for the engine to be careful when binding two suspended
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% variables. Basically, in this case the engine must be sure to wake
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% up one of the goals, as they may make dif fail. The way the engine
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% does so is by searching the list of suspended variables, and search
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% whether they share a common suspended goal. If they do, that
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% suspended goal is added to the WokenList.
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%
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% Second, thanks to dif we may try to suspend on the same variable
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% several times. dif calls a special version of freeze that checks
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% whether that is in fact the case.
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%
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prolog:dif(X, Y) :-
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'$can_unify'(X, Y, LVars), !,
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LVars = [_|_],
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dif_suspend_on_lvars(LVars, redo_dif(_Done, X, Y)).
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prolog:dif(_, _).
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dif_suspend_on_lvars([], _).
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dif_suspend_on_lvars([H|T], G) :-
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internal_freeze(H, G),
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dif_suspend_on_lvars(T, G).
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%
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% This predicate is called whenever a variable dif was suspended on is
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% bound. Note that dif may have already executed successfully.
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%
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% Three possible cases: dif has executed and Done is bound; we redo
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% dif and the two terms either unify, hence we fail, or may unify, and
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% we try to increase the number of suspensions; last, the two terms
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% did not unify, we are done, so we succeed and bind the Done variable.
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%
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redo_dif(Done, _, _) :- nonvar(Done), !.
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redo_dif(Done, X, Y) :-
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'$can_unify'(X, Y, LVars), !,
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LVars = [_|_],
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dif_suspend_on_lvars(LVars, redo_dif(Done, X, Y)).
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redo_dif('$done', _, _).
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% If you called nonvar as condition for when, then you may find yourself
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% here.
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%
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% someone else (that is Cond had ;) did the work, do nothing
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%
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redo_freeze(Done, _, _) :- nonvar(Done), !.
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%
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% We still have some more conditions: continue the analysis.
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%
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redo_freeze(Done, _, '$when'(C, G, Done)) :- !,
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'$when'(C, G, Done).
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%
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% check if the variable was really bound
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%
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redo_freeze(Done, V, G) :- var(V), !,
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internal_freeze(V, redo_freeze(Done,V,G)).
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%
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% I can't believe it: we're done and can actually execute our
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% goal. Notice we have to say we are done, otherwise someone else in
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% the disjunction might decide to wake up the goal themselves.
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%
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redo_freeze('$done', _, G) :-
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'$execute'(G).
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%
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% eq is a combination of dif and freeze
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redo_eq(Done, _, _, _, _) :- nonvar(Done), !.
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redo_eq(_, X, Y, _, G) :-
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'$can_unify'(X, Y, LVars),
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LVars = [_|_], !,
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dif_suspend_on_lvars(LVars, G).
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redo_eq(Done, _, _, when(C, G, Done), _) :- !,
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when(C, G, Done).
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redo_eq('$done', _ ,_ , Goal, _) :-
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'$execute'(Goal).
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%
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% ground is similar to freeze
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redo_ground(Done, _, _) :- nonvar(Done), !.
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redo_ground(Done, X, Goal) :-
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'$non_ground'(X, Var), !,
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internal_freeze(Var, redo_ground(Done, X, Goal)).
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redo_ground(Done, _, when(C, G, Done)) :- !,
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when(C, G, Done).
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redo_ground('$done', _, Goal) :-
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'$execute'(Goal).
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%
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% support for when/2 built-in
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%
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prolog:when(Conds,Goal) :-
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'$current_module'(Mod),
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prepare_goal_for_when(Goal, Mod, ModG),
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when(Conds, ModG, Done, [], LG), !,
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%write(vsc:freezing(LG,Done)),nl,
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suspend_when_goals(LG, Done).
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prolog:when(_,Goal) :-
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'$execute'(Goal).
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%
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% support for when/2 like declaration.
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%
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%
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% when will block on a conjunction or disjunction of nonvar, ground,
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% ?=, where ?= is both terms being bound together
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%
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%
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'$declare_when'(Cond, G) :-
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generate_code_for_when(Cond, G, Code),
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'$current_module'(Module),
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'$$compile'(Code, Code, 5, Module), fail.
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'$declare_when'(_,_).
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%
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% use a meta interpreter for now
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%
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generate_code_for_when(Conds, G,
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( G :- when(Conds, ModG, Done, [], LG), !,
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suspend_when_goals(LG, Done)) ) :-
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'$current_module'(Mod),
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prepare_goal_for_when(G, Mod, ModG).
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%
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% make sure we have module info for G!
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%
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prepare_goal_for_when(G, Mod, Mod:call(G)) :- var(G), !.
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prepare_goal_for_when(M:G, _, M:G) :- !.
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prepare_goal_for_when(G, Mod, Mod:G).
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%
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% now for the important bit
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%
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% Done is used to synchronise: when it is bound someone else did the
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% goal and we can give up.
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%
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% $when/5 and $when_suspend succeds when there is need to suspend a goal
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%
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%
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when(V, G, Done, LG0, LGF) :- var(V), !,
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'$do_error'(instantiation_error,when(V,G)).
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when(nonvar(V), G, Done, LG0, LGF) :-
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when_suspend(nonvar(V), G, Done, LG0, LGF).
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when(?=(X,Y), G, Done, LG0, LGF) :-
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when_suspend(?=(X,Y), G, Done, LG0, LGF).
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when(ground(T), G, Done, LG0, LGF) :-
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when_suspend(ground(T), G, Done, LG0, LGF).
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when((C1, C2), G, Done, LG0, LGF) :-
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% leave it open to continue with when.
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(
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when(C1, when(C2, G, Done), Done, LG0, LGI)
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->
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LGI = LGF
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;
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% we solved C1, great, now we just have to solve C2!
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when(C2, G, Done, LG0, LGF)
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).
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when((G1 ; G2), G, Done, LG0, LGF) :-
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when(G1, G, Done, LG0, LGI),
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when(G2, G, Done, LGI, LGF).
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%
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% Auxiliary predicate called from within a conjunction.
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% Repeat basic code for when, as inserted in first clause for predicate.
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%
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when(_, _, Done) :-
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nonvar(Done), !.
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when(Cond, G, Done) :-
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when(Cond, G, Done, [], LG),
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!,
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suspend_when_goals(LG, Done).
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when(_, G, '$done') :-
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'$execute'(G).
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%
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% Do something depending on the condition!
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%
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% some one else did the work.
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%
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when_suspend(_, _, Done, _, []) :- nonvar(Done), !.
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%
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% now for the serious stuff.
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%
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when_suspend(nonvar(V), G, Done, LG0, LGF) :-
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try_freeze(V, G, Done, LG0, LGF).
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when_suspend(?=(X,Y), G, Done, LG0, LGF) :-
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try_eq(X, Y, G, Done, LG0, LGF).
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when_suspend(ground(X), G, Done, LG0, LGF) :-
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try_ground(X, G, Done, LG0, LGF).
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try_freeze(V, G, Done, LG0, LGF) :-
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var(V),
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LGF = ['$coroutining':internal_freeze(V, redo_freeze(Done, V, G))|LG0].
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try_eq(X, Y, G, Done, LG0, LGF) :-
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'$can_unify'(X, Y, LVars), LVars = [_|_],
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LGF = ['$coroutining':dif_suspend_on_lvars(LVars, redo_eq(Done, X, Y, G))|LG0].
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try_ground(X, G, Done, LG0, LGF) :-
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'$non_ground'(X, Var), % the C predicate that succeds if
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% finding out the term is nonground
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% and gives the first variable it
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% finds. Notice that this predicate
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% must know about svars.
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LGF = ['$coroutining':internal_freeze(Var, redo_ground(Done, X, G))| LG0].
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%
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% When executing a when, if nobody succeeded, we need to create suspensions.
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%
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suspend_when_goals([], _).
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suspend_when_goals(['$coroutining':internal_freeze(V, G)|Ls], Done) :-
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var(Done), !,
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internal_freeze(V, G),
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suspend_when_goals(Ls, Done).
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suspend_when_goals([dif_suspend_on_lvars(LVars, G)|LG], Done) :-
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var(Done), !,
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dif_suspend_on_lvars(LVars, G),
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suspend_when_goals(LG, Done).
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suspend_when_goals([_|_], _).
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%
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% Support for wait declarations on goals.
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% Or we also use the more powerful, SICStus like, "block" declarations.
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%
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% block or wait declarations must precede the first clause.
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%
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%
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% I am using the simplest solution now: I'll add an extra clause at
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% the beginning of the procedure to do this work. This creates a
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% choicepoint and make things a bit slower, but it's probably not as
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% significant as the remaining overheads.
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%
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prolog:'$block'(Conds) :-
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generate_blocking_code(Conds, _, Code),
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'$current_module'(Module),
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'$$compile'(Code, Code, 5, Module), fail.
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prolog:'$block'(_).
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generate_blocking_code(Conds, G, Code) :-
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'$extract_head_for_block'(Conds, G),
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'$recorded'('$blocking_code','$code'(G,OldConds),R), !,
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erase(R),
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functor(G, Na, Ar),
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'$current_module'(M),
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abolish(M:Na, Ar),
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generate_blocking_code((Conds,OldConds), G, Code).
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generate_blocking_code(Conds, G, (G :- (If, !, when(When, G)))) :-
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extract_head_for_block(Conds, G),
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recorda('$blocking_code','$code'(G,Conds),_),
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generate_body_for_block(Conds, G, If, When).
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%
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% find out what we are blocking on.
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%
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extract_head_for_block((C1, _), G) :- !,
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extract_head_for_block(C1, G).
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extract_head_for_block(C, G) :-
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functor(C, Na, Ar),
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functor(G, Na, Ar).
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%
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% If we suspend on the conditions, we should continue
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% execution. If we don't suspend we should fail so that we can take
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% the next clause. To
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% know what we have to do we just test how many variables we suspended
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% on ;-).
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%
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%
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% We generate code as follows:
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%
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% block a(-,-,?)
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%
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% (var(A1), var(A2) -> true ; fail), !, when((nonvar(A1);nonvar(A2)),G).
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%
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% block a(-,-,?), a(?,-, -)
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%
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% (var(A1), var(A2) -> true ; (var(A2), var(A3) -> true ; fail)), !,
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% when(((nonvar(A1);nonvar(A2)),(nonvar(A2);nonvar(A3))),G).
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generate_body_for_block((C1, C2), G, (Code1 -> true ; Code2), (WhenConds,OtherWhenConds)) :- !,
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generate_for_cond_in_block(C1, G, Code1, WhenConds),
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generate_body_for_block(C2, G, Code2, OtherWhenConds).
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generate_body_for_block(C, G, (Code -> true ; fail), WhenConds) :-
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generate_for_cond_in_block(C, G, Code, WhenConds).
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generate_for_cond_in_block(C, G, Code, Whens) :-
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C =.. [_|Args],
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G =.. [_|GArgs],
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fetch_out_variables_for_block(Args,GArgs,L0Vars),
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add_blocking_vars(L0Vars, LVars),
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generate_for_each_arg_in_block(LVars, Code, Whens).
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add_blocking_vars([], [_]) :- !.
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add_blocking_vars(LV, LV).
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fetch_out_variables_for_block([], [], []).
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fetch_out_variables_for_block(['?'|Args], [_|GArgs], LV) :-
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fetch_out_variables_for_block(Args, GArgs, LV).
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fetch_out_variables_for_block(['-'|Args], [GArg|GArgs],
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[GArg|LV]) :-
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fetch_out_variables_for_block(Args, GArgs, LV).
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generate_for_each_arg_in_block([], false, true).
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generate_for_each_arg_in_block([V], var(V), nonvar(V)) :- !.
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generate_for_each_arg_in_block([V|L], (var(V),If), (nonvar(V);Whens)) :-
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generate_for_each_arg_in_block(L, If, Whens).
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%
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% The wait declaration is a simpler and more efficient version of block.
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%
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prolog:'$wait'(Na/Ar) :-
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functor(S, Na, Ar),
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arg(1, S, A),
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'$current_module'(M),
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'$$compile'((S :- var(A), !, freeze(A, S)), (S :- var(A), !, freeze(A, S)), 5, M), fail.
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prolog:'$wait'(_).
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frozen(V, G) :- nonvar(V), !,
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'$do_error'(type_error(variable,V),frozen(V,G)).
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frozen(V, LG) :-
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'$attributes':get_conj_from_attvars([V], LG).
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%internal_freeze(V,G) :-
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% attributes:get_att(V, 0, Gs), write(G+Gs),nl,fail.
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internal_freeze(V,G) :-
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update_att(V, G).
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update_att(V, G) :-
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attributes:get_module_atts(V, '$coroutining'(_,Gs)),
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not_vmember(G, Gs), !,
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attributes:put_module_atts(V, '$coroutining'(_,[G|Gs])).
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update_att(V, G) :-
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attributes:put_module_atts(V, '$coroutining'(_,[G])).
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not_vmember(_, []).
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not_vmember(V, [V1|DonesSoFar]) :-
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V \== V1,
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not_vmember(V, DonesSoFar).
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first_att(T, V) :-
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term_variables(T, Vs),
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check_first_attvar(Vs, V).
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check_first_attvar(V.Vs, V0) :- attvar(V), !, V == V0.
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check_first_attvar(_.Vs, V0) :-
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check_first_attvar(Vs, V0).
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