1/*  Part of SWI-Prolog
    2
    3    Author:        Tom Schrijvers, K.U.Leuven
    4    E-mail:        Tom.Schrijvers@cs.kuleuven.ac.be
    5    WWW:           http://www.swi-prolog.org
    6    Copyright (c)  2004-2016, K.U.Leuven
    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
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   19       distribution.
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   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
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   33*/
   34
   35:- module(when,
   36          [ when/2                      % +Condition, :Goal
   37          ]).   38:- set_prolog_flag(generate_debug_info, false).   39
   40:- meta_predicate
   41    when(+, 0),
   42    suspend_list(+, 0),
   43    trigger(+, 0),
   44    trigger_disj(+, 0),
   45    trigger_conj(+, +, 0).

    ...
    when(ground(Expr), 0 =:= Expr),

 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

   84when(Condition, Goal) :-
   85    '$eval_when_condition'(Condition, Optimised),
   86    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).

   98trigger_first(true, Goal) :-
   99    !,
  100    call(Goal).
  101trigger_first(nonvar(X), Goal) :-
  102    !,
  103    '$suspend'(X, when, trigger_nonvar(X, Goal)).
  104trigger_first(Cond, Goal) :-
  105    trigger(Cond, Goal).
  106
  107trigger(nonvar(X),Goal) :-
  108    trigger_nonvar(X,Goal).
  109trigger(ground(X),Goal) :-
  110    trigger_ground(X,Goal).
  111trigger(?=(X,Y),Goal) :-
  112    trigger_determined(X,Y,Goal).
  113trigger((G1,G2),Goal) :-
  114    trigger_conj(G1,G2,Goal).
  115trigger(or(GL),Goal) :-
  116    trigger_disj(GL, check_disj(_DisjID,GL,Goal)).
  117
  118trigger_nonvar(X, Goal) :-
  119    (   nonvar(X)
  120    ->  call(Goal)
  121    ;   '$suspend'(X, when, trigger_nonvar(X, Goal))
  122    ).

 trigger_ground(@Term, :Goal)

Trigger Goal when Term becomes ground. The current implementation uses nonground/2, waiting for an atribtrary variable and re-check Term when this variable is bound. Previous version used term_variables and suspended on a term constructed from these variables. It is clear that either approach performs better on certain types of terms. The term_variables/2 based approach wins on large terms that are almost ground. Possibly we need a nonground that also returns the number of tests performed and switch to the term_variables/2 based approach if this becomes large.

  136trigger_ground(X, Goal) :-
  137    (   nonground(X, V)
  138    ->  '$suspend'(V, when, trigger_ground(X, Goal))
  139    ;   call(Goal)
  140    ).
  141
  142trigger_determined(X, Y, Goal) :-
  143    unifiable(X, Y, Unifier),
  144    !,
  145    (   Unifier == []
  146    ->  call(Goal)
  147    ;   put_attr(Det, when, det(trigger_determined(X,Y,Goal))),
  148        suspend_list(Unifier, wake_det(Det))
  149    ).
  150trigger_determined(_, _, Goal) :-
  151    call(Goal).
  152
  153
  154wake_det(Det) :-
  155    ( var(Det) ->
  156            get_attr(Det,when,Attr),
  157            del_attr(Det,when),
  158            Det = (-),
  159            Attr = det(Goal),
  160            call(Goal)
  161    ;
  162            true
  163    ).
  164
  165trigger_conj(G1,G2,Goal) :-
  166    trigger(G1, trigger(G2,Goal)).
  167
  168trigger_disj([],_).
  169trigger_disj([H|T], G) :-
  170    trigger(H, G),
  171    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.

  183check_disj(Disj,_,Goal) :-
  184    (   Disj == (-)
  185    ->  true
  186    ;   Disj = (-),
  187        call(Goal)
  188    ).
  189
  190suspend_list([],_Goal).
  191suspend_list([V=W|Unifier],Goal) :-
  192    '$suspend'(V, when, Goal),
  193    (   var(W)
  194    ->  '$suspend'(W, when, Goal)
  195    ;   true
  196    ),
  197    suspend_list(Unifier,Goal).
  198
  199attr_unify_hook(call(Goal), Other) :-
  200    (   get_attr(Other, when, call(GOTher))
  201    ->  del_attr(Other, when),
  202        Goal, GOTher
  203    ;   Goal
  204    ).
  205
  206
  207%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
  208attribute_goals(V) -->
  209    { get_attr(V, when, Attr) },
  210    when_goals(Attr).
  211
  212when_goals(det(trigger_determined(X, Y, G))) -->
  213    !,
  214    (   { disj_goal(G, Disj, DG) }
  215    ->  disj_or(Disj, DG)
  216    ;   { G = when:trigger(C, Goal) }
  217    ->  [ when((?=(X,Y),C), Goal) ]
  218    ;   [ when(?=(X,Y), G) ]
  219    ).
  220when_goals(call(Conj)) -->
  221    when_conj_goals(Conj).
  222
  223when_conj_goals((A,B)) -->
  224    !,
  225    when_conj_goals(A),
  226    when_conj_goals(B).
  227when_conj_goals(when:G) -->
  228    when_goal(G).
  229
  230when_goal(trigger_nonvar(X, G)) -->
  231    (   { disj_goal(G, Disj, DG) }
  232    ->  disj_or(Disj, DG)
  233    ;   { G = when:trigger(C, Goal) }
  234    ->  [ when((nonvar(X),C), Goal) ]
  235    ;   [ when(nonvar(X),G) ]
  236    ).
  237when_goal(trigger_ground(X, G)) -->
  238    (   { disj_goal(G, Disj, DG) }
  239    ->  disj_or(Disj, DG)
  240    ;   { G = when:trigger(C, Goal) }
  241    ->  [ when((ground(X),C), Goal) ]
  242    ;   [ when(ground(X),G) ]
  243    ).
  244when_goal(wake_det(_)) -->
  245    [].
  246
  247disj_goal(when:check_disj(X, _, _), [], -) :- X == (-).
  248disj_goal(when:check_disj(-, Or, DG), Or, DG).
  249
  250disj_or([], _) --> [].
  251disj_or(List, DG) -->
  252    { or_list(List, Or) },
  253    [when(Or, DG)].
  254
  255or_list([H], H) :- !.
  256or_list([H|T], (H;OT)) :-
  257    or_list(T, OT).
  258
  259:- multifile sandbox:safe_meta_predicate/1.  260
  261sandbox:safe_meta_predicate(when:when/2)