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) 2020, 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(intercept, 36 [ intercept/3, % :Goal, ?Ball, :Handler 37 intercept/4, % :Goal, ?Ball, :Handler, +Arg 38 intercept_all/4, % +Templ, :Goal, ?Ball, -List 39 nb_intercept_all/4, % +Templ, :Goal, ?Ball, -List 40 send_signal/1, % +Ball 41 send_silent_signal/1 % +Ball 42 ]). 43:- autoload(library(error),[must_be/2]).

Intercept and signal interface

This library allows for creating an execution context (goal) which defines how calls to send_signal/1 are handled. This library is typically used to fetch values from the context or process results depending on the context.

For example, assume we parse a (large) file using a grammar (see phrase_from_file/3) that has some sort of record structure. What should we do with the recognised records? We can return them in a list, but if the input is large this is a huge overhead if the records are to be asserted or written to a file. Using this interface we can use

document -->
    record(Record),
    !,
    { send_signal(record(Record)) },
    document.
document -->
    [].

Given the above, we can assert all records into the database using the following query:

    ...,
    intercept(phrase_from_file(File, document),
              record(Record),
              assertz(Record)).

Or, we can collect all records in a list using intercept_all/4:

    ...,
    intercept_all(Record,
                  phrase_from_file(File, document), record(Record),
                  Records).

132intercept(Goal, Ball, Handler, Context) :- 133 do_intercept(Goal, Ball, Handler, args(Context)). 134 135do_intercept(Goal, Ball, Handler, Context) :- 136 Goal, 137 no_lco(Ball, Handler, Context). 138 139no_lco(_,_,_).

?- intercept_all(I, enum(1,6), emit(I), List). List = [1, 2, 3, 4, 5, 6]. ?- intercept_all(I, (between(1,3,Max),enum(1,Max)), emit(I), List). Max = 1, List = [1] ; Max = 2, List = [1, 2] ; Max = 3, List = [1, 2, 3].

166intercept_all(Template, Goal, Ball, List) :- 167 List0 = [_], 168 State = list(List0, List0), 169 intercept(Goal, Ball, add_ball(Template), State), 170 arg(1, State, [_|List]). 171 172add_ball(Elem, State) :- 173 Tail = [Elem], 174 arg(2, State, List), 175 setarg(2, List, Tail), 176 setarg(2, State, Tail).

193nb_intercept_all(Template, Goal, Ball, List) :- 194 List0 = [_], 195 State = list(List0, List0), 196 intercept(Goal, Ball, nb_add_ball(Template), State), 197 arg(1, State, [_|List]). 198 199nb_add_ball(Elem, State) :- 200 duplicate_term(Elem, Copy), 201 Tail = [Copy], 202 arg(2, State, List), 203 nb_linkarg(2, List, Tail), 204 nb_linkarg(2, State, Tail).

214send_signal(Signal) :- 215 must_be(nonvar, Signal), 216 prolog_current_frame(Frame), 217 ( interceptor(Frame, Signal, Handler, Context) 218 -> call_handler(Context, Handler) 219 ; throw(error(unintercepted_signal(Signal), _)) 220 ).

227send_silent_signal(Signal) :- 228 must_be(nonvar, Signal), 229 prolog_current_frame(Frame), 230 ( interceptor(Frame, Signal, Handler, Context) 231 -> call_handler(Context, Handler) 232 ; true 233 ). 234 235call_handler(args, Handler) :- 236 call(Handler). 237call_handler(args(A0), Handler) :- 238 call(Handler, A0). 239 240interceptor(Frame, Signal, Handler, Context) :- 241 prolog_frame_attribute(Frame, parent_goal(Next), 242 intercept:do_intercept(_Goal, Signal0, Handler0, Context)), 243 ( copy_term(Signal0+Handler0, Signal+Handler) 244 -> true 245 ; interceptor(Next, Signal, Handler, Context) 246 ). 247 248 249 /******************************* 250 * SANDBOX * 251 *******************************/ 252 253:- multifile 254 sandbox:safe_meta_predicate/1, 255 sandbox:safe_primitive/1. 256 257sandbox:safe_meta_predicate(intercept:intercept/3). 258sandbox:safe_meta_predicate(intercept:intercept/4). 259sandbox:safe_meta_predicate(intercept:intercept_all/4). 260sandbox:safe_meta_predicate(intercept:nb_intercept_all/4). 261 262sandbox:safe_primitive(intercept:send_signal(_)). 263sandbox:safe_primitive(intercept:send_silent_signal(_))