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) 2015-2017, 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(solution_sequences, 36 [ distinct/1, % :Goal 37 distinct/2, % ?Witness, :Goal 38 reduced/1, % :Goal 39 reduced/3, % ?Witness, :Goal, +Options 40 limit/2, % +Limit, :Goal 41 offset/2, % +Offset, :Goal 42 order_by/2, % +Spec, :Goal 43 group_by/4 % +By, +Template, :Goal, -Bag 44 ]). 45:- use_module(library(nb_set)). 46:- use_module(library(error)). 47:- use_module(library(apply)). 48:- use_module(library(lists)). 49:- use_module(library(ordsets)). 50:- use_module(library(option)).

Modify solution sequences

The meta predicates of this library modify the sequence of solutions of a goal. The modifications and the predicate names are based on the classical database operations DISTINCT, LIMIT, OFFSET, ORDER BY and GROUP BY.

These predicates were introduced in the context of the SWISH Prolog browser-based shell, which can represent the solutions to a predicate as a table. Notably wrapping a goal in distinct/1 avoids duplicates in the result table and using order_by/2 produces a nicely ordered table.

However, the predicates from this library can also be used to stay longer within the clean paradigm where non-deterministic predicates are composed from simpler non-deterministic predicates by means of conjunction and disjunction. While evaluating a conjunction, we might want to eliminate duplicates of the first part of the conjunction. Below we give both the classical solution for solving variations of (a(X), b(X)) and the ones using this library side-by-side.

Avoid duplicates of earlier steps

  setof(X, a(X), Xs),               distinct(a(X)),
  member(X, Xs),                    b(X)
  b(X).

Note that the distinct/1 based solution returns the first result of distinct(a(X)) immediately after a/1 produces a result, while the setof/3 based solution will first compute all results of a/1.

Only try b(X) only for the top-10 a(X)

  setof(X, a(X), Xs),               limit(10, order_by([desc(X)], a(X))),
  reverse(Xs, Desc),                b(X)
  first_max_n(10, Desc, Limit),
  member(X, Limit),
  b(X)

Here we see power of composing primitives from this library and staying within the paradigm of pure non-deterministic relational predicates.

See also: - all solution predicates findall/3, bagof/3 and setof/3.; - library(aggregate) */

103:- meta_predicate 104 distinct(0), 105 distinct(?, 0), 106 reduced(0), 107 reduced(?, 0, +), 108 limit(+, 0), 109 offset(+, 0), 110 order_by(+, 0), 111 group_by(?, ?, 0, -).

132distinct(Goal) :- 133 distinct(Goal, Goal). 134distinct(Witness, Goal) :- 135 term_variables(Witness, Vars), 136 Witness1 =.. [v|Vars], 137 empty_nb_set(Set), 138 call(Goal), 139 add_nb_set(Witness1, Set, true).

156reduced(Goal) :- 157 reduced(Goal, Goal, []). 158reduced(Witness, Goal, Options) :- 159 option(size_limit(SizeLimit), Options, 10_000), 160 term_variables(Witness, Vars), 161 Witness1 =.. [v|Vars], 162 empty_nb_set(Set), 163 State = state(Set), 164 call(Goal), 165 reduced_(State, Witness1, SizeLimit). 166 167reduced_(State, Witness1, SizeLimit) :- 168 arg(1, State, Set), 169 add_nb_set(Witness1, Set, true), 170 size_nb_set(Set, Size), 171 ( Size > SizeLimit 172 -> empty_nb_set(New), 173 nb_setarg(1, State, New) 174 ; true 175 ).

184limit(Count, Goal) :- 185 Count > 0, 186 State = count(0), 187 call(Goal), 188 arg(1, State, N0), 189 N is N0+1, 190 ( N =:= Count 191 -> ! 192 ; nb_setarg(1, State, N) 193 ).

201offset(Count, Goal) :- 202 Count > 0, 203 !, 204 State = count(0), 205 call(Goal), 206 arg(1, State, N0), 207 ( N0 >= Count 208 -> true 209 ; N is N0+1, 210 nb_setarg(1, State, N), 211 fail 212 ). 213offset(Count, Goal) :- 214 Count =:= 0, 215 !, 216 call(Goal). 217offset(Count, _) :- 218 domain_error(not_less_than_zero, Count).

232order_by(Spec, Goal) :- 233 must_be(list, Spec), 234 non_empty_list(Spec), 235 maplist(order_witness, Spec, Witnesses0), 236 join_orders(Witnesses0, Witnesses), 237 non_witness_template(Goal, Witnesses, Others), 238 reverse(Witnesses, RevWitnesses), 239 maplist(x_vars, RevWitnesses, WitnessVars), 240 Template =.. [v,Others|WitnessVars], 241 findall(Template, Goal, Results), 242 order(RevWitnesses, 2, Results, OrderedResults), 243 member(Template, OrderedResults). 244 245order([], _, Results, Results). 246order([H|T], N, Results0, Results) :- 247 order1(H, N, Results0, Results1), 248 N2 is N + 1, 249 order(T, N2, Results1, Results). 250 251order1(asc(_), N, Results0, Results) :- 252 sort(N, @=<, Results0, Results). 253order1(desc(_), N, Results0, Results) :- 254 sort(N, @>=, Results0, Results). 255 256non_empty_list([]) :- 257 !, 258 domain_error(non_empty_list, []). 259non_empty_list(_). 260 261order_witness(Var, _) :- 262 var(Var), 263 !, 264 instantiation_error(Var). 265order_witness(asc(Term), asc(Witness)) :- 266 !, 267 witness(Term, Witness). 268order_witness(desc(Term), desc(Witness)) :- 269 !, 270 witness(Term, Witness). 271order_witness(Term, _) :- 272 domain_error(order_specifier, Term). 273 274x_vars(asc(Vars), Vars). 275x_vars(desc(Vars), Vars). 276 277witness(Term, Witness) :- 278 term_variables(Term, Vars), 279 Witness =.. [v|Vars].

286join_orders([], []). 287join_orders([asc(O1)|T0], [asc(O)|T]) :- 288 !, 289 ascs(T0, OL, T1), 290 join_witnesses(O1, OL, O), 291 join_orders(T1, T). 292join_orders([desc(O1)|T0], [desc(O)|T]) :- 293 !, 294 descs(T0, OL, T1), 295 join_witnesses(O1, OL, O), 296 join_orders(T1, T). 297 298ascs([asc(A)|T0], [A|AL], T) :- 299 !, 300 ascs(T0, AL, T). 301ascs(L, [], L). 302 303descs([desc(A)|T0], [A|AL], T) :- 304 !, 305 descs(T0, AL, T). 306descs(L, [], L). 307 308join_witnesses(O, [], O) :- !. 309join_witnesses(O, OL, R) :- 310 term_variables([O|OL], VL), 311 R =.. [v|VL].

318non_witness_template(Goal, Witness, Template) :- 319 ordered_term_variables(Goal, AllVars), 320 ordered_term_variables(Witness, WitnessVars), 321 ord_subtract(AllVars, WitnessVars, TemplateVars), 322 Template =.. [t|TemplateVars]. 323 324ordered_term_variables(Term, Vars) :- 325 term_variables(Term, Vars0), 326 sort(Vars0, Vars).

336group_by(By, Template, Goal, Bag) :- 337 ordered_term_variables(Goal, GVars), 338 ordered_term_variables(By+Template, UVars), 339 ord_subtract(GVars, UVars, ExVars), 340 bagof(Template, ExVars^Goal, Bag)