File: lazy_lists.pl

package info (click to toggle)
swi-prolog 8.0.2+dfsg-3+deb10u1
  • links: PTS, VCS
  • area: main
  • in suites: buster
  • size: 72,036 kB
  • sloc: ansic: 349,612; perl: 306,654; java: 5,208; cpp: 4,436; sh: 3,042; ruby: 1,594; yacc: 845; makefile: 136; xml: 82; sed: 12; sql: 6
file content (461 lines) | stat: -rw-r--r-- 15,839 bytes parent folder | download | duplicates (2)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
/*  Part of SWI-Prolog

    Author:        Jan Wielemaker
    E-mail:        J.Wielemaker@vu.nl
    WWW:           http://www.swi-prolog.org
    Copyright (c)  2016, VU University Amsterdam
                         CWI Amsterdam
    All rights reserved.

    Redistribution and use in source and binary forms, with or without
    modification, are permitted provided that the following conditions
    are met:

    1. Redistributions of source code must retain the above copyright
       notice, this list of conditions and the following disclaimer.

    2. Redistributions in binary form must reproduce the above copyright
       notice, this list of conditions and the following disclaimer in
       the documentation and/or other materials provided with the
       distribution.

    THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
    "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
    LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
    FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
    COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
    INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
    BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
    LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
    CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
    LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
    ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
    POSSIBILITY OF SUCH DAMAGE.
*/

:- module(lazy_lists,
          [ lazy_list/2,                        % :Next, -List
            lazy_list/3,                        % :Next, +State0, -List
                                                % Utilities
            lazy_list_materialize/1,            % ?List
            lazy_list_length/2,                 % +List, -Len

            lazy_findall/3,                     % ?Templ, :Goal, -List
            lazy_findall/4,                     % +ChunkSize, ?Templ, :Goal, -List
                                                % Interators
            lazy_get_codes/4,                   % +Stream, +N, -List, -Tail
            lazy_read_terms/4,                  % +Stream, +Options, -List, -Tail
            lazy_read_lines/4,                  % +Stream, +Options, -List, -Tail

            lazy_message_queue/4,               % +Queue, +Options, -List, -Tail
            lazy_engine_next/4,                 % +Engine, +N, -List, -Tail

            lazy_list_iterator/4                % +Iterator, -Next, :GetNext,
                                                % :TestEnd
          ]).
:- use_module(library(option)).
:- use_module(library(lists)).
:- use_module(library(error)).

:- meta_predicate
    lazy_list(2, -),
    lazy_list(3, +, -),
    lazy_findall(?, 0, -),
    lazy_findall(+, ?, 0, -).

/** <module> Lazy list handling

This module builds a lazy list from   a predicate that fetches a _slice_
of this list. In addition it provides _interactors_ (slice constructors)
for several common use cases for lazy  lists, such as reading objects of
several sizes from files (characters,   lines,  terms), reading messages
from message queues and reading answers from _engines_.

Lazy lists are lists that  end  in   a  constraint.  Trying to unify the
constraint forces the next slice of the list  to be fetched and added to
the list.

The typical use case for lazy lists is to   run a DCG grammar on it. For
example, an _agent_ may be listening on a socket and turn the line-based
message protocol into a list using the fragment below.

```
        ...,
        tcp_open(Socket, Read, Write),
        lazy_list(lazy_read_lines(Read, [as(codes)]), List),
        phrase(action, List).
```

Typically, the iterator works on a globally allocated object that is not
always subject to garbage collection.  In such cases, the skeleton usage
follows the pattern below:

```
        setup_call_cleanup(
            <open resource>(R),
            (  lazy_list(<iterator>(R), List),
               process_list(List)
            ),
            <close resource>(R))
```

This is rather unfortunately, but there is no way we can act on the fact
that `List` is no further accessed. In  some cases, e.g., message queues
or engines, the resource is subject to (atom) garbage collection.
*/

:- predicate_options(lazy_read_terms/4, 2,
                     [ chunk(positive_integer),
                       pass_to(read_term/3, 3)
                     ]).
:- predicate_options(lazy_read_lines/4, 2,
                     [ chunk(positive_integer),
                       as(oneof([atom,string,codes,chars]))
                     ]).
:- predicate_options(lazy_message_queue/4, 2,
                     [ chunk(positive_integer),
                       pass_to(thread_get_message/3, 3)
                     ]).

%!  lazy_list(:Next, -List)
%
%   Create a lazy list from a callback. Next is called repeatedly to
%   extend the list. It is called   as call(Next, List, Tail), where
%   the _difference list_ List\Tail produces the   next slice of the
%   list. If the end of  the  input   is  reached,  `List` must be a
%   proper list and `Tail` must be `[]`.
%
%   @bug The content returned  by  the   iterator  is  duplicated in
%   nb_setarg/3. This is  needed  by  avoid   the  risk  of  trailed
%   assignments in the structure. Avoiding   this  duplication would
%   significantly reduce the overhead.

lazy_list(Next, List) :-
    put_attr(List, lazy_lists, lazy_list(Next, _)).

attr_unify_hook(State, Value) :-
    State = lazy_list(Next, Read),
    (   var(Read)
    ->  call(Next, NewList, Tail),
        (   Tail == []
        ->  nb_setarg(2, State, NewList)
        ;   lazy_list(Next, Tail),
            nb_setarg(2, State, NewList)
        ),
        arg(2, State, Value)
    ;   Value = Read
    ).

attribute_goals(X) -->
    { get_attr(X, lazy_lists, lazy_list(Next, _)) },
    [lazy_list(Next, X)].

%!  lazy_list(:Next, +State0, -List)
%
%   Create a lazy list where the next element is defined by
%
%       call(Next, State0, State1, Head)
%
%   The example below uses this  predicate   to  define  a lazy list
%   holding the Fibonacci numbers. Our state  keeps the two previous
%   Fibonacci numbers.
%
%     ```
%     fibonacci_numbers(L) :-
%         lazy_list(fib, state(-,-), L).
%
%     fib(state(-,-), state(0,-), 0) :- !.
%     fib(state(0,-), state(1,0), 1) :- !.
%     fib(state(P,Q), state(F,P), F) :-
%         F is P+Q.
%     ```
%
%   The above can be used to retrieve   the Nth Fibonacci number. As
%   fib/2 provides no access  to  the   complete  list  of Fibonacci
%   numbers, this can be used to generate large Fibonacci numbers.
%
%     ```
%     fib(N, F) :-
%         fibonacci_numbers(L),
%         nth1(N, L, F).
%     ```

lazy_list(Next, State0, List) :-
    lazy_list(lazy_state(Next, s(State0)), List).

lazy_state(Pred, LState, [H|T], T) :-
    LState = s(State0),
    call(Pred, State0, State1, H),
    !,
    nb_setarg(1, LState, State1).
lazy_state(_, _, [], []).


                 /*******************************
                 *   OPERATIONS ON LAZY LISTS   *
                 *******************************/

%!  lazy_list_materialize(?List) is det.
%
%   Materialize the lazy list.

lazy_list_materialize(List) :-
    '$skip_list'(_, List, Tail),
    (   var(Tail),
        Tail = [_|T2]
    ->  lazy_list_materialize(T2)
    ;   Tail = []
    ->  true
    ;   type_error(list, Tail)
    ).

%!  lazy_list_length(+List, -Len) is det.
%
%   True if Len is the length of   the  materialized lazy list. Note
%   that length/2 reports the length   of the currently materialized
%   part and on backtracking longer lists.

lazy_list_length(List, Len) :-
    lazy_list_length(List, 0, Len).

lazy_list_length(List, L0, L) :-
    !,
    '$skip_list'(N, List, Tail),
    (   var(Tail),
        Tail = [_|T2]
    ->  L1 is L0+N+1,
        lazy_list_length(T2, L1, L)
    ;   Tail = []
    ->  L is L0+N
    ;   type_error(list, Tail)
    ).


                 /*******************************
                 *          INTERATORS          *
                 *******************************/

lazy_list_expand_handler(
    lazy_list_iterator(Handler, Next, Get1, TestEnd),
    Clauses) :-
    negate(TestEnd, NotTestEnd),
    extend_goal(Handler, [N, List, Tail], Head),
    extend_goal(Handler, [N2,T,Tail], Recurse),
    general_goal(Handler, Handler2),
    extend_goal(Handler2, [_, Tail,Tail], Head2),
    Clauses = [ (Head :-
                    succ(N2, N), !,
                    (   Get1,
                        NotTestEnd
                    ->  List = [Next|T],
                        Recurse
                    ;   List = [],
                        Tail = []
                    )),
                (Head2)
              ].

negate(A==B, A\==B) :- !.
negate(fail, true) :- !.
negate(false, true) :- !.
negate(Goal, \+ Goal).

extend_goal(Var, _, _) :-
    var(Var),
    !,
    instantiation_error(Var).
extend_goal(M:G, Args, M:GX) :-
    !,
    extend_goal(G, Args, GX).
extend_goal(Name, Args, GX) :-
    atom(Name),
    !,
    compound_name_arguments(GX, Name, Args).
extend_goal(G, XArgs, GX) :-
    compound_name_arguments(G, Name, Args0),
    append(Args0, XArgs, Args),
    compound_name_arguments(GX, Name, Args).

general_goal(Var, Var) :-
    var(Var),
    !.
general_goal(M:G, M:GG) :-
    !,
    general_goal(G, GG).
general_goal(Atom, Atom) :-
    atom(Atom),
    !.
general_goal(G, GG) :-
    !,
    compound_name_arity(G, Name, Arity),
    compound_name_arity(GG, Name, Arity).

:- multifile
    system:term_expansion/2.

system:term_expansion((:- lazy_list_iterator(It, One, GetNext, TestEnd)),
                      Expanded) :-
    lazy_list_expand_handler(
        lazy_list_iterator(It, One, GetNext, TestEnd),
        Expanded).

%!  lazy_list_iterator(+Iterator, -Next, :GetNext, :TestEnd)
%
%   Directive to create a lazy list  iterator from a predicate that
%   gets a single next value.

lazy_list_iterator(Iterator, Next, GetNext, TestEnd) :-
    throw(error(context_error(nodirective,
                              lazy_list_iterator(Iterator, Next,
                                                  GetNext, TestEnd)),
                _)).

%!  lazy_get_codes(+Stream, +N, -List, -Tail)
%
%   Lazy list iterator to get character   codes  from a stream.
%
%   @see library(pure_input) The predicate lazy_get_codes/4 provides
%   similar functionality to what   stream_to_lazy_list/2 does while
%   in addition library(pure_input) is faster due to the use of more
%   low-level primitives and supports fetching   the location in the
%   stream.

:- lazy_list_iterator(lazy_get_codes(Stream), Code,
                      get_code(Stream, Code),
                      Code == -1).

%!  lazy_read_terms(+Stream, +Options, -List, -Tail)
%
%   Turn a stream into a lazy list of Prolog terms.  Options are
%   passed to read_term/3, except for:
%
%     - chunk(ChunkSize)
%     Determines the read chunk size.  Default is 10.

lazy_read_terms(Stream, Options, List, Tail) :-
    select_option(chunk(N), Options, ReadOptions, 10),
    lazy_read_terms_(Stream, ReadOptions, N, List, Tail).

:- lazy_list_iterator(lazy_read_terms_(Stream, Options), Term,
                      read_term(Stream, Term, Options),
                      Term == end_of_file).

%!  lazy_read_lines(+Stream, +Options, -List, -Tail) is det.
%
%   Lazy list iterator to read lines from Stream.  Options include:
%
%     - chunk(ChunkSize)
%     Determines the read chunk size.  Default is 10.
%     - as(+Type)
%     Determine the output type for each line.  Valid values are
%     `atom`, `string`, `codes` or `chars`.  Default is `string`.

lazy_read_lines(Stream, Options, List, Tail) :-
    option(chunk(ChunkSize), Options, 10),
    option(as(Type), Options, string),
    must_be(positive_integer, ChunkSize),
    must_be(oneof([atom,string,codes,chars]), Type),
    lazy_read_lines(Type, Stream, ChunkSize, List, Tail).

lazy_read_lines(string, Stream, ChunkSize, List, Tail) :-
    lazy_read_string_lines(Stream, ChunkSize, List, Tail).
lazy_read_lines(atom, Stream, ChunkSize, List, Tail) :-
    lazy_read_atom_lines(Stream, ChunkSize, List, Tail).
lazy_read_lines(codes, Stream, ChunkSize, List, Tail) :-
    lazy_read_codes_lines(Stream, ChunkSize, List, Tail).
lazy_read_lines(chars, Stream, ChunkSize, List, Tail) :-
    lazy_read_chars_lines(Stream, ChunkSize, List, Tail).

:- lazy_list_iterator(lazy_read_string_lines(Stream), Line,
                      read_line_to_string(Stream, Line),
                      Line == end_of_file).
:- lazy_list_iterator(lazy_read_codes_lines(Stream), Line,
                      read_line_to_codes(Stream, Line),
                      Line == end_of_file).
:- lazy_list_iterator(lazy_read_chars_lines(Stream), Line,
                      read_line_to_chars(Stream, Line),
                      Line == end_of_file).
:- lazy_list_iterator(lazy_read_atom_lines(Stream), Line,
                      read_line_to_atom(Stream, Line),
                      Line == -1).

read_line_to_chars(Stream, Chars) :-
    read_line_to_string(Stream, String),
    (   String == end_of_file
    ->  Chars = String
    ;   string_chars(String, Chars)
    ).

read_line_to_atom(Stream, Atom) :-
    read_line_to_string(Stream, String),
    (   String == end_of_file
    ->  Atom = -1
    ;   atom_string(Atom, String)
    ).

%!  lazy_message_queue(+Queue, +Options, -List, -Tail) is det.
%
%   Lazy list iterator for message  queues.   Options  are passed to
%   thread_get_message/3. In addition,  the   following  options are
%   processed:
%
%     - chunk(ChunkSize)
%     Determines the read chunk size.  Default is 1.
%
%   A thread can listen to its own message queue using
%
%   ```
%           thread_self(Me),
%           lazy_list(lazy_message_queue(Me, []), List),
%           phrase(action(List)).
%   ```

lazy_message_queue(Queue, Options, List, Tail) :-
    select_option(chunk(ChunkSize), Options, QueueOptions, 1),
    lazy_message_queue_(Queue, QueueOptions, ChunkSize, List, Tail).

:- lazy_list_iterator(lazy_message_queue_(Queue, Options), Message,
                      thread_get_message(Queue, Message, Options),
                      fail).


%!  lazy_engine_next(+Engine, +N, -List, -Tail)
%
%   Lazy list iterator for  engines.  This   is  used  to  implement
%   lazy_findall/3,4.

:- lazy_list_iterator(lazy_engine_next(Engine), Answer,
                      engine_next(Engine, Answer),
                      fail).

%!  lazy_findall(?Templ, :Goal, -List) is det.
%!  lazy_findall(+ChunkSize, ?Templ, :Goal, -List) is det.
%
%   True when List is a lazy  list containing the instantiations for
%   Template for each  answer  of  Goal.   Goal  is  executed  in an
%   _engine_ (see engine_create/3).
%
%   @bug    Engines are reclaimed by atom garbage collection.  As
%           they can be quite expensive, a large amount of resources
%           may be waiting for collection.  If the list is fully
%           materialized only the dead engine remains, which is
%           fairly cheap.

lazy_findall(Templ, Goal, List) :-
    lazy_findall(1, Templ, Goal, List).
lazy_findall(Chunk, Templ, Goal, List) :-
    engine_create(Templ, Goal, Engine),
    lazy_list(lazy_engine_next(Engine, Chunk), List).


                 /*******************************
                 *            SANDBOX           *
                 *******************************/

:- multifile
    sandbox:safe_meta_predicate/1.

sandbox:safe_meta_predicate(lazy_lists:lazy_findall/3).
sandbox:safe_meta_predicate(lazy_lists:lazy_findall/4).
sandbox:safe_meta_predicate(lazy_lists:lazy_list/2).
sandbox:safe_meta_predicate(lazy_lists:lazy_list/3).