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    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)  2016, VU University Amsterdam
    7                         CWI Amsterdam
    8    All rights reserved.
    9
   10    Redistribution and use in source and binary forms, with or without
   11    modification, are permitted provided that the following conditions
   12    are met:
   13
   14    1. Redistributions of source code must retain the above copyright
   15       notice, this list of conditions and the following disclaimer.
   16
   17    2. Redistributions in binary form must reproduce the above copyright
   18       notice, this list of conditions and the following disclaimer in
   19       the documentation and/or other materials provided with the
   20       distribution.
   21
   22    THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
   23    "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
   24    LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
   25    FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
   26    COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
   27    INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
   28    BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
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   34*/
   35
   36:- module(lazy_lists,
   37          [ lazy_list/2,                        % :Next, -List
   38            lazy_list/3,                        % :Next, +State0, -List
   39                                                % Utilities
   40            lazy_list_materialize/1,            % ?List
   41            lazy_list_length/2,                 % +List, -Len
   42
   43            lazy_findall/3,                     % ?Templ, :Goal, -List
   44            lazy_findall/4,                     % +ChunkSize, ?Templ, :Goal, -List
   45                                                % Interators
   46            lazy_get_codes/4,                   % +Stream, +N, -List, -Tail
   47            lazy_read_terms/4,                  % +Stream, +Options, -List, -Tail
   48            lazy_read_lines/4,                  % +Stream, +Options, -List, -Tail
   49
   50            lazy_message_queue/4,               % +Queue, +Options, -List, -Tail
   51            lazy_engine_next/4,                 % +Engine, +N, -List, -Tail
   52
   53            lazy_list_iterator/4                % +Iterator, -Next, :GetNext,
   54                                                % :TestEnd
   55          ]).   56:- autoload(library(error),
   57	    [type_error/2,instantiation_error/1,must_be/2]).   58:- autoload(library(lists),[append/3]).   59:- autoload(library(option),[select_option/4,option/3]).   60:- autoload(library(readutil),
   61	    [read_line_to_string/2,read_line_to_codes/2]).   62
   63
   64:- meta_predicate
   65    lazy_list(2, -),
   66    lazy_list(3, +, -),
   67    lazy_findall(?, 0, -),
   68    lazy_findall(+, ?, 0, -).

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. */

  111:- predicate_options(lazy_read_terms/4, 2,
  112                     [ chunk(positive_integer),
  113                       pass_to(read_term/3, 3)
  114                     ]).  115:- predicate_options(lazy_read_lines/4, 2,
  116                     [ chunk(positive_integer),
  117                       as(oneof([atom,string,codes,chars]))
  118                     ]).  119:- predicate_options(lazy_message_queue/4, 2,
  120                     [ chunk(positive_integer),
  121                       pass_to(thread_get_message/3, 3)
  122                     ]).
 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.
  137lazy_list(Next, List) :-
  138    put_attr(List, lazy_lists, lazy_list(Next, _)).
  139
  140% (*) We need a copy of the  list   where  the copy must include the new
  141% attributed  variable  to  avoid  that   backtracking  makes  the  list
  142% non-lazy.  We do want to avoid copying `Next`.  So, we add a dummy and
  143% then replace this using nb_linkarg/3 with our Next.
  144
  145attr_unify_hook(State, Value) :-
  146    State = lazy_list(Next, Read),
  147    (   var(Read)
  148    ->  call(Next, NewList, Tail),
  149        (   Tail == []
  150        ->  nb_setarg(2, State, NewList)
  151        ;   put_attr(Tail, lazy_lists, lazy_list(dummy, _)),  % See (*)
  152            nb_setarg(2, State, NewList),
  153            arg(2, State, NewListCP),
  154            '$skip_list'(_, NewListCP, TailCP),
  155            get_attr(TailCP, lazy_lists, LazyList),
  156            nb_linkarg(1, LazyList, Next)
  157        ),
  158        arg(2, State, Value)
  159    ;   Value = Read
  160    ).
  161
  162attribute_goals(X) -->
  163    { get_attr(X, lazy_lists, lazy_list(Next, _)) },
  164    [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).
  196lazy_list(Next, State0, List) :-
  197    lazy_list(lazy_state(Next, s(State0)), List).
  198
  199lazy_state(Pred, LState, [H|T], T) :-
  200    LState = s(State0),
  201    call(Pred, State0, State1, H),
  202    !,
  203    nb_setarg(1, LState, State1).
  204lazy_state(_, _, [], []).
  205
  206
  207                 /*******************************
  208                 *   OPERATIONS ON LAZY LISTS   *
  209                 *******************************/
 lazy_list_materialize(?List) is det
Materialize the lazy list.
  215lazy_list_materialize(List) :-
  216    '$skip_list'(_, List, Tail),
  217    (   var(Tail),
  218        Tail = [_|T2]
  219    ->  lazy_list_materialize(T2)
  220    ;   Tail = []
  221    ->  true
  222    ;   type_error(list, Tail)
  223    ).
 lazy_list_length(+List, -Len) is det
True if Len is the length of the materialized lazy list. Note that length/2 reports the length of the currently materialized part and on backtracking longer lists.
  231lazy_list_length(List, Len) :-
  232    lazy_list_length(List, 0, Len).
  233
  234lazy_list_length(List, L0, L) :-
  235    !,
  236    '$skip_list'(N, List, Tail),
  237    (   var(Tail),
  238        Tail = [_|T2]
  239    ->  L1 is L0+N+1,
  240        lazy_list_length(T2, L1, L)
  241    ;   Tail = []
  242    ->  L is L0+N
  243    ;   type_error(list, Tail)
  244    ).
  245
  246
  247                 /*******************************
  248                 *          INTERATORS          *
  249                 *******************************/
  250
  251lazy_list_expand_handler(
  252    lazy_list_iterator(Handler, Next, Get1, TestEnd),
  253    Clauses) :-
  254    negate(TestEnd, NotTestEnd),
  255    extend_goal(Handler, [N, List, Tail], Head),
  256    extend_goal(Handler, [N2,T,Tail], Recurse),
  257    general_goal(Handler, Handler2),
  258    extend_goal(Handler2, [_, Tail,Tail], Head2),
  259    Clauses = [ (Head :-
  260                    succ(N2, N), !,
  261                    (   Get1,
  262                        NotTestEnd
  263                    ->  List = [Next|T],
  264                        Recurse
  265                    ;   List = [],
  266                        Tail = []
  267                    )),
  268                (Head2)
  269              ].
  270
  271negate(A==B, A\==B) :- !.
  272negate(fail, true) :- !.
  273negate(false, true) :- !.
  274negate(Goal, \+ Goal).
  275
  276extend_goal(Var, _, _) :-
  277    var(Var),
  278    !,
  279    instantiation_error(Var).
  280extend_goal(M:G, Args, M:GX) :-
  281    !,
  282    extend_goal(G, Args, GX).
  283extend_goal(Name, Args, GX) :-
  284    atom(Name),
  285    !,
  286    compound_name_arguments(GX, Name, Args).
  287extend_goal(G, XArgs, GX) :-
  288    compound_name_arguments(G, Name, Args0),
  289    append(Args0, XArgs, Args),
  290    compound_name_arguments(GX, Name, Args).
  291
  292general_goal(Var, Var) :-
  293    var(Var),
  294    !.
  295general_goal(M:G, M:GG) :-
  296    !,
  297    general_goal(G, GG).
  298general_goal(Atom, Atom) :-
  299    atom(Atom),
  300    !.
  301general_goal(G, GG) :-
  302    !,
  303    compound_name_arity(G, Name, Arity),
  304    compound_name_arity(GG, Name, Arity).
  305
  306:- multifile
  307    system:term_expansion/2.  308
  309system:term_expansion((:- lazy_list_iterator(It, One, GetNext, TestEnd)),
  310                      Expanded) :-
  311    lazy_list_expand_handler(
  312        lazy_list_iterator(It, One, GetNext, TestEnd),
  313        Expanded).
 lazy_list_iterator(+Iterator, -Next, :GetNext, :TestEnd)
Directive to create a lazy list iterator from a predicate that gets a single next value.
  320lazy_list_iterator(Iterator, Next, GetNext, TestEnd) :-
  321    throw(error(context_error(nodirective,
  322                              lazy_list_iterator(Iterator, Next,
  323                                                  GetNext, TestEnd)),
  324                _)).
 lazy_get_codes(+Stream, +N, -List, -Tail)
Lazy list iterator to get character codes from a stream.
See also
- 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.
  336:- lazy_list_iterator(lazy_get_codes(Stream), Code,
  337                      get_code(Stream, Code),
  338                      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.
  348lazy_read_terms(Stream, Options, List, Tail) :-
  349    select_option(chunk(N), Options, ReadOptions, 10),
  350    lazy_read_terms_(Stream, ReadOptions, N, List, Tail).
  351
  352:- lazy_list_iterator(lazy_read_terms_(Stream, Options), Term,
  353                      read_term(Stream, Term, Options),
  354                      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.
  366lazy_read_lines(Stream, Options, List, Tail) :-
  367    option(chunk(ChunkSize), Options, 10),
  368    option(as(Type), Options, string),
  369    must_be(positive_integer, ChunkSize),
  370    must_be(oneof([atom,string,codes,chars]), Type),
  371    lazy_read_lines(Type, Stream, ChunkSize, List, Tail).
  372
  373lazy_read_lines(string, Stream, ChunkSize, List, Tail) :-
  374    lazy_read_string_lines(Stream, ChunkSize, List, Tail).
  375lazy_read_lines(atom, Stream, ChunkSize, List, Tail) :-
  376    lazy_read_atom_lines(Stream, ChunkSize, List, Tail).
  377lazy_read_lines(codes, Stream, ChunkSize, List, Tail) :-
  378    lazy_read_codes_lines(Stream, ChunkSize, List, Tail).
  379lazy_read_lines(chars, Stream, ChunkSize, List, Tail) :-
  380    lazy_read_chars_lines(Stream, ChunkSize, List, Tail).
  381
  382:- lazy_list_iterator(lazy_read_string_lines(Stream), Line,
  383                      read_line_to_string(Stream, Line),
  384                      Line == end_of_file).  385:- lazy_list_iterator(lazy_read_codes_lines(Stream), Line,
  386                      read_line_to_codes(Stream, Line),
  387                      Line == end_of_file).  388:- lazy_list_iterator(lazy_read_chars_lines(Stream), Line,
  389                      read_line_to_chars(Stream, Line),
  390                      Line == end_of_file).  391:- lazy_list_iterator(lazy_read_atom_lines(Stream), Line,
  392                      read_line_to_atom(Stream, Line),
  393                      Line == -1).  394
  395read_line_to_chars(Stream, Chars) :-
  396    read_line_to_string(Stream, String),
  397    (   String == end_of_file
  398    ->  Chars = String
  399    ;   string_chars(String, Chars)
  400    ).
  401
  402read_line_to_atom(Stream, Atom) :-
  403    read_line_to_string(Stream, String),
  404    (   String == end_of_file
  405    ->  Atom = -1
  406    ;   atom_string(Atom, String)
  407    ).
 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)).
  426lazy_message_queue(Queue, Options, List, Tail) :-
  427    select_option(chunk(ChunkSize), Options, QueueOptions, 1),
  428    lazy_message_queue_(Queue, QueueOptions, ChunkSize, List, Tail).
  429
  430:- lazy_list_iterator(lazy_message_queue_(Queue, Options), Message,
  431                      thread_get_message(Queue, Message, Options),
  432                      fail).
 lazy_engine_next(+Engine, +N, -List, -Tail)
Lazy list iterator for engines. This is used to implement lazy_findall/3,4.
  440:- lazy_list_iterator(lazy_engine_next(Engine), Answer,
  441                      engine_next(Engine, Answer),
  442                      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.
  457lazy_findall(Templ, Goal, List) :-
  458    lazy_findall(1, Templ, Goal, List).
  459lazy_findall(Chunk, Templ, Goal, List) :-
  460    engine_create(Templ, Goal, Engine),
  461    lazy_list(lazy_engine_next(Engine, Chunk), List).
  462
  463
  464                 /*******************************
  465                 *            SANDBOX           *
  466                 *******************************/
  467
  468:- multifile
  469    sandbox:safe_meta_predicate/1.  470
  471sandbox:safe_meta_predicate(lazy_lists:lazy_findall/3).
  472sandbox:safe_meta_predicate(lazy_lists:lazy_findall/4).
  473sandbox:safe_meta_predicate(lazy_lists:lazy_list/2).
  474sandbox:safe_meta_predicate(lazy_lists:lazy_list/3)