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lazy_lists.pl -- Lazy list handling
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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:

            <open resource>(R),
            (  lazy_list(<iterator>(R), 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.

Source 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 [].
- 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.
Source 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) :-
    nth1(N, L, F).
Source lazy_list_materialize(?List) is det
Materialize the lazy list.
Source 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.
Source lazy_list_iterator(+Iterator, -Next, :GetNext, :TestEnd)
Directive to create a lazy list iterator from a predicate that gets a single next value.
Source 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.
Source 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:
Determines the read chunk size. Default is 10.
Source lazy_read_lines(+Stream, +Options, -List, -Tail) is det
Lazy list iterator to read lines from Stream. Options include:
Determines the read chunk size. Default is 10.
Determine the output type for each line. Valid values are atom, string, codes or chars. Default is string.
Source 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:
Determines the read chunk size. Default is 1.

A thread can listen to its own message queue using

        lazy_list(lazy_message_queue(Me, []), List),
Source lazy_engine_next(+Engine, +N, -List, -Tail)
Lazy list iterator for engines. This is used to implement lazy_findall/3,4.
Source lazy_findall(?Templ, :Goal, -List) is det
Source 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).
- 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.