/* 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.
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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,
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*/
:- 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
]).
:- autoload(library(error),
[type_error/2,instantiation_error/1,must_be/2]).
:- autoload(library(lists),[append/3]).
:- autoload(library(option),[select_option/4,option/3]).
:- autoload(library(readutil),
[read_line_to_string/2,read_line_to_codes/2]).
:- 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, _)).
% (*) We need a copy of the list where the copy must include the new
% attributed variable to avoid that backtracking makes the list
% non-lazy. We do want to avoid copying `Next`. So, we add a dummy and
% then replace this using nb_linkarg/3 with our Next.
attr_unify_hook(State, Value) :-
State = lazy_list(Next, Read),
( var(Read)
-> call(Next, NewList, Tail),
( Tail == []
-> nb_setarg(2, State, NewList)
; put_attr(Tail, lazy_lists, lazy_list(dummy, _)), % See (*)
nb_setarg(2, State, NewList),
arg(2, State, NewListCP),
'$skip_list'(_, NewListCP, TailCP),
get_attr(TailCP, lazy_lists, LazyList),
nb_linkarg(1, LazyList, Next)
),
arg(2, State, Value)
; Value = Read
).
attribute_goals(X) -->
{ get_attr(X, lazy_lists, lazy_list(Next, _)) },
[lazy_list(Next, X)].
%! lazy_list(:Next, +State0, -List)
%
% Create a lazy list where the next element is defined by
%
% call(Next, State0, State1, Head)
%
% The example below uses this predicate to define a lazy list
% holding the Fibonacci numbers. Our state keeps the two previous
% Fibonacci numbers.
%
% ```
% fibonacci_numbers(L) :-
% lazy_list(fib, state(-,-), L).
%
% fib(state(-,-), state(0,-), 0) :- !.
% fib(state(0,-), state(1,0), 1) :- !.
% fib(state(P,Q), state(F,P), F) :-
% F is P+Q.
% ```
%
% The above can be used to retrieve the Nth Fibonacci number. As
% fib/2 provides no access to the complete list of Fibonacci
% numbers, this can be used to generate large Fibonacci numbers.
%
% ```
% fib(N, F) :-
% fibonacci_numbers(L),
% nth1(N, L, F).
% ```
lazy_list(Next, State0, List) :-
lazy_list(lazy_state(Next, s(State0)), List).
lazy_state(Pred, LState, [H|T], T) :-
LState = s(State0),
call(Pred, State0, State1, H),
!,
nb_setarg(1, LState, State1).
lazy_state(_, _, [], []).
/*******************************
* OPERATIONS ON LAZY LISTS *
*******************************/
%! lazy_list_materialize(?List) is det.
%
% Materialize the lazy list.
lazy_list_materialize(List) :-
'$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).