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Predicates for parsing HTML and XML documents.
Written 2020-2022 by Markus Triska (triska@metalevel.at)
Part of Scryer Prolog.
Currently, two predicates are provided:
- load_html(+Source, -Es, +Options)
- load_xml(+Source, -Es, +Options)
These predicates parse HTML and XML documents, respectively.
Source must be one of:
- a list of characters with the document contents
- stream(S), specifying a stream S from which to read the content
- file(Name), where Name is a list of characters specifying a file name.
Es is unified with the abstract syntax tree of the parsed document,
represented as a list of elements where each is of the form:
* a list of characters, representing text
* element(Name, Attrs, Children)
- Name, an atom, is the name of the tag
- Attrs is a list of Key=Value pairs:
Key is an atom, and Value is a list of characters
- Children is a list of elements as specified here.
Currently, Options are ignored. In the future, more options may be
provided to control parsing.
Example:
?- load_html("Hello!", Es, []).
Yielding:
Es = [element(html,[],
[element(head,[],
[element(title,[],
["Hello!"])]),
element(body,[],[])])].
library(xpath) provides convenient reasoning about parsed documents.
For example, to fetch the title of the document above, we can use:
?- load_html("Hello!", Es, []),
xpath(Es, //title(text), T).
Yielding T = "Hello!".
Use http_open/3 from library(http/http_open) to read answers from
web servers via streams.
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:- module(sgml, [load_html/3,
load_xml/3]).
:- use_module(library(iso_ext)).
:- use_module(library(error)).
:- use_module(library(dcgs)).
:- use_module(library(pio)).
:- use_module(library(charsio)).
load_html(Source, Es, Options) :-
must_be_source(Source, load_html/3),
must_be(list, Options),
load_structure_(Source, Es, Options, html).
load_xml(Source, Es, Options) :-
must_be_source(Source, load_xml/3),
must_be(list, Options),
load_structure_(Source, Es, Options, xml).
must_be_source(Source, Context) :-
( var(Source) -> instantiation_error(Context)
; is_sgml_source(Source) -> true
; domain_error(sgml_source, Source, Context)
).
is_sgml_source(file(Fs)) :- must_be(chars, Fs).
is_sgml_source(stream(_)).
is_sgml_source([]).
is_sgml_source([C|Cs]) :- must_be(chars, [C|Cs]).
load_structure_([], [], _, _).
load_structure_([C|Cs], [E], Options, What) :-
load_(What, [C|Cs], E, Options).
load_structure_(file(Fs), [E], Options, What) :-
once(phrase_from_file(seq(Cs), Fs)),
load_(What, Cs, E, Options).
load_structure_(stream(Stream), [E], Options, What) :-
get_n_chars(Stream, _, Cs),
load_(What, Cs, E, Options).
load_(html, Cs, E, Options) :- '$load_html'(Cs, E, Options).
load_(xml, Cs, E, Options) :- '$load_xml'(Cs, E, Options).