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