- Reference manual
- SWI-Prolog HTTP support
2.1 library(http/http_open): HTTP client library
- See also
- - load_html/3 and xpath/3
can be used to parse and navigate HTML documents.
- http_get/3 and http_post/4 provide an alternative interface that convert the reply depending on the
This library defines http_open/3, which opens an URL as a Prolog stream. The functionality of the library can be extended by loading two additional modules that act as plugins:
- Loading this library causes http_open/3
to handle HTTPS connections. Relevant options for SSL certificate
handling are handed to
ssl_context/3. This plugin is loaded
automatically if the scheme
httpsis requested using a default SSL context. See the plugin for additional information regarding security.
- Loading this library supports the
gziptransfer encoding. This plugin is lazily loaded if a connection is opened that claims this transfer encoding.
- Loading this library adds tracking cookies to http_open/3. Returned cookies are collected in the Prolog database and supplied for subsequent requests.
- This library adds support for chunked encoding and makes the http_open/3 advertise itself as HTTP/1.1 instead of HTTP/1.0.
Here is a simple example to fetch a web-page:
?- http_open('http://www.google.com/search?q=prolog', In, ), copy_stream_data(In, user_output), close(In). <!doctype html><head><title>prolog - Google Search</title><script> ...
The example below fetches the modification time of a web-page. Note
'' (the empty atom) if the
web-server does not provide a time-stamp for the resource. See also parse_time/2.
modified(URL, Stamp) :- http_open(URL, In, [ method(head), header(last_modified, Modified) ]), close(In), Modified \== '', parse_time(Modified, Stamp).
Then next example uses Google search. It exploits
to manage URIs,
library(sgml) to load an HTML document and
to navigate the parsed HTML. Note that you may need to adjust the XPath
queries if the data returned by Google changes (this example indeed no
longer works and currently fails at the first xpath/3
:- use_module(library(http/http_open)). :- use_module(library(xpath)). :- use_module(library(sgml)). :- use_module(library(uri)). google(For, Title, HREF) :- uri_encoded(query_value, For, Encoded), atom_concat('http://www.google.com/search?q=', Encoded, URL), http_open(URL, In, ), call_cleanup( load_html(In, DOM, ), close(In)), xpath(DOM, //h3(@class=r), Result), xpath(Result, //a(@href=HREF0, text), Title), uri_components(HREF0, Components), uri_data(search, Components, Query), uri_query_components(Query, Parts), memberchk(q=HREF, Parts).
An example query is below:
?- google(prolog, Title, HREF). Title = 'SWI-Prolog', HREF = 'http://www.swi-prolog.org/' ; Title = 'Prolog - Wikipedia', HREF = 'https://nl.wikipedia.org/wiki/Prolog' ; Title = 'Prolog - Wikipedia, the free encyclopedia', HREF = 'https://en.wikipedia.org/wiki/Prolog' ; Title = 'Pro-Log is logistiek dienstverlener m.b.t. vervoer over water.', HREF = 'http://www.pro-log.nl/' ; Title = 'Learn Prolog Now!', HREF = 'http://www.learnprolognow.org/' ; Title = 'Free Online Version - Learn Prolog ...
- [det]http_open(+URL, -Stream, +Options)
- Open the data at the HTTP server as a Prolog stream. URL is
either an atom specifying a URL or a list representing a
broken-down URL as specified below. After this predicate
succeeds the data can be read from Stream. After completion
this stream must be closed using the built-in Prolog predicate
close/1. Options provides
true), do not try to automatically authenticate the client if a 401 (Unauthorized) status code is received.
- Send authorization. See also http_set_authorization/2.
- basic(+User, +Password)
- HTTP Basic authentication.
- HTTP Bearer authentication.
- digest(+User, +Password)
- HTTP Digest authentication. This option is only provided if the plugin
library(http/http_digest)is also loaded.
- Connect to the given Unix domain socket. In this scenario the host name
and port or ignored. If the server replies with a redirect
message and the host differs from the original host as normal TCP
connection is used to handle the redirect. This option is inspired by
- Specify the
Connectionheader. Default is
close. The alternative is
Keep-alive. This maintains a pool of available connections as determined by keep_connection/1. The
Keep-alive, Upgrade. Keep-alive connections can be closed explicitly using http_close_keep_alive/1. Keep-alive connections may significantly improve repetitive requests on the same server, especially if the IP route is long, HTTPS is used or the connection uses a proxy.
- Unify FinalURL with the final destination. This differs from the original URL if the returned head of the original indicates an HTTP redirect (codes 301, 302 or 303). Without a redirect, FinalURL is the same as URL if URL is an atom, or a URL constructed from the parts.
- header(Name, -AtomValue)
- If provided, AtomValue is unified with the value of the indicated field in the reply header. Name is matched case-insensitive and the underscore (_) matches the hyphen (-). Multiple of these options may be provided to extract multiple header fields. If the header is not available AtomValue is unified to the empty atom ('').
- If provided, List is unified with a list of Name(Value) pairs
corresponding to fields in the reply header. Name and Value follow the
same conventions used by the
header(Name,Value)option. A pseudo header
status_code(Code)is added to provide the HTTP status as an integer. See also
raw_headers(-List)which provides the entire HTTP reply header in unparsed representation.
- One of
headmessage can be used in combination with the
header(Name, Value)option to access information on the resource without actually fetching the resource itself. The returned stream must be closed immediately.
post(Data)is provided, the default is
- Size is unified with the integer value of
Content-Lengthin the reply header.
- Version is a pair
Major-Minor, where Major and Minor are integers representing the HTTP version in the reply header.
- Ask for partial content. Range is a term Unit(From,To),
where From is an integer and To is either an
integer or the atom
end. HTTP 1.1 only supports Unit =
bytes. E.g., to ask for bytes 1000-1999, use the option
- Do not install a decoding filter for Encoding. For example,
raw_encoding('applocation/gzip')the system will not decompress the stream if it is compressed using
- Unify Lines with a list of strings that represents the
complete reply header returned by the server. See also
true), do not automatically redirect if a 3XX code is received. Must be combined with
status_code(Code)and one of the header options to read the redirect reply. In particular, without
status_code(Code)a redirect is mapped to an exception.
- If this option is present and Code unifies with the HTTP status code, do not translate errors (4xx, 5xx) into an exception. Instead, http_open/3 behaves as if 2xx (success) is returned, providing the application to read the error document from the returned stream.
- Unify the output stream with Out and do not close it. This can be used to upgrade a connection.
- If provided, set a timeout on the stream using set_stream/2.
With this option if no new data arrives within Timeout
seconds the stream raises an exception. Default is to wait forever (
- Issue a
POSTrequest on the HTTP server. Data is handed to http_post_data/3.
- Use an HTTP proxy to connect to the outside world. See also socket:proxy_for_url/3. This option overrules the proxy specification defined by socket:proxy_for_url/3.
- proxy(+Host, +Port)
- Synonym for
- Send authorization to the proxy. Otherwise the same as the
true, bypass proxy hooks. Default is
- Additional name-value parts are added in the order of appearance to the HTTP request header. No interpretation is done.
- Sets the maximum length of a redirection chain. This is needed for some
IRIs that redirect indefinitely to other IRIs without looping (e.g.,
redirecting to IRIs with a random element in them).
Max must be either a non-negative integer or the atom
infinite. The default value is
- Defines the value of the
User-Agentfield of the HTTP header. Default is
The hook http:open_options/2 can be used to provide default options based on the broken-down URL. The option
status_code(-Code)is particularly useful to query REST interfaces that commonly return status codes other than
200that need to be be processed by the client code.
URL is either an atom or string (url) or a list of parts.
When provided, this list may contain the fields
query_string(whose argument is an atom) or
search(whose argument is a list of
Name=Valuecompound terms). Only
hostis mandatory. The example below opens the URL
http://www.example.com/my/path?q=Hello%20World&lang=en. Note that values must not be quoted because the library inserts the required quotes.
http_open([ host('www.example.com'), path('/my/path'), search([ q='Hello world', lang=en ]) ])
error(existence_error(url, Id),Context)is raised if the HTTP result code is not in the range 200..299. Context has the shape
context(Message, status(Code, TextCode)), where Code is the numeric HTTP code and TextCode is the textual description thereof provided by the server. Message may provide additional details or may be unbound.
- See also
- ssl_context/3 for SSL related options if
- [multifile]map_method(+MethodID, -Method)
- Support additional
METHODkeywords. Default are the official HTTP methods as defined by the various RFCs.
- Do not use the
Transfer-encodingencoding for specific values of ContentType. This predicate is multifile and can thus be extended by the user.
- [det]http_set_authorization(+URL, +Authorization)
- Set user/password to supply with URLs that have URL as
prefix. If Authorization is the atom
-, possibly defined authorization is cleared. For example:
?- http_set_authorization('http://www.example.com/private/', basic('John', 'Secret'))
- To be done
- Move to a separate module, so http_get/3, etc. can use this too.
- [semidet,multifile]iostream:open_hook(+Spec, +Mode, -Stream, -Close, +Options0, -Options)
- Hook implementation that makes open_any/5
Mode == read.
- Close all keep-alive connections matching Address. Address
is of the form Host:Port. In particular,
http_close_keep_alive(_)closes all currently known keep-alive connections.
- [nondet,multifile]http:open_options(+Parts, -Options)
- This hook is used by the HTTP client library to define default options
based on the the broken-down request-URL. The following example
redirects all trafic, except for localhost over a proxy:
:- multifile http:open_options/2. http:open_options(Parts, Options) :- option(host(Host), Parts), Host \== localhost, Options = [proxy('proxy.local', 3128)].
This hook may return multiple solutions. The returned options are combined using merge_options/3 where earlier solutions overrule later solutions.
- [semidet,multifile]http:write_cookies(+Out, +Parts, +Options)
- Emit a
Cookie:header for the current connection. Out is an open stream to the HTTP server, Parts is the broken-down request (see uri_components/2) and Options is the list of options passed to http_open. The predicate is called as if using ignore/1.
- See also
- - complements http:update_cookies/3.
library(http/http_cookie)implements cookie handling on top of these hooks.
- [semidet,multifile]http:update_cookies(+CookieData, +Parts, +Options)
- Update the cookie database. CookieData is the value of the
Set-Cookiefield, Parts is the broken-down request (see uri_components/2) and Options is the list of options passed to http_open.
- See also
- - complements http:write_cookies
library(http/http_cookies)implements cookie handling on top of these hooks.