A.50 library(url): Analysing and constructing URL

author

- Jan Wielemaker
- Lukas Faulstich

deprecated

New code should use library(uri), provided by the clib package.

This library deals with the analysis and construction of a URL, Universal Resource Locator. URL is the basis for communicating locations of resources (data) on the web. A URL consists of a protocol identifier (e.g. HTTP, FTP, and a protocol-specific syntax further defining the location. URLs are standardized in RFC-1738.

The implementation in this library covers only a small portion of the defined protocols. Though the initial implementation followed RFC-1738 strictly, the current is more relaxed to deal with frequent violations of the standard encountered in practical use.

[det]global_url(+URL, +Base, -Global)

Translate a possibly relative URL into an absolute one.

Errors

syntax_error(illegal_url) if URL is not legal.

is_absolute_url(+URL)

True if URL is an absolute URL. That is, a URL that starts with a protocol identifier.

http_location(?Parts, ?Location)

Construct or analyze an HTTP location. This is similar to parse_url/2, but only deals with the location part of an HTTP URL. That is, the path, search and fragment specifiers. In the HTTP protocol, the first line of a message is

<Action> <Location> HTTP/<version>

Location

Atom or list of character codes.

[det]parse_url(?URL, ?Attributes)

Construct or analyse a URL. URL is an atom holding a URL or a variable. Attributes is a list of components. Each component is of the format Name(Value). Defined components are:

protocol(Protocol)

The used protocol. This is, after the optional url:, an identifier separated from the remainder of the URL using :. parse_url/2 assumes the http protocol if no protocol is specified and the URL can be parsed as a valid HTTP url. In addition to the RFC-1738 specified protocols, the file protocol is supported as well.

host(Host)

Host-name or IP-address on which the resource is located. Supported by all network-based protocols.

port(Port)

Integer port-number to access on the \arg{Host}. This only appears if the port is explicitly specified in the URL. Implicit default ports (e.g., 80 for HTTP) do not appear in the part-list.

path(Path)

(File-) path addressed by the URL. This is supported for the ftp, http and file protocols. If no path appears, the library generates the path /.

search(ListOfNameValue)

Search-specification of HTTP URL. This is the part after the ?, normally used to transfer data from HTML forms that use the HTTP GET method. In the URL it consists of a www-form-encoded list of Name=Value pairs. This is mapped to a list of Prolog Name=Value terms with decoded names and values.

fragment(Fragment)

Fragment specification of HTTP URL. This is the part after the # character.

The example below illustrates all of this for an HTTP URL.

?- parse_url('http://www.xyz.org/hello?msg=Hello+World%21#x',
       P).

P = [ protocol(http),
      host('www.xyz.org'),
      fragment(x),
      search([ msg = 'Hello World!'
             ]),
      path('/hello')
    ]

By instantiating the parts-list this predicate can be used to create a URL.

[det]parse_url(+URL, +BaseURL, -Attributes)

Similar to parse_url/2 for relative URLs. If URL is relative, it is resolved using the absolute URL BaseURL.

[det]www_form_encode(+Value, -XWWWFormEncoded)

[det]www_form_encode(-Value, +XWWWFormEncoded)

En/decode to/from application/x-www-form-encoded. Encoding encodes all characters except RFC 3986 unreserved (ASCII alnum (see code_type/2)), and one of "-._~" using percent encoding. Newline is mapped to %OD%OA. When decoding, newlines appear as a single newline (10) character.

Note that a space is encoded as %20 instead of +. Decoding decodes both to a space.

deprecated

Use uri_encoded/3 for new code.

[semidet]set_url_encoding(?Old, +New)

Query and set the encoding for URLs. The default is utf8. The only other defined value is iso_latin_1.

To be done

Having a global flag is highly inconvenient, but a work-around for old sites using ISO Latin 1 encoding.

[det]url_iri(+Encoded, -Decoded)

[det]url_iri(-Encoded, +Decoded)

Convert between a URL, encoding in US-ASCII and an IRI. An IRI is a fully expanded Unicode string. Unicode strings are first encoded into UTF-8, after which %-encoding takes place.

[det]parse_url_search(?Spec, ?Fields:list(Name=Value))

Construct or analyze an HTTP search specification. This deals with form data using the MIME-type application/x-www-form-urlencoded as used in HTTP GET requests.

[det]file_name_to_url(+File, -URL)

[semidet]file_name_to_url(-File, +URL)

Translate between a filename and a file:// URL.

To be done

Current implementation does not deal with paths that need special encoding.