A.38 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 GET protocol. 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.