## 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.