URI support in Allegro CL

This document contains the following sections:

1.0 Introduction
2.0 The URI API definition
3.0 Parsing, escape decoding/encoding and the path
4.0 Interning URIs
5.0 Allegro CL implementation notes
6.0 Examples

This version of the Allegro CL URI support documentation is for distribution with the Open Source version of the URI code. Links to Allegro CL documentation other than URI-specific files have been supressed. To see Allegro CL documentation, see http://www.franz.com/support/documentation/, which is the Allegro CL documentation page of the franz inc. website. Links to Allegro CL documentation can be found on that page.

1.0 Introduction

URI stands for Universal Resource Identifier. For a description of URIs, see RFC2396, which can be found in several places, including the IETF web site (http://www.ietf.org/rfc/rfc2396.txt) and the UCI/ICS web site (http://www.ics.uci.edu/pub/ietf/uri/rfc2396.txt). We prefer the UCI/ICS one as it has more examples.

URIs are a superset in functionality and syntax to URLs (Universal Resource Locators) and URNs (Universal Resource Names). That is, RFC2396 updates and merges RFC1738 and RFC1808 into a single syntax, called the URI. It does exclude some portions of RFC1738 that define specific syntax of individual URL schemes.

In URL slang, the scheme is usually called the `protocol', but it is called scheme in RFC1738. A URL `host' corresponds to the URI `authority.' The URL slang `bookmark' or `anchor' is `fragment' in URI lingo.

The URI facility was available as a patch to Allegro CL 5.0.1 and is included with release 6.0. the URI facility might not be in an Allegro CL image. Evaluate (require :uri) to ensure the facility is loaded (that form returns nil if the URI module is already loaded).

Broadly, the URI facility creates a Lisp object that represents a URI, and provides setters and accessors to fields in the URI object. The URI object can also be interned, much like symbols in CL are. This document describes the facility and the related operators.

Aside from the obvious slots which are called out in the RFC, URIs also have a property list. With interning, this is another similarity between URIs and CL symbols.

2.0 The URI API definition

Symbols naming objects (functions, variables, etc.) in the uri module are exported from the net.uri package.

URIs are represented by CLOS objects. Their slots are:

scheme 
host 
port 
path 
query
fragment 
plist

The host and port slots together correspond to the authority (see RFC2396). There is an accessor-like function, uri-authority, that can be used to extract the authority from a URI. See the RFC2396 specifications pointed to at the beginning of the 1.0 Introduction for details of all the slots except plist. The plist slot contains a standard Common Lisp property list.

All symbols are external in the net.uri package, unless otherwise noted. Brief descriptions are given in this document, with complete descriptions in the individual pages.

uri: the class of URI objects.
urn: the class of URN objects.
uri-p
Arguments: object

Returns true if object is an instance of class uri.
copy-uri
Arguments: uri &key place scheme host port path query fragment plist

Copies the specified URI object. See the description page for information on the keyword arguments.
uri-scheme
uri-host
uri-port
uri-path
uri-query
uri-fragment
uri-plist

Arguments: uri-object

These accessors return the value of the associated slots of the uri-object
uri-authority
Arguments: uri-object

Returns the authority of uri-object. The authority combines the host and port.
render-uri
Arguments: uri stream

Print to stream the printed representation of uri.
parse-uri
Arguments: string &key (class 'uri)

Parse string into a URI object.
merge-uris
Arguments: uri base-uri &optional place

Return an absolute URI, based on uri, which can be relative, and base-uri which must be absolute.
enough-uri
Arguments: uri base

Converts uri into a relative URI using base as the base URI.
uri-parsed-path
Arguments: uri

Return the parsed representation of the path.
uri
Arguments: object

Defined methods: if argument is a uri object, return it; create a uri object if possible and return it, or error if not possible.

3.0 Parsing, escape decoding/encoding and the path

The method uri-path returns the path portion of the URI, in string form. The method uri-parsed-path returns the path portion of the URI, in list form. This list form is discussed below, after a discussion of decoding/encoding.

RFC2396 lays out a method for inserting into URIs reserved characters. You do this by escaping the character. An escaped character is defined like this:

escaped = "%" hex hex 

hex = digit | "A" | "B" | "C" | "D" | "E" | "F" | "a" | "b" | "c" | "d" | "e" | "f"

In addition, the RFC defines excluded characters:

"<" | ">" | "#" | "%" | <"> | "{" | "}" | "|" | "\" | "^" | "[" | "]" | "`"

The set of reserved characters are:

";" | "/" | "?" | ":" | "@" | "&" | "=" | "+" | "$" | ","

with the following exceptions:

within the authority component, the characters ";", ":", "@", "?", and "/" are reserved.
within a path segment, the characters "/", ";", "=", and "?" are reserved.
within a query component, the characters ";", "/", "?", ":", "@", "&", "=", "+", ",", and "$" are reserved.

From the RFC, there are two important rules about escaping and unescaping (encoding and decoding):

decoding should only happen when the URI is parsed into component parts;
encoding can only occur when a URI is made from component parts (ie, rendered for printing).

The implication of this is that to decode the URI, it must be in a parsed state. That is, you can't convert %2f (the escaped form of "/") until the path has been parsed into its component parts. Another important desire is for the application viewing the component parts to see the decoded values of the components. For example, consider:

http://www.franz.com/calculator/3%2f2

This might be the implementation of a calculator, and how someone would execute 3/2. Clearly, the application that implements this would want to see path components of "calculator" and "3/2". "3%2f2" would not be useful to the calculator application.

For the reasons given above, a parsed version of the path is available and has the following form:

([:absolute | :relative] component1 [component2...])

where components are:

element | (element param1 [param2 ...])

and element is a path element, and the param's are path element parameters. For example, the result of

(uri-parsed-path (parse-uri "foo;10/bar:x;y;z/baz.htm"))

(:relative ("foo" "10") ("bar:x" "y" "z") "baz.htm")

There is a certain amount of canonicalization that occurs when parsing:

A path of (:absolute) or (:absolute "") is equivalent to a nil path. That is, http://a/ is parsed with a nil path and printed as http://a.
Escaped characters that are not reserved are not escaped upon printing. For example, "foob%61r" is parsed into "foobar" and appears as "foobar" when the URI is printed.

4.0 Interning URIs

This section describes how to intern URIs. Interning is not mandatory. URIs can be used perfectly well without interning them.

Interned URIs in Allegro are like symbols. That is, a string representing a URI, when parsed and interned, will always yield an eq object. For example:

(eq (intern-uri "http://www.franz.com") 
    (intern-uri "http://www.franz.com"))

is always true. (Two strings with identical contents may or may not be eq in Common Lisp, note.)

The functions associated with interning are:

make-uri-space
Arguments: &key size

Make a new hash-table object to contain interned URIs.
uri-space
Arguments:

Return the object into which URIs are currently being interned.
uri=
Arguments: uri1 uri2

Returns true if uri1 and uri2 are equivalent.
intern-uri
Arguments: uri-name &optional uri-space

Intern the uri object specified in the uri-space specified. Methods exist for strings and uri objects.
unintern-uri
Arguments: uri &optional uri-space

Unintern the uri object specified or all uri objects (in uri-space if specified) if uri is t.
do-all-uris
Arguments: (var &optional uri-space result) &body body

Bind var to all currently defined uris (in uri-space if specified) and evaluate body.

5.0 Allegro CL implementation notes

The following are true:
(uri= (parse-uri "http://www.franz.com/")
(parse-uri "http://www.franz.com"))
(eq (intern-uri "http://www.franz.com/")
(intern-uri "http://www.franz.com"))
The following is true:
(eq (intern-uri "http://www.franz.com:80/foo/bar.htm")
(intern-uri "http://www.franz.com/foo/bar.htm"))
(I.e. specifying the default port is the same as specifying no port at all. This is specific in RFC2396.)
The scheme and authority are case-insensitive. In Allegro CL, the scheme is a keyword that appears in the normal case for the Lisp in which you are executing.
#u"..." is shorthand for (parse-uri "...") but if an existing #u dispatch macro definition exists, it will not be overridden.
The interaction between setting the scheme, host, port, path, query, and fragment slots of URI objects, in conjunction with interning URIs will have very bad and unpredictable results.
The printable representation of URIs is cached, for efficiency. This caching is undone when the above slots are changed. That is, when you create a URI the printed representation is cached. When you change one of the above mentioned slots, the printed representation is cleared and calculated when the URI is next printed. For example:

user(10): (setq u #u"http://foo.bar.com/foo/bar") 
#<uri http://foo.bar.com/foo/bar> 
user(11): (setf (net.uri:uri-host u) "foo.com") 
"foo.com" 
user(12): u 
#<uri http://foo.com/foo/bar> 
user(13):

This allows URIs behavior to follow the principle of least surprise.

6.0 Examples

uri(10): (use-package :net.uri)
t
uri(11): (parse-uri "foo")
#<uri foo>
uri(12): #u"foo"
#<uri foo>
uri(13): (setq base (intern-uri "http://www.franz.com/foo/bar/"))
#<uri http://www.franz.com/foo/bar/>
uri(14): (merge-uris (parse-uri "foo.htm") base)
#<uri http://www.franz.com/foo/bar/foo.htm>
uri(15): (merge-uris (parse-uri "?foo") base)
#<uri http://www.franz.com/foo/bar/?foo>
uri(16): (setq base (intern-uri "http://www.franz.com/foo/bar/baz.htm"))
#<uri http://www.franz.com/foo/bar/baz.htm>
uri(17): (merge-uris (parse-uri "foo.htm") base)
#<uri http://www.franz.com/foo/bar/foo.htm>
uri(18): (merge-uris #u"?foo" base)
#<uri http://www.franz.com/foo/bar/?foo>
uri(19): (describe #u"http://www.franz.com")
#<uri http://www.franz.com> is an instance of #<standard-class net.uri:uri>:
 The following slots have :instance allocation:
  scheme        :http
  host          "www.franz.com"
  port          nil
  path          nil
  query         nil
  fragment      nil
  plist         nil
  escaped       nil
  string        "http://www.franz.com"
  parsed-path   nil
  hashcode      nil
uri(20): (describe #u"http://www.franz.com/")
#<uri http://www.franz.com> is an instance of #<standard-class net.uri:uri>:
 The following slots have :instance allocation:
  scheme        :http
  host          "www.franz.com"
  port          nil
  path          nil
  query         nil
  fragment      nil
  plist         nil
  escaped       nil
  string        "http://www.franz.com"
  parsed-path   nil
  hashcode      nil
uri(21): #u"foobar#baz%23xxx"
#<uri foobar#baz#xxx>