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1.1 URL Structure
path/ param
1.2 URL Functions
combine-url/ relative
netscape/ string->url
call/ input-url
1.3 URL Unit
1.4 URL Signature
url+ scheme^

1 URLs and HTTP

The net/url library provides utilities to parse and manipulate URIs, as specified in RFC 2396 [RFC2396], and to use the HTTP protocol.

To access the text of a document from the web, first obtain its URL as a string. Convert the address into a url structure using string->url. Then, open the document using get-pure-port or get-impure-port, depending on whether or not you wish to examine its MIME headers. At this point, you have a regular input port with which to process the document, as with any other file.

Currently the only supported protocols are "http", "https", and sometimes "file".

1.1 URL Structure

The URL structure types are provided by the net/url-structs library, and re-exported by net/url.

(struct url (scheme
  #:extra-constructor-name make-url)
  scheme : (or/c false/c string?)
  user : (or/c false/c string?)
  host : (or/c false/c string?)
  port : (or/c false/c exact-nonnegative-integer?)
  path-absolute? : boolean?
  path : (listof path/param?)
  query : (listof (cons/c symbol? (or/c false/c string?)))
  fragment : (or/c false/c string?)
The basic structure for all URLs, which is explained in RFC 3986 [RFC3986]. The following diagram illustrates the parts:


  {-1}   {2} {3} {4}{---5---------} {6} {----7-------------} {8}


  1 = scheme, 2 = user, 3 = host, 4 = port,

  5 = path (two elements),  6 = param (of second path element),

  7 = query, 8 = fragment

The strings inside the user, path, query, and fragment fields are represented directly as Racket strings, without URL-syntax-specific quoting. The procedures string->url and url->string translate encodings such as %20 into spaces and back again.

By default, query associations are parsed with either ; or & as a separator, and they are generated with & as a separator. The current-alist-separator-mode parameter adjusts the behavior.

An empty string at the end of the path list corresponds to a URL that ends in a slash. For example, the result of (string->url "http://racket-lang.org/a/") has a path field with strings "a" and "", while the result of (string->url "http://racket-lang.org/a") has a path field with only the string "a".

When a "file" URL is represented by a url structure, the path field is mostly a list of path elements. For Unix paths, the root directory is not included in path; its presence or absence is implicit in the path-absolute? flag. For Windows paths, the first element typically represents a drive, but a UNC path is represented by a first element that is "" and then successive elements complete the drive components that are separated by / or \.

(struct path/param (path param)
  #:extra-constructor-name make-path/param)
  path : (or/c string? (one-of/c 'up 'same))
  param : (listof string?)
A pair that joins a path segment with its params in a URL.

1.2 URL Functions

An HTTP connection is created as a pure port or a impure port. A pure port is one from which the MIME headers have been removed, so that what remains is purely the first content fragment. An impure port is one that still has its MIME headers.

(string->url str)  url?
  str : string?
Parses the URL specified by str into a url struct. The string->url procedure uses form-urlencoded->alist when parsing the query, so it is sensitive to the current-alist-separator-mode parameter for determining the association separator.

If str starts with "file:", then the path is always parsed as an absolute path, and the parsing details depend on file-url-path-convention-type:

(combine-url/relative base relative)  url?
  base : url?
  relative : string?
Given a base URL and a relative path, combines the two and returns a new URL as per the URL combination specification. They are combined according to the rules in RFC 3986 [RFC3986].

This function does not raise any exceptions.

(netscape/string->url str)  url?
  str : string?
Turns a string into a URL, applying (what appear to be) Netscape’s conventions on automatically specifying the scheme: a string starting with a slash gets the scheme "file", while all others get the scheme "http".

(url->string URL)  string?
  URL : url?
Generates a string corresponding to the contents of a url struct. For a "file:" URL, the URL must not be relative, the result always starts file://, and the interpretation of the path depends on the value of file-url-path-convention-type:

The url->string procedure uses alist->form-urlencoded when formatting the query, so it is sensitive to the current-alist-separator-mode parameter for determining the association separator. The default is to separate associations with a &.

(path->url path)  url?
  path : (or/c path-string? path-for-some-system?)
Converts a path to a url.

(url->path URL [kind])  path-for-some-system?
  URL : url?
  kind : (one-of/c 'unix 'windows)
   = (system-path-convention-type)
Converts URL, which is assumed to be a "file" URL, to a path.

(file-url-path-convention-type)  (one-of/c 'unix 'windows)
(file-url-path-convention-type kind)  void?
  kind : (one-of/c 'unix 'windows)
Determines the default conversion to and from strings for "file" URLs. See string->url and url->string.

(get-pure-port URL [header])  input-port?
  URL : url?
  header : (listof string?) = null
(head-pure-port URL [header])  input-port?
  URL : url?
  header : (listof string?) = null
(delete-pure-port URL [header])  input-port?
  URL : url?
  header : (listof string?) = null
Initiates a GET/HEAD/DELETE request for URL and returns a pure port corresponding to the body of the response. The optional list of strings can be used to send header lines to the server.

The GET method is used to retrieve whatever information is identified by URL.

The HEAD method is identical to GET, except the server must not return a message body. The meta-information returned in a response to a HEAD request should be identical to the information in a response to a GET request.

The DELETE method is used to delete the entity identified by URL.

Beware: By default, "https" scheme handling does not verify a server’s certificate (i.e., it’s equivalent of clicking through a browser’s warnings), so communication is safe, but the identity of the server is not verified. To validate the server’s certificate, set current-https-protocol to a context created with ssl-make-client-context, and enable certificate validation in the context with ssl-set-verify!.

The "file" scheme for URLs is handled only by get-pure-port, which uses open-input-file, does not handle exceptions, and ignores the optional strings.

(get-impure-port URL [header])  input-port?
  URL : url?
  header : (listof string?) = null
(head-impure-port URL [header])  input-port?
  URL : url?
  header : (listof string?) = null
(delete-impure-port URL [header])  input-port?
  URL : url?
  header : (listof string?) = null
Like get-pure-port, etc., but the resulting impure port contains both the returned headers and the body. The "file" URL scheme is not handled by these functions.

(post-pure-port URL post [header])  input-port?
  URL : url?
  post : bytes?
  header : (listof string?) = null
(put-pure-port URL post [header])  input-port?
  URL : url?
  post : bytes?
  header : (listof string?) = null
Initiates a POST/PUT request for URL and sends the post byte string. The result is a pure port, which contains the body of the response is returned. The optional list of strings can be used to send header lines to the server.

Beware: See get-pure-port for warnings about "https" certificate validation.

(post-impure-port URL post [header])  input-port?
  URL : url?
  post : bytes?
  header : (listof string?) = null
(put-impure-port URL post [header])  input-port?
  URL : url?
  post : bytes?
  header : (listof string?) = null
Like post-pure-port and put-pure-port, but the resulting impure port contains both the returned headers and body.

(display-pure-port in)  void?
  in : input-port?
Writes the output of a pure port, which is useful for debugging purposes.

(purify-port in)  string?
  in : input-port?
Purifies a port, returning the MIME headers, plus a leading line for the form HTTP/vers code message›, where ‹vers› is something like 1.0 or 1.1, ‹code› is an exact integer for the response code, and ‹message› is arbitrary text without a return or newline.

The net/head library provides procedures, such as extract-field for manipulating the header.

Since web servers sometimes return mis-formatted replies, purify-port is liberal in what it accepts as a header. as a result, the result string may be ill formed, but it will either be the empty string, or it will be a string matching the following regexp:


(call/input-url URL connect handle)  any
  URL : url?
  connect : (url? . -> . input-port?)
  handle : (input-port? . -> . any)
(call/input-url URL connect handle header)  any
  URL : url?
  connect : (url? (listof string?) . -> . input-port?)
  handle : (input-port? . -> . any)
  header : (listof string?)
Given a URL and a connect procedure like get-pure-port to convert the URL to an input port (either a pure port or impure port), calls the handle procedure on the port and closes the port on return. The result of the handle procedure is the result of call/input-url.

When a header argument is supplied, it is passed along to the connect procedure.

The connection is made in such a way that the port is closed before call/input-url returns, no matter how it returns. In particular, it is closed if handle raises an exception, or if the connection process is interruped by an asynchronous break exception.

  (listof (list/c string? string? (integer-in 0 65535)))
(current-proxy-servers mapping)  void?
  mapping : (listof (list/c string? string? (integer-in 0 65535)))
A parameter that determines a mapping of proxy servers used for connections. Each mapping is a list of three elements:

Currently, the only proxiable scheme is "http". The default mapping is the empty list (i.e., no proxies).

A parameter that determines the connection mode for "https" connections; the parameter value is passed as the third argument to ssl-connect when creating an "https" connection. Set this parameter to validate a server’s certificates, for example, as described with get-pure-port.

1.3 URL Unit

 (require net/url-unit)

url@ : unit?
Imports tcp^, exports url+scheme^.

The url+scheme^ signature contains current-connect-scheme, which url@ binds to a parameter. The parameter is set to the scheme of a URL when tcp-connect is called to create a connection. A tcp-connect variant linked to url@ can check this parameter to choose the connection mode; in particular, net/url supplies a tcp-connect that actually uses ssl-connect when (current-connect-scheme) produces "https".

Note that net/url does not provide the current-connect-scheme parameter.

1.4 URL Signature

 (require net/url-sig)

url^ : signature

Includes everything exported by the net/url module except current-https-protocol. Note that the exports of net/url and the url^ signature do not include current-connect-scheme.

url+scheme^ : signature

Adds current-connect-scheme to url^.