▼Web Applications in Racket
On this page:
page
embed/  url
lambda/  page
define/  page
current-request
binding-id/  c
binding-format/  c
get-binding
get-bindings
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8 Page: Short-hand for Common Patterns

 (require web-server/page) package: web-server-lib

The Web Server provides a simple utility library for building Web applications that consist mostly of send/suspend/dispatch-created pages and request handling.

Most Web applications rely heavily on send/suspend/dispatch and typically use the pattern:
(send/suspend/dispatch
 (λ (my-embed/url)
   .... (my-embed/url other-page) ....))

syntax

(page e ...)

The page macro automates this by expanding (page e ...) to a usage of send/suspend/dispatch where the syntax parameter embed/url is bound to the argument of send/suspend/dispatch.

syntax

embed/url

When used inside page syntactically, a rename transformer for the procedure embedding function; otherwise, a syntax error.

A simple example:
(page
 (response/xexpr
  `(html
    (body
     (a ([href
          ,(embed/url
            (λ (req)
              "You clicked!"))])
        "Click me")))))

Similarly, many Web applications make use almost exclusively of functions that are arguments to embed/url and immediately invoke send/suspend/dispatch.

syntax

(lambda/page formals e ...)

syntax

(define/page (id . formals) e ...)

The lambda/page and define/page automate this by expanding to functions that accept a request as the first argument (followed by any arguments specified in formals) and immediately wrap their body in page. This functions also cooperate with get-binding by binding the request to the current-request parameter.

The binding interface of web-server/http is powerful, but subtle to use conveniently due to its protection against hostile clients.

parameter

(current-request) → request?

(current-request req) → void?
  req : request?

value

binding-id/c : contract?

value

binding-format/c : contract?

procedure

(get-binding id [req #:format format])

 → (or/c false/c string? bytes? binding?)
  id : binding-id/c
  req : request? = (current-request)
  format : binding-format/c = 'string

procedure

(get-bindings id [req #:format format])

 → (listof (or/c string? bytes? binding?))
  id : binding-id/c
  req : request? = (current-request)
  format : binding-format/c = 'string
The get-binding(s) interface attempts to resolve this by providing a powerful interface with convenient defaults.

get-binding extracts the first binding of a form input from a request, while get-bindings extracts them all.

They accept a form identifier (id) as either a byte string, a string, or a symbol. In each case, the user input is compared in a case-sensitive way with the form input.

They accept an optional request argument (req) that defaults to the value of the current-request parameter used by lambda/page and define/page.

Finally, they accept an optional keyword argument (format) that specifies the desired return format. The default, 'string, produces a UTF-8 string (or #f if the byte string cannot be converted to UTF-8.) The 'bytes format always produces the raw byte string. The 'file format produces the file upload content (or #f if the form input was not an uploaded file.) The 'binding format produces the binding object.

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