8 Page: Short-hand for Common Patterns

▼ |  | Web Applications in Racket

1 | Running Web Servlets
2 | Stateful Servlets
3 | Stateless Servlets
4 | HTTP: Hypertext Transfer Protocol
5 | URL-Based Dispatch
6 | Formlets: Functional Form Abstraction
7 | Templates: Separation of View
8 | Page: Short-hand for Common Patterns
9 | Testing Servlets
10 | Troubleshooting and Tips
| Index

On this page:

page
embed/ url
lambda/ page
define/ page
current-request
binding-id/ c
binding-format/ c
get-binding
get-bindings

Racket
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## 8 Page: Short-hand for Common Patterns🔗ℹ

(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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