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►5 URL-Based Dispatch
On this page:
5.1 Using web-server/ dispatch
5.2 API Reference
dispatch-rules
dispatch-rules+ applies
dispatch-case
dispatch-url
serve/ dispatch
5.3 Built-in URL patterns
number-arg
integer-arg
real-arg
string-arg
symbol-arg
5.4 Extending web-server/ dispatch
define-bidi-match-expander
bidi-match-going-in?
define-coercion-match-expander
make-coerce-safe?
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5 URL-Based Dispatch

 (require web-server/dispatch)

The library allows the creation of two-way mappings between permanent URLs and request-handling procedures.

This library was inspired by the (planet untyped/dispatch) package.

5.1 Using web-server/dispatch

Suppose you are writing a blog application and want pretty URLs for different views of the site. You would define some URL dispatching rules as follows:

  > (define-values (blog-dispatch blog-url)
      (dispatch-rules
       [("") list-posts]
       [("posts" (string-arg)) review-post]
       [("archive" (integer-arg) (integer-arg)) review-archive]
       [else list-posts]))

And define your request handlers as follows:
  > (define (list-posts req) `(list-posts))
  > (define (review-post req p) `(review-post ,p))
  > (define (review-archive req y m) `(review-archive ,y ,m))

Now when a request is sent to your application, it will be directed to the appropriate handler:
  > (define (url->request u)
      (make-request #"GET" (string->url u) empty
                    (delay empty) #f "1.2.3.4" 80 "4.3.2.1"))
  > (blog-dispatch
     (url->request "http://www.chrlsnchrg.com"))

  '(list-posts)

  > (blog-dispatch
     (url->request "http://www.chrlsnchrg.com/"))

  '(list-posts)

  > (blog-dispatch
     (url->request
      "http://www.chrlsnchrg.com/posts/Extracurricular-Activity"))

  '(review-post "Extracurricular-Activity")

  > (blog-dispatch
     (url->request "http://www.chrlsnchrg.com/archive/1984/10"))

  '(review-archive 1984 10)

  > (blog-dispatch
     (url->request "http://www.chrlsnchrg.com/contact"))

  '(list-posts)

You can also generate these pretty URLs from procedure calls:
  > (blog-url list-posts)

  "/"

  > (blog-url review-post "Another-Saturday-Night")

  "/posts/Another-Saturday-Night"

  > (blog-url review-archive 1984 11)

  "/archive/1984/11"

After mastering the world of blogging software, you decide to put the ubiquitous Add-Two-Numbers.com out of business with Sum.com:
  > (define-values (sum-dispatch sum-url)
      (dispatch-rules
       [((integer-arg) ...) sum]
       [else (lambda (req) (sum req empty))]))
  > (define (sum req is)
      (apply + is))
  > (sum-dispatch (url->request "http://www.sum.com/"))

  0

  > (sum-dispatch (url->request "http://www.sum.com/2"))

  2

  > (sum-dispatch (url->request "http://www.sum.com/2/3/4"))

  9

  > (sum-dispatch (url->request "http://www.sum.com/5/10/15/20"))

  50

  > (sum-url sum empty)

  "/"

  > (sum-url sum (list 1))

  "/1"

  > (sum-url sum (list 2 3 5 7))

  "/2/3/5/7"

5.2 API Reference

(dispatch-rules
 [dispatch-pattern dispatch-fun]
 ...
 [else else-fun])
(dispatch-rules
 [dispatch-pattern dispatch-fun]
 ...)
 
  else-fun : (request? . -> . response/c)
  dispatch-fun : (request? any/c ... . -> . response/c)
Returns two values: the first is a dispatching function with the contract (-> request? response/c) that calls the appropriate dispatch-fun based on the first dispatch-pattern that matches the request’s URL; the second is a URL-generating function with the contract (-> procedure? any/c ... string?) that generates a URL using dispatch-pattern for the dispatch-fun given as its first argument.

If else-fun is left out, one is provided that calls (next-dispatcher) to signal to the Web Server that this dispatcher does not apply.

  dispatch-pattern = ()
  | (string . dispatch-pattern)
  | (bidi-match-expander ... . dispatch-pattern)
  | (bidi-match-expander . dispatch-pattern)

(dispatch-rules+applies
 [dispatch-pattern dispatch-fun]
 ...
 [else else-fun])
(dispatch-rules+applies
 [dispatch-pattern dispatch-fun]
 ...)
 
  else-fun : (request? . -> . response/c)
  dispatch-fun : (request? any/c ... . -> . response/c)
Like dispatch-rules, except returns a third value with the contract (-> request? boolean?) that returns #t if the dispatching rules apply to the request and #f otherwise.

(dispatch-case
 [dispatch-pattern dispatch-fun]
 ...
 [else else-fun])
(dispatch-case
 [dispatch-pattern dispatch-fun]
 ...)
 
  else-fun : (request? . -> . response/c)
  dispatch-fun : (request? any/c ... . -> . response/c)
Returns a dispatching function as described by dispatch-rules.

(dispatch-url
 [dispatch-pattern dispatch-fun]
 ...)
 
  dispatch-fun : (request? any/c ... . -> . response/c)
Returns a URL-generating function as described by dispatch-rules.

(serve/dispatch dispatch) → void
  dispatch : (request? . -> . response/c)
Calls serve/servlet with appropriate arguments so that every request is handled by dispatch.

5.3 Built-in URL patterns

web-server/dispatch builds in a few useful URL component patterns.

(number-arg)
A bi-directional match expander that parses a number? from the URL and generates a URL with a number’s encoding as a string.

(integer-arg)
A bi-directional match expander that parses a integer? from the URL and generates a URL with a integer’s encoding as a string.

(real-arg)
A bi-directional match expander that parses a real? from the URL and generates a URL with a real’s encoding as a string.

(string-arg)
A bi-directional match expander that parses a string? from the URL and generates a URL containing the string.

(symbol-arg)
A bi-directional match expander that parses a symbol? from the URL and generates a URL with a symbol’s encoding as a string.

5.4 Extending web-server/dispatch

 (require web-server/dispatch/extend)

You can create new URL component patterns by defining bi-directional match expanders.

(define-bidi-match-expander id in-xform out-xform)
Binds id to a bi-directional match expander where in-xform is a match expander (defined by define-match-expander) that is used when parsing URLs and out-xform is one used when generating URLs.

Both in-xform and out-xform should use the syntax (xform arg ... id) where the args are specific to id and compatible with both in-xform and out-xform. id will typically be provided automatically by dispatch-rules.

bidi-match-going-in?
A syntax parameter used by bi-directional match expanders to determine if a URL is being parsed or generated.

When defining new patterns, you may find it useful to use these helper functions:

(define-coercion-match-expander id test? coerce)
Binds id to a match expander that expands (id x) to (? test? (app coerce x)) (i.e., uses test? to determine if the pattern matches and coerce to transform the binding.)

(make-coerce-safe? coerce) → (any/c . -> . boolean?)
  coerce : (any/c . -> . any/c)
Returns a function that returns #t if coerce would not throw an exception or return #f on its input.

Examples:

  > (define string->number? (make-coerce-safe? string->number))
  > (string->number? "1")

  #t

  > (string->number? "1.2")

  #t

  > (string->number? "+inf.0")

  #t

  > (string->number? "one")

  #f

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