[racket] passing a list of dispatch clauses to dispatch-rules

From: Janos Tobias Locsei (jtlocsei at cantab.net)
Date: Wed Dec 18 15:42:29 EST 2013

Thanks Jay, your explanation with match helped me get a better
understanding of what's going on.

For the specific case of match, you can make the b-helper example work if
you pass the failure procedure to b-helper and have b-helper return it:

(define (b in)
  (match in
    [x (=> failure-proc)
       (b-helper x failure-proc)]
     "It's home!"]

(define (b-helper x fp)
  (define y (string->number x))
  (if y

(module+ test
  (require rackunit)
  (check-equal? (b "4") 4)
  (check-equal? (b "home") "It's home!")
  (check-equal? (b "five") "Unknown"))

But I don't think there's a way to do this with dispatch-url because afaik
it doesn't give you the option of specifying a failure procedure. So, what
you suggested in terms of using serve/launch/wait,  dispatch sequencer and
dispatch servlet looks like the way to do it if I was set on a particular
sequence (which for the time being I'm not).


On 18 December 2013 09:56, Jay McCarthy <jay.mccarthy at gmail.com> wrote:

> There's an annoying terminological problem.
> The core Web server has a concept of a dispatcher sequence and
> (next-dispatcher) jumps to the next of those. The web-server/dispatch
> library helps you write a function that does pattern matching on the
> URL of a request. What you want is that after you take a branch in
> that function, you'd like to have the branch fail and then the pattern
> matching try again.
> racket/match [which web-server/dispatch is built on] supports
> something like that:
> (define (a in)
>   (match in
>     [x
>      (define y (string->number x))
>      (if y
>        y
>        (failure-cont))]
>     ["home"
>      "It's home!"]
>     [_
>      "Unknown"]))
> (module+ test
>   (require rackunit)
>   (check-equal? (a "4") 4)
>   (check-equal? (a "home") "It's home!")
>   (check-equal? (a "five") "Unknown"))
> But the use of (failure-cont) has to be syntactically within the
> match, i.e. the following does not work:
> (define (b-helper x)
>   (define y (string->number x))
>   (if y
>     y
>     (failure-cont)))
> (define (b in)
>   (match in
>     [x
>      (b-helper y)]
>     ["home"
>      "It's home!"]
>     [_
>      "Unknown"]))
> If you really need to sequence them this way, then you could use
> 1. serve/launch/wait
> 2. sequence dispatcher
> 3. dispatch/servlet handling files
> 4. and then dispatch/servlet handling everything else
> So that the (next-dispatcher) in step 3 jumped to step 4.
> Jay
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