[racket] Problem with response/output

From: Matthew Butterick (mb at mbtype.com)
Date: Fri Mar 6 19:26:18 EST 2015

> From the error message, I changed the lambda to return (void) and then it
> worked.
> I think maybe the contract is wrong but frankly I don't understand much
> about contracts.

The contract, by definition, is always right ;)

In this case, `response/output` takes as its first argument a procedure
that accepts an output-port and returns void. [1] In contract-speak this is
commonly written with dot notation as:

(output-port? . -> . void?)

but it appears in error messages, like the one above, with the equivalent

(-> output-port? void?)

Note also that the error says the contract violation was "in the range of
the 1st argument of" ... [giant contract follows]. That helps track down
the error. The contract for the first argument is '(-> output-port?
void?)'. This is a procedure contract. And "the range of" a procedure
contract means the contract on the return value. Which in this case is
`void?`. So the complaint is that the procedure given in the first argument
is returning '11' when it should be returning void.

Why is (-> output-port? void?) the contract for the first arg of
`response/output`? The idea is that you write to the output-port directly
rather than returning a value to the caller. Insisting on void as a return
value imposes an extra measure of discipline, and sets an expectation.

The reason your 'not-working' dispatcher is not working is that
`write-bytes` does two things: it sends bytes to the output port, but then
also returns the number of bytes written. [2] So this procedure:

(λ (op) (write-bytes #"Hello world" op))

is defective because it returns the number of bytes. Meaning, it breaks the
contract, which demands void. (That's also why your error is '11': that's
the number of bytes in "Hello world").

But your revised procedure:

(λ (op) (write-bytes #"Hello world" op) (void))

Meets the contract because it ignores the return value from `write-bytes`
and returns (void) instead.

You should repeat this technique whenever you use `response/output`.

You can also look into `response/full` and `response/xexpr`, which can be a
more convenient way of making simple HTML or text responses.


which cross-references



which cross-references


On Fri, Mar 6, 2015 at 3:48 PM, André Matheus <amatheus at mac.com> wrote:

> Hi, starting a project of mine, I've setup a dispatch rule and a function
> to return the response.
> To make things simple, I've used response/output, with a lambda writing to
> the output-port.
> However, I've got the error:
> response/output: contract violation
>   expected: void?
>   given: 11
>   in: the range of
>       the 1st argument of
>       (->*
>        ((-> output-port? void?))
>        (#:code
>         number?
>         #:headers
>         (listof header?)
>         #:message
>         bytes?
>         #:mime-type
>         (or/c bytes? #f)
>         #:seconds
>         number?)
>        response?)
>   contract from:
>       <pkgs>/web-server-lib/web-server/http/response-structs.rkt
>   blaming: /home/amatheus/Dropbox/focus/todagendas/teste.rkt
>    (assuming the contract is correct)
>   at: <pkgs>/web-server-lib/web-server/http/response-structs.rkt:41.2
>   context...:
>    /usr/share/racket/collects/racket/contract/private/blame.rkt:143:0:
> raise-blame-error16
> /usr/share/racket/pkgs/web-server-lib/web-server/http/response.rkt:115:12
> So I've tried to use a response, with the same lambda, and it worked.
> From the error message, I changed the lambda to return (void) and then it
> worked.
> I think maybe the contract is wrong but frankly I don't understand much
> about contracts.
> I've setup some code that exposes the problem. Navigating to "/working"
> and "/fixed" works
> fine; navigating to "/not-working" exposes the problem.
> #lang racket
> (require web-server/dispatch
>          web-server/servlet-env
>          net/url
>          web-server/http/request-structs
>          web-server/http/response-structs)
> (define (not-working req)
>   (response/output (λ (op) (write-bytes #"Hello world" op))))
> (define (working req)
>   (response
>    301 #"OK"
>    (current-seconds) TEXT/HTML-MIME-TYPE
>    empty
>    (λ (op) (write-bytes #"Hello world" op))))
> (define (fixed req)
>   (response/output (λ (op) (write-bytes #"Hello world" op) (void))))
> (define (url->request u)
>     (make-request #"GET" (string->url u) empty
>                   (delay empty) #f "" 80 ""))
> (define-values (agenda-dispatch agenda-url)
>   (dispatch-rules
>    [("working") working]
>    [("not-working")  not-working]
>    [("fixed") fixed]))
> (define (main)
>   (serve/servlet agenda-dispatch
>                  #:servlet-regexp #rx""
>                  #:servlet-path ""))
> (module+ main
>   (main))
> The contracts in response-structs.rkt:
> (provide/contract
>  [struct response
>          ([code number?]
>           [message bytes?]
>           [seconds number?]
>           [mime (or/c false/c bytes?)]
>           [headers (listof header?)]
>           [output (output-port? . -> . void)])]
>  [response/full (-> number? bytes? number? (or/c false/c bytes?) (listof
> header?) (listof bytes?) response?)]
>  [response/output (->* ((-> output-port? void?))
>                        (#:code number?
>                         #:message bytes?
>                         #:seconds number?
>                         #:mime-type (or/c bytes? #f)
>                         #:headers (listof header?))
>                        response?)]
>  [TEXT/HTML-MIME-TYPE bytes?])
> Is the contract wrong or am I doing something weird?
> Thanks,
> André
> ____________________
>   Racket Users list:
>   http://lists.racket-lang.org/users
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