[racket] Contract weirdness: "broke contract [...]; expected <part?>, given: #<part>"

On Sat, Nov 6, 2010 at 12:06 PM, Carlos Scheidegger
<carlos.scheidegger at gmail.com> wrote:
> Hi,
>
> This must be a simple question, but I'm having some trouble convincing
> Racket that one of my procedure calls respects the required contract,
> and I'm not quite sure how to solve it. I only experience the problem
> when running the code with serve/servlet, so I have a hunch that some
> interaction with dynamic loading might be to blame here.
>
> Specifically, there's a part? procedure (from scribble/core) which
> returns true when I run the code from the interactive racket prompt,
> but returns false when I run the same code via a procedure passed to
> serve/servlet. I end up getting a servlet error claiming I "broke the
> contract (-> part? (or/c #f content?)) on part-title-content; expected
> <part?>, given: #<part>".
>
> The full servlet trace is here:
>
> Exception
>
> The application raised an exception with the message:
>
> <collects>/scribble/core.rkt:
>  (file
>   /home/cscheid/code/racket/cscheid.net/src/generate-xexpr.rkt)
>  broke the contract (-> part? (or/c #f content?)) on
> part-title-content; expected <part?>, given: #<part>
>
> Stack trace:
>
> raise-blame-error at:
>  line 54, column 0, in file
> /home/cscheid/code/racket/plt-5.0.1/collects/racket/contract/private/blame.rkt
> <unknown procedure> at:
>  line 1445, column 3, in file
> /home/cscheid/code/racket/plt-5.0.1/collects/racket/contract/private/arrow.rkt
> part->xexpr at:
>  line 82, column 0, in file
> /home/cscheid/code/racket/cscheid.net/src/generate-xexpr.rkt
> map at:
>  line 18, column 11, in file
> /home/cscheid/code/racket/plt-5.0.1/collects/racket/private/map.rkt
> render-main-page at:
>  line 8, column 0, in file /home/cscheid/code/racket/cscheid.net/src/serve.rkt
> <unknown procedure> at:
>  line 1445, column 3, in file
> /home/cscheid/code/racket/plt-5.0.1/collects/racket/contract/private/arrow.rkt
> <unknown procedure> at:
>  line 1445, column 3, in file
> /home/cscheid/code/racket/plt-5.0.1/collects/racket/contract/private/arrow.rkt
> <unknown procedure> at:
>  line 1445, column 3, in file
> /home/cscheid/code/racket/plt-5.0.1/collects/racket/contract/private/arrow.rkt
> <unknown procedure> at:
>  line 1445, column 3, in file
> /home/cscheid/code/racket/plt-5.0.1/collects/racket/contract/private/arrow.rkt
> <unknown procedure> at:
>  line 1445, column 3, in file
> /home/cscheid/code/racket/plt-5.0.1/collects/racket/contract/private/arrow.rkt
> <unknown procedure> at:
>  line 57, column 2, in file
> /home/cscheid/code/racket/plt-5.0.1/collects/web-server/dispatchers/dispatch-servlets.rkt
> <unknown procedure> at:
>  line 1445, column 3, in file
> /home/cscheid/code/racket/plt-5.0.1/collects/racket/contract/private/arrow.rkt
> <unknown procedure> at:
>  line 1445, column 3, in file
> /home/cscheid/code/racket/plt-5.0.1/collects/racket/contract/private/arrow.rkt
> <unknown procedure> at:
>  line 1445, column 3, in file
> /home/cscheid/code/racket/plt-5.0.1/collects/racket/contract/private/arrow.rkt
> <unknown procedure> at:
>  line 1445, column 3, in file
> /home/cscheid/code/racket/plt-5.0.1/collects/racket/contract/private/arrow.rkt
> <unknown procedure> at:
>  line 1445, column 3, in file
> /home/cscheid/code/racket/plt-5.0.1/collects/racket/contract/private/arrow.rkt
> select-handler/no-breaks at:
>  line 173, column 2, in file
> /home/cscheid/code/racket/plt-5.0.1/collects/racket/private/more-scheme.rkt
> <unknown procedure> at:
>  line 1445, column 3, in file
> /home/cscheid/code/racket/plt-5.0.1/collects/racket/contract/private/arrow.rkt
> <unknown procedure> at:
>  line 1445, column 3, in file
> /home/cscheid/code/racket/plt-5.0.1/collects/racket/contract/private/arrow.rkt
> <unknown procedure> at:
>  line 1445, column 3, in file
> /home/cscheid/code/racket/plt-5.0.1/collects/racket/contract/private/arrow.rkt
> <unknown procedure> at:
>  line 1445, column 3, in file
> /home/cscheid/code/racket/plt-5.0.1/collects/racket/contract/private/arrow.rkt
> connection-loop at:
>  line 62, column 2, in file
> /home/cscheid/code/racket/plt-5.0.1/collects/web-server/private/dispatch-server-unit.rkt
>
> I'm sure I'm doing some things which are not exactly kosher - I'm
> dynamically loading Scribble files and converting them to xexprs. My
> wild guess is that scribble/core is somehow loading twice, and that
> the contract is checking a structure built with one of the
> instantiations against the predicate of the other instantiation. That
> the setup works in an interactive Racket prompt gives me hope for a
> solution. It's been about 6 years since I've done anything with
> Racket, so there's a good chance I'm being terribly dense.

That's exactly what the problem is. This is what the
#:make-servlet-namespace and #:servlet-namespace keyword arguments to
the servlet dispatching functions are all about. Each servlet normally
gets its own new namespace, but you can specifically share modules
among servlets (and the server).

Jay

>
> Is there a simpler way to achieve what I'm going for? That is, I have
> a list of scribble files, all in the same directory, and I want to
> load each of them. Right now, I'm doing this roughly via:
>
> ; path denotes the path to the list of files,
> ; scribble-file-list holds a list of filenames which I want to load
> (parameterize ([current-load-relative-directory (normalize-path path)])
>  (map (lambda (filename) (dynamic-require filename 'doc)) scribble-file-list))
>
> Thank you very much in advance,
> -carlos
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-- 
Jay McCarthy <jay at cs.byu.edu>
Assistant Professor / Brigham Young University
http://faculty.cs.byu.edu/~jay

"The glory of God is Intelligence" - D&C 93