[racket-dev] Removing Xexpr preference from Web Server

Who should be blamed if the coercion does not return a response?

Is there a contract on current-response/c? (I assume that the "/c"
there is a misnomer and it really is a parameter that holds a
contact/coercion, not a contract.)

Robby

On Mon, Dec 6, 2010 at 10:55 AM, Jay McCarthy <jay.mccarthy at gmail.com> wrote:
> Maybe dynamic/c isn't clear enough... its definition is pretty short:
> (define (dynamic/c pre parameter post)
>   (define pre-ctc (coerce-contract 'pre pre))
>   (define post-ctc (coerce-contract 'post post))
>   (make-contract
>    #:name (build-compound-type-name 'dynamic pre-ctc parameter post-ctc)
>    #:projection
>    (λ (b)
>      (define pre-proj ((contract-projection pre-ctc) b))
>      (define post-proj ((contract-projection post-ctc) b))
>      (λ (x)
>        (define dyn-proj
>          ((contract-projection (coerce-contract 'dynamic (parameter))) b))
>        (post-proj
>         (dyn-proj
>          (pre-proj
>           x)))))))
> The system provides pre and post, so it can offer protection to the coercion
> as well as receive protection FROM the coercion. But the coercion comes from
> a parameter which is exposed to the user.
> The one I use in the web-server is:
> (dynamic/c any/c current-response/c response?)
> where response? is the data structure predicate that the internal plumbing
> uses.
> Jay
> On Mon, Dec 6, 2010 at 9:51 AM, Jay McCarthy <jay.mccarthy at gmail.com> wrote:
>>
>> That's why dynamic/c has a pre/c and post/c. Before it uses the user's
>> contract, it applies pre/c. After it applies post/c. This ensures that the
>> user's contract actually coerces to a response?
>> Jay
>>
>> On Mon, Dec 6, 2010 at 9:25 AM, Robby Findler
>> <robby at eecs.northwestern.edu> wrote:
>>>
>>> On Mon, Dec 6, 2010 at 9:23 AM, Jay McCarthy <jay.mccarthy at gmail.com>
>>> wrote:
>>> > Yes, since I am allowing users to customize the coercion behavior, I
>>> > could
>>> > either have them provide two functions: a coercion-applies? function
>>> > and a
>>> > coercion function; OR I could have them just provide the coercion
>>> > function
>>> > and I will check the answer and re-run it inside of the function body.
>>> >
>>> > The other issue is that finding all the places where I should apply the
>>> > coercion inside the body of the function is difficult, because I need
>>> > to do
>>> > it at every place where a response/c could flow in (relatively easy)
>>> > and
>>> > every place where a response/c could flow out (much hard, esp. with
>>> > continuations). Contracts on functions are very nice in their ability
>>> > to do
>>> > stuff to inputs and outputs.
>>>
>>>
>>> I think I need more help to understand the programming problem better.
>>> Why are your users supplying you a contract that you are using to
>>> protect your functions? That is how can you use anything about that
>>> contract to avoid errors in your programs?
>>>
>>> Robby
>>>
>>> > Jay
>>> >
>>> > On Mon, Dec 6, 2010 at 8:19 AM, Matthias Felleisen
>>> > <matthias at ccs.neu.edu>
>>> > wrote:
>>> >>
>>> >> The string->number primitive is probably closer to what Jay wants to
>>> >> do.
>>> >>
>>> >> The only contract I can think of for string->number is
>>> >>
>>> >>  ;; Number -> Boolean
>>> >>  (define (string->number-able? x)
>>> >>    (number? (string->number x)))
>>> >>
>>> >> So the real problem is a performance problem, which a lazy
>>> >> interpretation
>>> >> of contracts by the compiler might be able to eliminate.
>>> >>
>>> >> Is this the true problem Jay -- Matthias
>>> >>
>>> >>
>>> >>
>>> >>
>>> >>
>>> >>
>>> >>
>>> >>
>>> >> On Dec 6, 2010, at 9:45 AM, Robby Findler wrote:
>>> >>
>>> >> > Let's be clear here: our inability to enforce projectionness is in
>>> >> > no
>>> >> > way condoning the two coercianlike contracts that you have now
>>> >> > written.
>>> >> >
>>> >> > That said, the only value I see to contracts that only signal errors
>>> >> > (or do nothing) is that programmers know what to expect from them.
>>> >> > The
>>> >> > downsides you mention are well taken, of course.
>>> >> >
>>> >> > In this specific case, your message seems slightly confused:
>>> >> > certainly
>>> >> > you should be able to use a contract to ensure that the coercion
>>> >> > will
>>> >> > always succeed. Let's assume you have done that and now discuss only
>>> >> > where the coercing bit of the "contract" goes. Is it in a higher
>>> >> > order
>>> >> > position? Is it something that describes an interface to your module
>>> >> > or can it be considered an internal detail?
>>> >> >
>>> >> > As a possible guide by analogy, consider the path-string? Predicate.
>>> >> > It is the contract on many functions the ultimately is connected to
>>> >> > some kind of a coercion somehwere buried inside the racket
>>> >> > primitives
>>> >> > for dealing with the filesystem. Is that like what you want to do?
>>> >> > If
>>> >> > so, how would your arguments hold up for that part of our system?
>>> >> >
>>> >> > Robby
>>> >> >
>>> >> > On Monday, December 6, 2010, Jay McCarthy <jay.mccarthy at gmail.com>
>>> >> > wrote:
>>> >> >> These contracts are not thrown "at dynamic places". The contract is
>>> >> >> always at the module boundary/etc, but its meaning if affected by
>>> >> >> the
>>> >> >> dynamic context of the particular boundary crossing. [1]
>>> >> >>
>>> >> >> I'm been thinking about why I want to use contracts for this
>>> >> >> purpose.
>>> >> >> The alternative is to put an any/c contract in all the places I
>>> >> >> currently have response/c and as the first thing in all those
>>> >> >> functions call
>>> >> >> current-any->response [or as the last thing on returns] on the
>>> >> >> input
>>> >> >> argument. I would then have to put a note in all the documentation
>>> >> >> of those
>>> >> >> any/c that it doesn't REALLY accept anything, instead in other
>>> >> >> accepts
>>> >> >> things that the dynamic current-any->response will turn into a
>>> >> >> response. If
>>> >> >> the coercion failed, then I would have to throw an error, which be
>>> >> >> purely
>>> >> >> dynamic with no blame information because it would not be
>>> >> >> associated with a
>>> >> >> contract boundary.
>>> >> >>
>>> >> >> In contrast, using a contract for this purpose allows me to
>>> >> >> centralize
>>> >> >> the documentation and behavior of these arguments, get correct
>>> >> >> blame on
>>> >> >> places where the coercion fails, and abstract the coercion out of
>>> >> >> the code
>>> >> >> that is using it into its interface. These are all great wins.
>>> >> >>
>>> >> >> In my opinion, if I did not use contracts, the only elegant thing
>>> >> >> to do
>>> >> >> would be to recreate something almost exactly like the contract
>>> >> >> system but
>>> >> >> called the coercion system. That is absurd to me when contracts
>>> >> >> already do
>>> >> >> exactly this.
>>> >> >>
>>> >> >> Am I just not clever enough to think of another elegant way?
>>> >> >> Why is there so much resistance to using the contract system in a
>>> >> >> perfectly legal way according to its own definition & contracts?
>>> >> >> [2] [i.e.
>>> >> >> "projection" functions are not forced to be projections by any
>>> >> >> means. /
>>> >> >> contracts already break eq?/equal?-ness / etc]
>>> >> >>
>>> >> >> Jay
>>> >> >> 1. We already have such context-sensitive contracts:
>>> >> >>
>>> >> >>
>>> >> >> http://docs.racket-lang.org/xml/index.html#(def._((lib._xml/main..rkt)._permissive/c))
>>> >> >>
>>> >> >> permissive/c exists to allow DrRacket to embed more snips inside
>>> >> >> the
>>> >> >> XML boxes, which are otherwise not XML elements.
>>> >> >> 2. make-contract's projection keyword has the contract (-> any/c
>>> >> >> any/c)
>>> >> >>
>>> >> >> The example of make-contract coerces the procedure by restricting
>>> >> >> how
>>> >> >> many arguments rather than checking that when it is given that
>>> >> >> number of
>>> >> >> arguments it is used properly, etc.
>>> >> >>
>>> >> >> Only flat and chaperone contracts attempt to enforce
>>> >> >> projection-ness.
>>> >> >> On Sun, Dec 5, 2010 at 9:31 AM, Matthias Felleisen
>>> >> >> <matthias at ccs.neu.edu> wrote:
>>> >> >>
>>> >> >> Jay, coercions aka casts in our world are compound words with -> in
>>> >> >> between them. Why do you need a new name?
>>> >> >>
>>> >> >> (There is an inconsistency in their behavior. To wit
>>> >> >>
>>> >> >> Welcome to Racket v5.0.99.4.
>>> >> >>> (integer->char 1000000000000000)
>>> >> >> integer->char: expects argument of type <exact integer in
>>> >> >> [0,#x10FFFF],
>>> >> >> not in [#xD800,#xDFFF]>; given 1000000000000000
>>> >> >>
>>> >> >>  === context ===
>>> >> >> /Users/matthias/plt/collects/racket/private/misc.rkt:78:7
>>> >> >>> (string->number "a10")
>>> >> >> #f
>>> >> >>
>>> >> >> But that is a historical problem.)
>>> >> >>
>>> >> >> ;; ---
>>> >> >>
>>> >> >> I am also reluctant to throw contracts at dynamic places. Contract
>>> >> >> boundaries should be syntactically distinct, e.g., module
>>> >> >> boundaries or
>>> >> >> define/contract.
>>> >> >>
>>> >> >> ;; ---
>>> >> >>
>>> >> >> I think you're really just checking an assertion. So perhaps you
>>> >> >> want
>>> >> >> to go with /a as a suffix.
>>> >> >>
>>> >> >>
>>> >> >> -- Matthias
>>> >> >>
>>> >> >>
>>> >> >>
>>> >> >>
>>> >> >> --
>>> >> >> 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
>>> >> >>
>>> >> >>
>>> >>
>>> >
>>> >
>>> >
>>> > --
>>> > 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
>>> >
>>
>>
>>
>> --
>> 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
>
>
>
> --
> 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
>