[racket-dev] Removing Xexpr preference from Web Server

On Mon, Dec 6, 2010 at 10:16 AM, Robby Findler
<robby at eecs.northwestern.edu>wrote:

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

The provider of the coercion should be blamed, but that is not possible [I
think] so the positive party of the whole dynamic/c is blamed.


> 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.)
>

current-response/c is contracted with (parameter/c contract?)

The /c is not a misnomer, you are just parsing it wrong. Read it as
(parameter (contract response)) not (contract (parameter response)), where
/c is the post-fix syntax for (contract x) and current- is the pre-fix
syntax for (parameter x)

Jay


>
> 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
> >
>



-- 
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
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