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?<div> </div><div> Jay <div class="gmail_quote">On Mon, Dec 6, 2010 at 9:25 AM, Robby Findler <<a href="mailto:robby@eecs.northwestern.edu">robby@eecs.northwestern.edu</a>> wrote: <blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;"> <div class="im">On Mon, Dec 6, 2010 at 9:23 AM, Jay McCarthy <<a href="mailto:jay.mccarthy@gmail.com">jay.mccarthy@gmail.com</a>> 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. </div>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 <div><div></div><div class="h5"> > Jay > > On Mon, Dec 6, 2010 at 8:19 AM, Matthias Felleisen <<a href="mailto:matthias@ccs.neu.edu">matthias@ccs.neu.edu</a>> > 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 <<a href="mailto:jay.mccarthy@gmail.com">jay.mccarthy@gmail.com</a>> >> > 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: >> >> >> >> <a href="http://docs.racket-lang.org/xml/index.html#(def._((lib._xml/main..rkt)._permissive/c))" target="_blank">http://docs.racket-lang.org/xml/index.html#(def._((lib._xml/main..rkt)._permissive/c))</a> >> >> >> >> 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 >> >> <<a href="mailto:matthias@ccs.neu.edu">matthias@ccs.neu.edu</a>> 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 <<a href="mailto:jay@cs.byu.edu">jay@cs.byu.edu</a>> >> >> Assistant Professor / Brigham Young University >> >> <a href="http://faculty.cs.byu.edu/~jay" target="_blank">http://faculty.cs.byu.edu/~jay</a> >> >> >> >> "The glory of God is Intelligence" - D&C 93 >> >> >> >> >> > > > > -- > Jay McCarthy <<a href="mailto:jay@cs.byu.edu">jay@cs.byu.edu</a>> > Assistant Professor / Brigham Young University > <a href="http://faculty.cs.byu.edu/~jay" target="_blank">http://faculty.cs.byu.edu/~jay</a> > > "The glory of God is Intelligence" - D&C 93 > </div></div></blockquote></div> -- Jay McCarthy <<a href="mailto:jay@cs.byu.edu" target="_blank">jay@cs.byu.edu</a>> Assistant Professor / Brigham Young University <a href="http://faculty.cs.byu.edu/~jay" target="_blank">http://faculty.cs.byu.edu/~jay</a> "The glory of God is Intelligence" - D&C 93 </div>