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Re: Extensible Serialization class with type classes
by Ralf Laemmel 27 Aug '04

27 Aug '04

[redirected to Haskell-cafe from ghc...] I think the point is not to define your class Seriliazer, but to define it as a generic function instead. After all, it is kind of show (because it is serialisation). Reading your earlier emails, it seems a remaining problem is to keep this function somewhat open as to allow to special cases afterwards. This is a nice topic. Here we go. The normal serialisation function with a serialisation result Q is of such a type serialise :: Data a => a -> Q Internally such a function would be probably defined by recursion such as serialise = ... gmapQ serialise ... In fact, some types would already be subject to special cases. So we have serialise = generic_case `extQ` specific_case1 `extQ` specific_case2 ... where generic_case = ... gmapQ serialise ... specific_case1 = ... % do something for my favourite type This is exactly the code skeleton of show. Now the question is how to make the whole business extensible. I normally do this with a type as follows: extensible_serialise :: Data a => (forall b. Data b => b -> Maybe Q) -> a -> Q That is, there is an extra parameter for the type specific cases to be ruled from the outside. Note the Maybe. Here, Nothing means that a specific type is not handled by the parameter, so the normal serialise has to be applied. Regarding your other problem, optional types, this seems to be solvable as well. Again, *don't* define this class class XSDType a with where xsdType :: a -> String xsdNS :: a -> String ...but rather define a generic function, this time is nothing but a type case of type forall x. Typeable x => x -> Maybe (String, String) Hope it helps. Conclusion: don't use classes, rather use generic functions. Ralf Simon David Foster wrote: >On Thu, 2004-08-26 at 20:52, Ralf Laemmel wrote: > > >>Hi Simon, >> >>I think you should be able to succeed with Scrap your boilerplate. >>(I am not sure that you need existentials.) >> >>For the overall XmlIsh style of type erasue and type validation see the >>XmlIsh example at: >>http://www.cs.vu.nl/boilerplate/ >> >> > >The main requirements for the XML serializer are; > >1) It should be scalable; so users should be able to import a compile >module with the basic XML infrastructure and add serializers for new >Complex and Simple (i.e. leaf node) types. > >2) It should be optionally typeable; there exists several systems for >typing an XML document, the most prominent of which is XSD. The main >requirement is that if the XML tree is to be typed, clearly each node in >the tree must be associated with mapping to an XSD name and name-space >(2 Strings) or some other data items if typing by another system. The >way I was planning on doing this was using a class; XSDType a with >functions xsdType :: a -> String and xsdNS :: a -> String. Then at >serialisation if a particular type and all it's constituent types were >XSD Typeable, extra data (i.e. an extra attribute type="nsp:name") can >(again optionally) be inserted into the tree. > >The XmlIsh example uses a number of functions for serialising different >data-types down the tree. I'm not sure how scalable doing it this way >would be; can you integrate a type-class system with it? The other >problem is dealing with Schematic extensions, such as SOAP Arrays which >have the requirement that the elements are always typeable but should >not be part of the core serialiser. > >My other question is, section 7 only mentions polymorphic types which >hold constraints Data, Typeable and Typeable1; can the new Generics deal >with other polymorphic constraints? This is the main problem; the >type-class system can deal fine with just serialisation of data-type >content, it just doesn't seem to be able to handle an optional >type-system. > >Ideally I need to produce a system where one XML namespace maps to one >module and that module provides data-types, serialisers and typers for >the given XML types, such that other modules which use those data-types >themselves should be able to import that module and its serialisers to >(heuristically) build serialisers for itself. > >-Si. > > > >_______________________________________________ >Glasgow-haskell-users mailing list >Glasgow-haskell-users(a)haskell.org >http://www.haskell.org/mailman/listinfo/glasgow-haskell-users > >

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ghc binaries for sunos, hpux, aix, irix, and osf1?
by Goetz Isenmann 24 Aug '04

24 Aug '04

Hi! Are there other binary ghc versions available besides those on http://www.haskell.org/ghc/download.html, that I can use for ghc bootstrap on sunos (5.5.1-5.9), hpux (10.20 and 11.x), aix (4.3-5.2), irix (6.5), and osf1 (4.0-5.1)? -- Goetz Isenmann

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Fun with multi-parameter type classes
by Sam Mason 22 Aug '04

22 Aug '04

Hi, I've been getting into Haskell over the past few months and have just come up against a bit of a brick wall when trying to encapsulate some of the data structures in my code nicely. Basically what I want to have, is a type class where one of the intermediate values is opaque with respect to code using its implementations. This is a simplified version of what I'm trying to accomplish: class Foo t where encode :: String -> t decode :: t -> String instance Foo Int where encode = read decode = show test = decode . encode This currently fails, because the type checker insists on trying to figure out what its type should be - even though it shouldn't be needed. Any suggestions on how to encode this sort of thing? And if it is possible, can it be extended to multiple type parameters? as this is really what I want to use it for. About the only way I can think of "fixing" it, is by turning the code inside out - sort of like the way an AST drives the compiler, but without knowing how it represents things internally. Thanks, Sam

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hugs, ghc portability
by Vadim Zaliva 20 Aug '04

20 Aug '04

Hi! I have following code: import Data.Bits xcomp :: Int -> Int -> Int xcomp a m = (xor a (complement m)) which compiles fine under GHC. However hugs does not like it, since they intentionally did not do: --instance Bits Int How can I resolve this to have this code work under GHC and hugs? Sincerely, Vadim -- "La perfection est atteinte non quand il ne reste rien a ajouter, mais quand il ne reste rien a enlever." (Antoine de Saint-Exupery)

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Logic programming in Haskell
by Stefan Monnier 20 Aug '04

20 Aug '04

Has anyone already tried to embed prolog-style logic programming as a Haskell library? Kind of like Schelog but for Haskell? I know about Curry, but I'd like a Haskell library rather than another language. Stefan

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Re: [Haskell-cafe] Fun with multi-parameter type classes
by Jon Cast 20 Aug '04

20 Aug '04

> Hi, > > I've been getting into Haskell over the past few months and have just > come up against a bit of a brick wall when trying to encapsulate > some of the data structures in my code nicely. Basically what I > want to have, is a type class where one of the intermediate values > is opaque with respect to code using its implementations. This is > a simplified version of what I'm trying to accomplish: > > class Foo t where > encode :: String -> t > decode :: t -> String > > instance Foo Int where > encode = read > decode = show > > test = decode . encode > > This currently fails, because the type checker insists on trying to > figure out what its type should be - even though it shouldn't be > needed. The intermediate type /is/ needed---it's a (hidden) parameter to your `encode' and `decode' functions. Why do you think it shouldn't be? <snip> Jon Cast

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Re: [Haskell-cafe] Logic programming in Haskell
by Benderjgdefault＠aol.com 20 Aug '04

20 Aug '04

In a message dated 8/19/04 11:11:00 AM, monnier(a)iro.umontreal.ca writes: >Has anyone already tried to embed prolog-style logic programming as >a Haskell library? Kind of like Schelog but for Haskell? > >I know about Curry, but I'd like a Haskell library rather than >another language. No doubt you are looking for an implementation, not papers, but I would like to point a selection of papers dealing with embedding logic programming in Haskell (on my Haskell bibliography site): http://haskell.readscheme.org/appl.html#logic This includes both the Seres and Spivey papers, but also a couple of papers by Ralf Hinze. Also, since you mention Schelog, perhaps you would also find interesting KANREN, a more recent logic-programming-in-Scheme system by Dan Friedman, Oleg and others: http://kanren.sourceforge.net Jim

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Haskell extension/improvement
by Ron de Bruijn 18 Aug '04

18 Aug '04

I was playing with QuickCheck and I just wanted to put all of my tests in a list, but that's not possible, because of the fact that a list can only contain values of the same type. So concrete I would like to write down this: prop_RevRev xs = reverse (reverse xs) == xs where types = xs::[Int] --trivial test prop_Simple = True == True whatIwant::(Testable a)=>[a] whatIwant = [prop_RevRev, prop_Simple] testAll::IO () testAll = do sequence_ $ map quickCheck whatIwant quickCheck can work on all Testable values, so why can't a list (any datastructure) can't hold Testable values? What this gives you is, a lot more flexibility. Just like you can define your functions on interfaces, now you can use interfaces in your datastructures. I can't think of any reason why it couldn't be implemented in any Haskell compiler. I saw this page: http://www.haskell.org/hawiki/ExistentialTypes but you still have to explicitly add the relation via Obj (and that's bad). Regards, Ron P.S. If it already exist in this exact way, how is it called? _______________________________ Do you Yahoo!? Express yourself with Y! Messenger! Free. Download now. http://messenger.yahoo.com

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puzzle: prove this floorSqrt correct
by blaetterrascheln＠web.de 18 Aug '04

18 Aug '04

-- Here's the discrete version of Newton's method for finding -- the square root. Does it always work? Any literature? floorSqrt :: Integer -> Integer floorSqrt n | n>=1 = detect (iterate (\x->(x+n`quot`x)`quot`2) 1) where detect (x:xs@(y:z:_)) | x==z = min x y | otherwise = detect xs isFloorSqrt n x = x*x <= n && n < (x+1)*(x+1) main = print$ all (\n->isFloorSqrt n (floorSqrt n)) [1..1000000] ________________________________________________________________ Verschicken Sie romantische, coole und witzige Bilder per SMS! Jetzt neu bei WEB.DE FreeMail: http://freemail.web.de/?mc=021193

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Haskell extension/improvement
by MR K P SCHUPKE 17 Aug '04

17 Aug '04

See http://www.cwi.nl/~ralf/HList for a paper which shows how to implement a heterogeneous list in haskell, and how to implement operations on it (amongst other things)... Keean.

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