- Haskell-Cafe - Haskell.org

Re: [Haskell-cafe] Logic programming in Haskell
by Benderjgdefault＠aol.com 19 Aug '04

19 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 17 Aug '04

17 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 16 Aug '04

16 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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automatic Haskell indentations
by Henning Thielemann 16 Aug '04

16 Aug '04

Looking for automatic indentation of Haskell code I found the thread http://www.haskell.org/pipermail/haskell-cafe/2003-August/005000.html It ended up with many suggestions for manual formatting of code. I wonder if there is some tool which simply formats a Haskell program. A tool like 'indent' (or 'm3pp' for Modula-3) which processes data from stdin to stdout could be nicely integrated into NEdit.

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RE: [Haskell-cafe] Hiding functions
by Simon Peyton-Jones 16 Aug '04

16 Aug '04

Yes that's true. See http://haskell.org/onlinereport/modules.html Section 5.5 S | -----Original Message----- | From: Lyle Kopnicky [mailto:lists@qseep.net] | Sent: 13 August 2004 23:37 | To: Simon Peyton-Jones | Cc: Haskell Cafe | Subject: Re: [Haskell-cafe] Hiding functions | | Simon, | | It also appears that if I don't call 'catch' from my module, I can | import both and not have a conflict. | | - Lyle | | Simon Peyton-Jones wrote: | | >It's an explicit Haskell 98 design choice | > http://haskell.org/onlinereport/modules.html | > | >"5.6.2 Shadowing Prelude Names | > | >The rules about the Prelude have been cast so that it is possible to use | >Prelude names for nonstandard purposes; however, every module that does | >so must have an import declaration that makes this nonstandard usage | >explicit." | > | >It's a decision one could debate, but it was an explicit choice. | > | >Simon | > | >

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Control.Monad.Error with a custom error type
by Brian Smith 15 Aug '04

15 Aug '04

I have: > data Reference = Ref [String] String > data ReferenceError = RefError > { expectedType :: String -- type of element we were looking for (e.g. "type","package") > , pointOfError :: Reference -- path to deepest parent element not found in path > } > type ReferenceMonad = Either ReferenceError I want to write functions that use "Either ReferenceError a" as as the error monad, instead of the more common "Either String a". In particular, I want to use be able to write: > type Model = [(String,Type)] > findType :: Model -> Reference -> ReferenceMonad Type > findType m -> r@(Ref [] name) = case lookup ((==name) . nameOf) m of > Nothing -> throwError r > Just x -> return x I know that I could make this work by making ReferenceError an instance of the Error class, but I cannot provide meaningful implementations of "noMsg" and "strMsg" for ReferenceError. So, it seems instead I need to make (Either ReferenceError) an instance of MonadError. However, when I try, I get: > instance MonadError (Either ReferenceError) Kind error: `Either ReferenceError' is not applied to enough type arguments When checking kinds in `MonadError (Either ReferenceError)' In the instance declaration for `MonadError (Either ReferenceError)' So, how do I get the effect I want for "findType"? Besides "throwError" I also want to use "catchError". Thanks, Brian (Haskell newbie)

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RE: [Haskell-cafe] Hiding functions
by Simon Peyton-Jones 14 Aug '04

14 Aug '04

It's an explicit Haskell 98 design choice http://haskell.org/onlinereport/modules.html "5.6.2 Shadowing Prelude Names The rules about the Prelude have been cast so that it is possible to use Prelude names for nonstandard purposes; however, every module that does so must have an import declaration that makes this nonstandard usage explicit." It's a decision one could debate, but it was an explicit choice. Simon | -----Original Message----- | From: haskell-cafe-bounces(a)haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of Lyle | Kopnicky | Sent: 12 August 2004 20:23 | To: Haskell Cafe | Subject: [Haskell-cafe] Hiding functions | | Hi all, | | I'm working on a program that uses my own brand of exceptions, and I | created two functions called 'throw' and 'catch'. In order for this to | work, I hide the Prelude 'catch' in my module, called 'Cont.hs'. Thus: | | module Cont where | import Prelude hiding (catch) | ... | throw = ... | catch = ... | | This works hunky-dory until I create another file that imports Cont. I | get a conflict when I use 'catch', so I have to hide the Prelude one again: | | import Prelude hiding (catch) | import Cont | ... | ... throw ... | ... catch ... | | So I'm a bit annoyed by this 'propagation' of hiding clauses. Then I | created a new file, that redefined throw: | | module ResumableExceptions where | import Cont hiding (throw) | import qualified Cont (throw) | ... | throw = ... Cont.throw ... | | Finally, I created a file using ResumableExceptions: | | import Cont hiding (throw) | import ResumableExceptions | ... | ... throw ... | | If I wanted to also use 'catch' I'd have to hide that from the Prelude | as well. | | I can't use type classes to solve this problem, because the types of the | two 'throw' functions are different. | | Perhaps I should just make up new names for these things, eh? But there | might be code that uses 'throw' and doesn't really care which one is | used, and it would be nice to just modify the import line and be done | with it. | | Any opinions? | | Thanks, | Lyle | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe(a)haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe

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RE: [Haskell-cafe] closed classes [was: Re: exceptions vs. Either]
by Simon Peyton-Jones 14 Aug '04

14 Aug '04

Closed classes are certainly interesting, but a better way to go in this case is to allow the programmer to declear new kinds, as well as new types. This is what Tim Sheard's language Omega lets you do, and I'm considering adding it to GHC. kind HNat = HZero | HSucc HNat class HNatC (a::HNat) instance HNatC HZero instance HNatC n => HNatC (HSucc n) Here the keyword 'kind' is like 'data', except that it introduces a new *kind* HNat with new *type* constructors HZero and HSucc, rather than a new *type* constructor with new *data* constructors. There is no way to construct a value of type HZero, or (HSucc HZero); these are simply phantom types. Today we are forced to say data HNat data HSucc a which loses the connection between the two. A merit of declaring a kind is that the kind is closed -- the only types of kind HNat are HZero, HSucc HZero, and so on. So the class doesn't need to be closed; no one can add new instances to HNatC because they don't have any more types of kind HNat. At the moment I'm only thinking of parameter-less kind declarations but one could easily imagine kind parameters, and soon we'll have kind polymorphism.... but one step at a time. Any thoughts? Simon | -----Original Message----- | From: haskell-cafe-bounces(a)haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of | Duncan Coutts | Sent: 06 August 2004 15:11 | To: MR K P SCHUPKE | Cc: Haskell Cafe | Subject: [Haskell-cafe] closed classes [was: Re: exceptions vs. Either] | | On Fri, 2004-08-06 at 14:05, MR K P SCHUPKE wrote: | > >You should include the definitions of the classes before saying | > | > HOrderedList l' just has to prove by induction that for any element | > in the list, the next element is greater, so the class is simply: | > | > class HOrderedList l | > instance HNil | > instance HCons a HNil | > instance (HOrderedList (HCons b l),HLe a b) => HCons a (HCons b l) | | Somewhat off-topic, | | It's when we write classes like these that closed classes would be | really useful. | | You really don't want people to add extra instances to this class, it'd | really mess up your proofs! | | I come across this occasionally, like when modelling external type | systems. For example the Win32 registry or GConf have a simple type | system, you can store a fixed number of different primitive types and in | the case of GConf, pairs and lists of these primitive types. This can be | modelled with a couple type classes and a bunch of instances. However | this type system is not extensible so it'd be nice if code outside the | defining module could not interfere with it. | | The class being closed might also allow fewer dictionaries and so better | run time performance. | | It would also have an effect on overlapping instances. In my GConf | example you can in particular store Strings and lists of any primitive | type. But now these two overlap because a String is a list. However | these don't really overlap because Char is not one of the primitive | types so we could never get instances of String in two different ways. | But because the class is open the compiler can't see that, someone could | always add an instance for Char in another module. If the class were | closed they couldn't and the compiler could look at all the instances in | deciding if any of them overlap. | | So here's my wishlist item: | | closed class GConfValue v where | ... | | Duncan | | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe(a)haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe

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RE: [Haskell-cafe] Hiding functions
by Bayley, Alistair 13 Aug '04

13 Aug '04

What is wrong with creating your own catch and throw with different names? e.g. data MyException = MyException ... deriving (Typeable, Show) catchMyEx :: IO a -> (MyException -> IO a) -> IO a catchMyEx = catchDyn throwMyEx :: MyException -> a throwMyEx = throwDyn > But there might be code that uses 'throw' and doesn't really > care which one is used, and it would be nice to just modify > the import line and be done with it. Can you expand on this? (more example code?) If you want to throw your own exceptions, then you must use throwDyn/catchDyn, so you have to use something other than catch/throw anyway. Alistair. > -----Original Message----- > From: Lyle Kopnicky [mailto:lists@qseep.net] > Sent: 12 August 2004 20:23 > To: Haskell Cafe > Subject: [Haskell-cafe] Hiding functions > > Hi all, > > I'm working on a program that uses my own brand of > exceptions, and I created two functions called 'throw' and > 'catch'. In order for this to work, I hide the Prelude > 'catch' in my module, called 'Cont.hs'. Thus: > > module Cont where > import Prelude hiding (catch) > ... > throw = ... > catch = ... > > This works hunky-dory until I create another file that > imports Cont. I get a conflict when I use 'catch', so I have > to hide the Prelude one again: > > import Prelude hiding (catch) > import Cont > ... > ... throw ... > ... catch ... > > So I'm a bit annoyed by this 'propagation' of hiding clauses. > Then I created a new file, that redefined throw: > > module ResumableExceptions where > import Cont hiding (throw) > import qualified Cont (throw) > ... > throw = ... Cont.throw ... > > Finally, I created a file using ResumableExceptions: > > import Cont hiding (throw) > import ResumableExceptions > ... > ... throw ... > > If I wanted to also use 'catch' I'd have to hide that from > the Prelude as well. > > I can't use type classes to solve this problem, because the > types of the two 'throw' functions are different. > > Perhaps I should just make up new names for these things, eh? > But there might be code that uses 'throw' and doesn't really > care which one is used, and it would be nice to just modify > the import line and be done with it. > > Any opinions? > > Thanks, > Lyle ----------------------------------------- ***************************************************************** Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. *****************************************************************

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