August 2004 - Haskell-Cafe

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 19 Aug '04

19 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 19 Aug '04

19 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 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 17 Aug '04

17 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 14 Aug '04

14 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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