May 2007 - Haskell-Cafe

Multiple DB drivers in HDBC
by kpierre 20 May '07

20 May '07

Hello. I have a problem with migration to HDBC 1.1. My application had transparent support for multiple database types, because of HDBC's Connection object. How can I preserve this feature with minimal code changes? I tried to use ConnWrapper, as migration guide suggest, but it seems there's no way to convert arbitrary IConnection object into ConnWrapper. Thanks in advance, pierre -- View this message in context: http://www.nabble.com/Multiple-DB-drivers-in-HDBC-tf3784996.html#a10703798 Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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New YAHT maintainer
by Eric Y. Kow 20 May '07

20 May '07

Hi, I just wanted to announce that Sam Hughes will be taking over as Keeper of the YAHT. YAHT is the venerable Yet Another Haskell Tutorial. It is available both as a wikibook and as a PDF (*). The Keeper of the YAHT has the grave duty of synchronising the YAHT wikibook and tex source, and releasing an updated PDF from time to time. You can send modifications to YAHT either by (a) updating the wikibook or (b) darcs send'ing a patch to the tex source (patches will go to the Haskell Café mailing list for community review). Many thanks to Sam for stepping up! * http://darcs.haskell.org/yaht/yaht.pdf -- Eric Kow http://www.loria.fr/~kow PGP Key ID: 08AC04F9 Merci de corriger mon français.

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idlelog
by Leandro Lisboa Penz 20 May '07

20 May '07

Hi I made a program that detects user presence in a linux box by checking for keyboard interruptions in /proc/interrupts. Problem is, it is supposed to run for a long time, and it keeps about 40MB for itself. Yeah, this is one more "help me with this memory problem" mails... The code can be found in http://hpaste.org/1907, and in "darcs get http://lpenz.50webs.com/idlelog I guess I'm having some conceptual problem. Cheers Leandro Penz

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Re: [Haskell-cafe] Pesky monads...
by Matthew Cox 20 May '07

20 May '07

It occurred to me that the predicate will generally be a monadic function itself, so here's a refined version: :: Monad m => (a -> m Bool) -> (a -> m a) -> a -> m a untilM pred f x = do c <- pred x if c then return x else f x >>= untilM pred f Think of a computation in the State monad, a predicate will probably want to consult that state. Matthew Cox ----- Original Message ----- From: Matthew Cox To: <haskell-cafe(a)haskell.org> Sent: Sat, 19 May 2007 13:10:40 -0600 Subject: Re: [Haskell-cafe] Pesky monads... You can define one: untilM :: Monad m => (a -> Bool) -> (a -> m a) -> a -> m a untilM pred f x | pred x = return x | otherwise = f x >>= untilM pred f Matthew Cox ----- Original Message ----- From: Andrew Coppin To: haskell-cafe(a)haskell.org Sent: Sat, 19 May 2007 19:49:40 +0100 Subject: [Haskell-cafe] Pesky monads... I've been getting some pretty weird complaints from the type checker. And I just figured out why! Grr... Is there any function that does the same thing as "until", but in a monad? _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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idlelog
by Leandro Lisboa Penz 19 May '07

19 May '07

Hi I made a program that detects user presence in a linux box by checking for keyboard interruptions in /proc/interrupts. Problem is, it is supposed to run for a long time, and it keeps about 40MB for itself. Yeah, this is one more "help me with this memory problem" mails... The code can be found in http://hpaste.org/1907, and in "darcs get http://lpenz.50webs.com/idlelog I guess I'm having some conceptual problem. Cheers Leandro Penz

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Who lives in a heap like this?
by Andrew Coppin 19 May '07

19 May '07

Greetings. I have just implemented a heap. But... um... I can't acutally figure out *which kind* of heap it is! LOL. Any ideas? (Seems to work really well, whatever it is. Oh, and I discovered that you can sort data just by shoving it all into a heap, and then taking it all out again. Apparently this is a standard algorithm, and it's known as a "heap sort", unsurprisingly. You learn something every day...) module Heap where import Data.List (intersperse, unfoldr) data Heap t = Node !t !(Heap t) !(Heap t) | Null instance (Show t) => Show (Heap t) where show = work "" where work p (Null) = p ++ "-" work p (Node v t0 t1) = concat $ intersperse "\n" $ [work (' ':p) t0, p ++ show v, work (' ':p) t1] empty = Null is_empty Null = True is_empty _ = False insert v (Null) = Node v Null Null insert v (Node v0 t0 t1) = let lo = min v v0 hi = max v v0 in Node hi (insert lo t1) t0 get_max (Null) = error "heap is empty" get_max (Node v _ _) = v delete_max (Null) = error "heap is empty" delete_max (Node _ Null Null) = Null delete_max (Node _ Null t1) = t1 delete_max (Node _ t0 Null) = t0 delete_max (Node _ t0 t1) | get_max t0 > get_max t1 = Node (get_max t0) (delete_max t0) t1 | otherwise = Node (get_max t1) t0 (delete_max t1) size (Null) = 0 size (Node _ t0 t1) = 1 + (size t0) + (size t1) from_list :: (Ord t) => [t] -> Heap t from_list = foldr insert empty to_list :: (Ord t) => Heap t -> [t] to_list = unfoldr (\h -> if is_empty h then Nothing else Just (get_max h, delete_max h)) heap_sort :: (Ord t) => [t] -> [t] heap_sort = to_list . from_list

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A special function
by Andrew Coppin 19 May '07

19 May '07

I'd like to share something with you. I call it "the evil ******* function from hell": (Results xss) >>= f = Results $ map concat $ diagonalise $ map (foldr longZip [] . map (unpack . f)) xss If you attempt to comprehend how this function works, you WILL in fact loose your mind! However, it does in fact work completely correctly. It's just not possible to know why. :-D newtype Results t = Results [[t]] unpack (Results xss) = xss longZip = similar to zip (++) diagonalise = transform an infinite list of infinite lists into an infinite list of finite lists.

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Haskell on OS X
by Johan Tibell 19 May '07

19 May '07

I just switched to OS X and was wondering if someone would like to share their setup. Install binaries from haskell.org or Mac Ports? Which emacs build? etc Johan

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Re: [Haskell-cafe] Pesky monads...
by Matthew Cox 19 May '07

19 May '07

You can define one: untilM :: Monad m => (a -> Bool) -> (a -> m a) -> a -> m a untilM pred f x | pred x = return x | otherwise = f x >>= untilM pred f Matthew Cox ----- Original Message ----- From: Andrew Coppin To: haskell-cafe(a)haskell.org Sent: Sat, 19 May 2007 19:49:40 +0100 Subject: [Haskell-cafe] Pesky monads... I've been getting some pretty weird complaints from the type checker. And I just figured out why! Grr... Is there any function that does the same thing as "until", but in a monad? _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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Pesky monads...
by Andrew Coppin 19 May '07

19 May '07

I've been getting some pretty weird complaints from the type checker. And I just figured out why! Grr... Is there any function that does the same thing as "until", but in a monad?

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