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ANNOUNCE: Haskell Hackathon 07 II: Freiburg: Oct 5-7
by dons＠cse.unsw.edu.au 17 Jul '07

17 Jul '07

------------------------------------------------------------------------ Hac 2007 II Haskell Hackathon 2007 II October 5-7, 2007 Freiburg, Germany http://haskell.org/haskellwiki/Hac_2007_II ------------------------------------------------------------------------ We are pleased to announce the 2nd Haskell Hackathon for 2007! The event will be held over 3 days, October 5-7 2007, at the University of Freiburg, in Germany, after ICFP. The plan is to hack on serious Haskell infrastructure, tools, libraries and compilers. To attend please register, and get ready to hack those lambdas! Code to hack on: * Hackage * Cabal * Porting foreign libraries * Bug squashing * You decide! At the last Hackathon, in January at Oxford, resulted in: * Cabal, Hackage, Haddock improvements * Data.Binary and DeferedBinary libraries * GHCi debugger improvements * ghc-api interface from Emacs * Crypto lirbary improvments * A native Haskell 'tar' implementation * And more And we hope to be similarly productive this time around. Before you attend, do start thinking and familiarising yourself with 1 or 2 projects you wish to work on, to ensure no wasted effort during the Hackathon. A list of possible projects is available on the website == Registration: We ask that you register you interest. Follow the instructions on the registration page: http://haskell.org/haskellwiki/Hac_2007_II/Register Once you've registered, do add your info to the attendees self-organising page, http://haskell.org/haskellwiki/Hac_2007_II/Attendees if you are looking to share costs, or meet up prior to the hackathon, with other attendees. N.B. if you already expressed interest via the wiki, do confirm by registering `officially' anyway. == Important dates: Hackathon: October 5-7, 2007 == Organisers: Duncan Coutts Ian Lynagh Don Stewart With local arrangements courtesy: Stefan Wehr Phillip Heidegger

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System.Posix.User.getAllUserEntries can be called only once
by Sascha Böhme 16 Jul '07

16 Jul '07

Hello, are there special constraints why the function System.Posix.User.getAllUserEntries of the unix package can be called only once? Beginning with the second call, it only returns the empty list. Thus, this function cannot be used for a long running process which needs to query the user database from time to time, again and again. Thanks, Sascha

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Functor (Either a)
by Conor McBride 16 Jul '07

16 Jul '07

Hi all I read TFM http://haskell.org/ghc/docs/latest/html/libraries/base/ Prelude.html#t%3AFunctor and it does rather suggest that there's an instance of Functor for (Either a). Yet, ___ ___ _ / _ \ /\ /\/ __(_) / /_\// /_/ / / | | GHC Interactive, version 6.6, for Haskell 98. / /_\\/ __ / /___| | http://www.haskell.org/ghc/ \____/\/ /_/\____/|_| Type :? for help. Loading package base ... linking ... done. Prelude> fmap not (Right True) <interactive>:1:0: No instance for (Functor (Either a)) arising from use of `fmap' at <interactive>:1:0-20 Am I being dim? Conor

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darcs patch: s6_addr32 is undefined on darwin. Added an AC check a...
by Thomas Schilling 13 Jul '07

13 Jul '07

Fri Jun 22 18:58:52 CEST 2007 Thomas Schilling <nominolo(a)gmail.com> * s6_addr32 is undefined on darwin. Added an AC check and a definition taken from the NetBSD sources.

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Proposal: Data.Stream
by Wouter Swierstra 13 Jul '07

13 Jul '07

Dear all, I just uploaded a fairly unspectacular package Data.Stream to Hackage. It implements quite a few operations on streams (infinite lists), analogous to those defined in Data.List. Sometimes it is just good to know a list is infinite. * Haddock documentation: http://www.cs.nott.ac.uk/~wss/repos/Stream/ dist/doc/html/ * Hackage homepage: http://hackage.haskell.org/cgi-bin/hackage- scripts/package/Stream-0.1 I know of a few libraries that implement this package themself (Ross Paterson's arrow library and my IOSpec spring to mind). I think it's fairly uncontroversial, and would like to see it added to base (I realize that there are also proposals to clean up base to speed up the build of ghc), or at least, the standard libraries. What's the right way to accomplish this? Send a darcs patch to Ian? Submit a trac ticket? Thanks, Wouter

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Re: Optimising words
by Neil Mitchell 11 Jul '07

11 Jul '07

Hi > > Now we have eliminated an extra isSpace test per character, and as it > > happens, isSpace is very slow. > > Isn't it possible to improve isSpace, too? Yes, most definitely. But unless we improve it to the stage where it takes 0 time, its still worth eliminating useless additional calls. Thanks Neil

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Optimising words
by Neil Mitchell 10 Jul '07

10 Jul '07

Hi While benchmarking a word count program I found that it wasn't running as fast as it could. I traced this back to the original definition of words, which isn't as good as it could be: This is the version in Base: words :: String -> [String] words s = case dropWhile isSpace s of "" -> [] s' -> w : words s'' where (w, s'') = break isSpace s' We can instantiate s' with s:ss, since we already pattern match: words :: String -> [String] words s = case dropWhile isSpace s of "" -> [] s:ss -> w : words s'' where (w, s'') = break isSpace (s:ss) Now we know that s is not a space, since if it was the dropWhile would have removed it. This means that we don't need to test it, and can put it straight on to w: words :: String -> [String] words s = case dropWhile isSpace s of "" -> [] s:ss -> (s:w) : words s'' where (w, s'') = break isSpace ss Now we have eliminated an extra isSpace test per character, and as it happens, isSpace is very slow. Is my reasoning correct? If so, can we make this optimisation? Thanks Neil

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Read TypeRep? standard for typeRepKey?
by Claus Reinke 10 Jul '07

10 Jul '07

is there a reason why there is a Show TypeRep, but no Read TypeRep? and isn't it time to standardise typeRepKeys? something based on package name/version, module, name, so that one could rely on being able to read in what one writes out, and on TypeRep-based maps being compatible accross programs (at least as long as the versions match)? i was thinking about improving the reading of existentially typed data (see haskell-cafe thread) of that name, and was surprised to find that Data.Typeable doesn't quite support these things. or is there a workaround i'm missing? claus

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Applicative Identity
by Dan Doel 07 Jul '07

07 Jul '07

Hello, I ran into a tiny snag recently, attempting to write a library for a monad + transformer. If one uses MTL, it typically nice to be able to do something like the following: newtype FooT m a = FooT { runFooT :: ... } ... instances for FooT newtype Foo a = Foo { unFoo :: FooT Identity a } deriving (Functor, Monad, ...) Since the normal monad is just the case of the transformer parameterized by the Identity monad. However, we now have the Control.Applicative library, which sits in between Functors and Monads, so it seems reasonable to provide instances of those, as well. However, Identity (and, for that matter, all of the monads in MTL, I think) lacks an appropriate instance for Applicative, so generalized deriving can't be used there. On the other side, Control.Applicative has a newtype WrappedMonad, which provides such instances for an arbitrary monad. However, it has the reverse problem: There are no Monad (and related) instances for WrappedMonad. So, it seems that the full gamut of instances can't be automatically derived in this way, as the libraries currently stand. Now, this clearly isn't a dire situation, but it might be nice to fill in one or both of these gaps so the libraries play a little nicer with each other. For instance, I think the Identity instance is as simple as: instance Applicative Identity where pure = Identity f <*> a = Identity $ runIdentity f (runIdentity a) And, of course, in general, one can fill in the MTL instances as easily as: instance Applicative MTLType where pure = return (<*>) = ap Although it may be less trivial with transformers (though I assume Applicative a => Applicative (T a) aren't too hard). Many thanks, Dan Doel

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Proposal #1464: add dropPrefix to Data.List
by Ian Lynagh 07 Jul '07

07 Jul '07

Hi all, I seem to have a copy of this function in everything I write sooner or later, so I'd like to propose its addition to Data.List. It strips a prefix from a list and, if successful, returns the tail. I often use it with pattern guards: foo :: String -> IO () foo x | Just y <- dropPrefix "foo=" = putStrLn ("foo is " ++ show y) foo _ = putStrLn "Unknown" but you can of course achieve the same by using case etc. The definition is: dropPrefix :: Eq a => [a] -> [a] -> Maybe [a] dropPrefix [] ys = Just ys dropPrefix (x:xs) (y:ys) | x == y = dropPrefix xs ys dropPrefix _ _ = Nothing Let's try 11 July for a discussion deadline. Thanks Ian

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