June 2010 - Beginners

database: haskelldb, hdbc, hsql
by prad 28 Jun '10

28 Jun '10

after spending some time looking into db access through haskell, it seems there are these 3. realworld haskell showed be how to use hdbc and sqlite3 (and i almost got things working there) can't figure out what hsql is yet though i suspect it is similar to hdbc. haskelldb looks like it lets you code in something that doesn't resemble sql. it also seems to sit on top of the other two. so my question is which one should i persue and why? most of my stuff will be done on postgresql (though i'm not averse to using sqlite3 for smaller things). -- In friendship, prad ... with you on your journey Towards Freedom http://www.towardsfreedom.com (website) Information, Inspiration, Imagination - truly a site for soaring I's

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Category Theory
by Matt Andrew 28 Jun '10

28 Jun '10

Hi all, I have spend some time over the last couple of days trying to get my head around category theory, in order to understand concepts like monads a little better, in order to understand Haskell a little better =) I have grasped, at least to a degree, the basic concepts of 'category,' 'functor' and 'natural transformation' but I am still fuzzy on a few specifics and had a couple of questions. I'm doing this for self-study and have no one to ask these questions of and so thought I'd ask them here. My first question is this: where are functors understood to 'live,' or exist? They operate on categories, are they therefore understood to exist outside of the categories they operate on? (I understand that functors can form a category themselves, but I'm asking this question in relation to the categories they operate on. Are they inside or outside of them?) In other words when I define a functor in Haskell, are the pair of functions 'fmap,' and whatever unary type constructor is defined along with it, then part of the morphisms of the Haskell category? Or are they outside of the Haskell category (where the Haskell category I've seen defined in the articles I've read is one where its objects are types and its morphisms functions on those types)? This leads me to my next question. It seems that all functors in Haskell (or at least all members of the functor typeclass) are covalent endofunctors. Does this mean that whenever a new functor is defined in Haskell, that the Haskell category (H) grows to accomodate the effects of that functor? In other words, for any covalent endofunctor F : H -> H, do H's morphisms then grow to include F(f) and objects to include F(a), where a is any object in H and f is any morphism in H? I have been working off the assumption that the answer to my last question is 'yes.' The reason for this is natural transformations. Where functors seem to be able to be 'outside' of the categories they operate on (whether they are in fact 'outside' or not is my first question), natural transformations appear to have to be 'inside.' My understanding of the theory is that a natural transformation is a collection of morphisms in the target category of two parallel functors (where such morphisms operate on the functors and meet some specific properties). Surely the only way such a collection of morphisms would make sense would be if the objects they map to/from (i.e. the structures imposed by the two functors) were part of that category also? I hope these questions make sense. I really appreciate anyone who takes to time to read this! Matt

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cabal on windows 2000
by corentin.dupont＠ext.mpsa.com 28 Jun '10

28 Jun '10

Hello, cabal from the new haskell platform fails on windows 2000: http://trac.haskell.org/haskell-platform/ticket/101 Do you know of a workaround? Downgrade Cabal? Best Corentin

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how to update cabal?
by Rohit M. 28 Jun '10

28 Jun '10

I just installed the .dmg fore the Haskell platform. When I run "cabal update" in terminal I get the following: Note: there is a new version of cabal-install available. To upgrade, run: cabal install cabal-install I then ran "cabal -V": cabal-install version 0.8.0 using version 1.8.0.2 of the Cabal library The latest version of cabal install is 0.8.2. When I run "cabal install cabal-install" it seems to work fine, without any errors but when I go back and check i.e. "cabal -V" i still get version 0.8.0. I tried "cabal list cabal-install": Synopsis: The command-line interface for Cabal and Hackage. Latest version available: 0.8.2 Latest version installed: [ Not installed ] Homepage: http://www.haskell.org/cabal/ License: BSD3 What am I missing? It is really annoying that cabal update keeps telling me about a new version of cabal-install that I can't actually get to work! Thanks Rohit

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maybe use for functors or arrows?
by Michael Mossey 27 Jun '10

27 Jun '10

Is there a way to write the function process :: [(Location,Item)] -> [(Location,ValuableItem)] given a function indicating which Item's to keep transform :: Item -> Maybe ValuableItem using functors and arrows? The value for location stays with any item that is kept. What I have is process inp = catMaybes (map g inp) where g (l,i) = case transform i of Nothing -> Nothing Just v -> Just (l,v) This looks like an arrow situation to me because you want to make a function that acts on the second value in a tuple, and a little bit like a Maybe functor. Thanks, Mike

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System.Process.createProcess does not terminate
by John Obbele 27 Jun '10

27 Jun '10

Hello everyones ! I was playing with some test frameworks recently and came across what looks like a pseudo-fork bomb. After digging things a little, it seems I am messing with the System.Process.createProcess function : none of my spawned process terminates and eventually my OS freezes. (trivia: well now, I know that Fedora cannot handle more than 1100 processes on my cheap laptop ^^) I've attached a small haskell file which can easily reproduces my problem. If you, carefully, run it from the GHCi prompt while monitoring your pool of processes, you will probably see a hundred of zombie processes appearing. Has anyone got some clues on what's wrong with my function ? Is it related to some strange lazy behavior or should I simply manage each spawned process myself and terminate it when needed ? regards, /John P.S.: speaking of test frameworks, I am desperate to find good examples or documentations on how to use QuickCheck2. Where could I get such resources ?

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type def messes up
by prad 26 Jun '10

26 Jun '10

sometimes i have copied a typedef from a tutorial or demo and get errors. i take the typedef out and the program runs fine. i don't think the problem lies with the person creating the typedef because they are very experienced. is it possible that the matter has to do with the interpreter eg hugs vs ghci and such? or possibly an update of the syntax over time? for instance, i'm going through rex page's "two dozen short lessons in haskell" from uni of oklahoma. they use hugs and the error message for reverse 'x' is completely different from what i get using ghci. -- In friendship, prad ... with you on your journey Towards Freedom http://www.towardsfreedom.com (website) Information, Inspiration, Imagination - truly a site for soaring I's

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a problem
by Michael Mossey 25 Jun '10

25 Jun '10

Can I get a suggestion for a concise way to write 'sortOutMusicData' as described here? This is MusicXML-related. data Music_Data_ = Music_Data_1 Note | Music_Data_4 Direction | Music_Data_9 Sound ... sortOutMusicData :: [Music_Data_] -> ([Note],[Direction],[Sound])

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Compiling
by Chris Saunders 25 Jun '10

25 Jun '10

I have just started trying to learn Haskell again (I seem to have a mental block against this language). Right now I'm using Haskell in 5 steps. I have created the file hello.hs exactly as in the section "Write your first Haskell program". I then opened a cmd (I'm using Windows) window in the directory where I created hello.hs and entered "ghc -o hello hello.hs". What follows is the output of the cmd window: hello.hs:1:7: Not in scope: data constructor `PutStrLn' Can someone tell me why I am having this problem? Regards Chris Saunders

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get date
by Luca Ciciriello 24 Jun '10

24 Jun '10

How can I get the current system date-time in Haskell? I've used the library Time, but on MacOS X 10.6.4 the year is always 0. Luca.

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