- Beginners - Haskell.org

how to feel free of the Implementation of Data structure
by 吴兴博 25 Mar '11

25 Mar '11

1. To use the Data.List is so easy as it's in the 'prelude'. but when I want to get some elements by index, I need to use Array, or sometime I want to use ByteString for some good reason. We all know It's some 'linear ordered things'. Is there some method for me to use just one form of 'list' with every possible function on it. 2. f1 :: (Integral a) => a -> String f2 :: (Integral a) => String -> a pipe = f1 . f2 It is clear that pipe is String -> String, but.(1)How can I know what instance that 'a' is. (2)How can I determine what instance of 'a' is. -- ---------------- 吴兴博

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:kind (->) - What do the question marks imply?
by Amitava Shee 24 Mar '11

24 Mar '11

Hello Haskell gurus, I am trying to understand "kind" Prelude> :k Maybe Maybe :: * -> * Prelude> :k (->) (->) :: ?? -> ? -> * What are the question marks? Why are they different from *? How is this related to rank-2 polymorphism? Thanks in advance, Regards, Amitava Shee

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Exporting constructors
by Patrick LeBoutillier 24 Mar '11

24 Mar '11

Hi all, Is it possible to export constructors from a module in a way that they can be used for pattern matching but not for creating types? Patrick -- ===================== Patrick LeBoutillier Rosemère, Québec, Canada

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cabal dependencies question
by Rob Nikander 23 Mar '11

23 Mar '11

Hi, I was unable to normally install the "mime" package -- I got an error that it needed "base <= 4". I have 4.3.1.0. So I ran `cabal unpack mime`, edited mime.cabal to say "base <= 4.3.1.0" and from that file's directory ran `cabal install'. It appeared to install it okay. But when I try to install another package that needs mime, I get the same kind of error: > cabal install http-server Resolving dependencies... cabal: cannot configure mime-0.3.2. It requires base >=3 && <=4 For the dependency on base >=3 && <=4 there are these packages: base-3.0.3.1 and base-3.0.3.2. However none of them are available. base-3.0.3.1 was excluded because base-4.3.1.0 was selected instead I'm wondering why it thinks the mime package wants base <= 4 when I just installed with a .cabal file that says base <= 4.3.1.0. ?? thanks, Rob

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How to structure your Haskell installation?
by frode k 23 Mar '11

23 Mar '11

Hi. I'm installing the Haskell Platform om my Linux box and need some tips on how to structure the files on the OS. I need to install and use both GHC 6.12.3 and GHC 7.0.2. Hence I want to separate these installations. I've setup two directories, one for each platform (related to the GHC): # ls -d -1 $PWD/*.* /usr/haskell/2010.2.0.0 /usr/haskell/2011.2.0.0 I've installed GHC-6.12.3 in "2010.2.0.0" and will install GHC-7.0.2 in "2011.2.0.0". My question is more related to where I should install the packages. When I install a package using "runhaskell Setup ..." I assumed this to ONLY be installed for the version of GHC I'm using when installing it. Am I right about this? Having $PATH pointing to GHC-6.12.3 I've done this when installing packages, to add them to "2010.2.0.0": - - - - - - - cd /usr/src wget http://hackage.haskell.org/packages/archive/transformers/0.2.2.0/transforme… tar xzf transformers-0.2.2.0.tar.gz cd transformers-0.2.2.0 runhaskell Setup configure --prefix=/usr/haskell/2010.2.0.0/ runhaskell Setup build runhaskell Setup install - - - - - - - Later when I switch to GHC-7.0.2 I plan to install the packages into the "2011.2.0.0" directory in stead. Is this a good way to structure several Haskell installations on the same system? Is there a "best practice" on how to structure the files? I want to get this right from the start, to avoid having to go back and re-arrange everything. Regards, Frode K [k]

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Re: [Haskell-beginners] Recursion in monad
by Adrian May 23 Mar '11

23 Mar '11

OK I did this: import System.Random walk :: Int -> IO Int walk i = randomRIO (0,1) >>= \r -> return (i+r*2-1) say :: Int -> IO () say i = putStrLn $ show i rep :: Int -> a -> (a -> IO a) -> (a -> IO ()) -> IO () rep n i w s | n<=0 = return () | otherwise = s i >> w i >>= \ii -> rep (n-1) ii w s main :: IO () main = rep 10 50 walk say Is that the easiest way? Adrian.

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small expression evaluator
by Petr Novotnik 23 Mar '11

23 Mar '11

Hello, on my journey through Haskell I've hit a problem where I don't know any further and would greatly appreciate your help. I've got this code so far: data Value a e = VConst a | VFunc (e -> a) evalV :: Value a e -> e -> a evalV (VConst x) = const x evalV (VFunc f) = f liftV :: (a -> a -> t) -> (Value a e) -> (Value a e) -> e -> t liftV f x y e = (evalV x e) `f` (evalV y e) (.==.), (./=.) :: (Eq a) => (Value a e) -> (Value a e) -> e -> Bool (.==.) = liftV (==) (./=.) = liftV (/=) (.&&.), (.||.) .... some more operators This allows me to write client code like this: data Person = Person { personName :: String , personAge :: Int } deriving (Show) exampleExpr :: Bool exampleExpr = (VConst 99) .==. (VFunc personAge) $ Person "pete" 99 I was wondering, whether it'd be possible to enable defining expression without the Value data constructors, i.e. 99 .==. personAge $ Person "pete" 99 I've tried using a type class to have a function for lifting different types into Values. class ToValue a e | a -> e where valueLift :: a -> Value a e then I tried defining an instance for functions: instance ToValue ((->) Person a) Person where valueLift f = undefined in ghci: > :t valueLift personName valueLift personName :: Value (Person -> String) Person and suddenly I realized this will not work because I would need the type `Value String Person'. Now, I'm stuck and don't know which path to take in order to come closer to being able writing "99 .==. personAge" or "personAge .==. 99". Maybe I'm approaching it in a completely wrong way or it's simply not possible. Do you have any ideas, hints? Many thanks in advance for any kind of feedback, pete.

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"cabal install wxcore --enable-shared" Error
by Paulo Pocinho 20 Mar '11

20 Mar '11

I'm trying to build an application using "ghc -dynamic mywxapp.hs". Everything not involving wx compiles with dll linking. However, I can't build the required dynamic libraries for wxcore. Using "cabal install wxcore --enable-shared" gives the following: collect2: ld returned 1 exit status Creating library file: dist\build\libHSwxcore-0.12.1.6-ghc7.0.2.dll.a cabal: Error: some packages failed to install: wxcore-0.12.1.6 failed during the building phase. The exception was: ExitFailure 1 Using "cabal install wxcore --enable-shared --enable-split-objs" gives the following: E:\Tools\Haskell Platform\2011.2.0.0\mingw\bin\ld.exe: BFD (GNU Binutils) 2.20.51.20100613 internal error, aborting at ../../binutils-2.20.51/bfd/coffcode.h line 954 in handle_COMDAT E:\Tools\Haskell Platform\2011.2.0.0\mingw\bin\ld.exe: Please report this bug. cabal: Error: some packages failed to install: wxcore-0.12.1.6 failed during the building phase. The exception was: ExitFailure 1 Is it possible to fix?

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Re: [Haskell-beginners] Lazy vs Strict ponderings...
by sean＠objitsu.com 20 Mar '11

20 Mar '11

>> 2) Don't worry about memory leaks until they actually appear. If they >> do, use a profiling tool to find out what's going on. Most likely, one >> of the two things above happened in an interesting way. At the risk of sounding stupid in Haskell mode, how would I be aware of this other than obvious messages or seg-faults about memory getting low ? > > It is likely that, if you develop a medium to large sized application for > a customer, memory leaks will appear at the customers site. It will often > be very difficult to reproduce the situation and find the leak. That scares me enough to not want to use Haskell for serious application development. With 26 years in the trade I have had too many times when a fault cannot be reproduced and despite an 'obvious fault in the source' being fixed and the problem never coming back, without being able to reproduce and thus confirm that you have eliminated the problem, you can't ever relax on a Saturday night! LOL I hate the word 'random' when clients describe problems too! > If you > want to produce quality software, it is best to be sure what you are doing > during the whole development process. There should be a set of guidelines, > on how to prevent space leaks. > I guess "being sure..." comes with experience of the language, like any other. I can 'think' in quite a few languages with complete confidence, knowing that things will just work. I guess Haskell is a bigger elephant sandwich! There is something beautiful about its syntax and the fact that everything is (or can be) so precise and concise, sometimes a little too concise for me to read. It *makes* me want to be in that club just so I can have that level of understaning about my job. I learned BASIC and UCSD-PASCAL at age 11 and I have never stopped being fascinated about *what* computing is all about. Sometimes I think that a CPU *is* a perfectly enlightened consciousness always in the *now*. Time for my medication and music therapy..... > See also "On the reliability of programs", > http://www.cs.utexas.edu/users/EWD/transcriptions/EWD03xx/EWD303.html Will do, thanks. Sean

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SQLite3 Row <-> Data models -- Is this a typical Haskell pattern ?
by Stef T 20 Mar '11

20 Mar '11

Hey Everyone, Greetings, I am a total newb to haskell, so, I have a strange question (perhaps). Consider I have a data model which looks like this; data Brand = Brand { id :: Int, name :: String, created_at :: String, updated_at :: String } Now, I have a sqlite3 function such as checkout :: Int -> IO (Either String [[Row Value]]) checkout a = do handle <- openConnection "/Users/stef/haskell/db/development.sqlite3" execStatement handle $ "SELECT * from brands where id = " ++ show a From this, I get a Row/Tuple. So far, so good, but, the question _I_ have is, how do you go about mapping a returned row to the data model ? Is this even a design pattern that is used in FP languages ? I admit, I have spent numerous years in the MVC world, so, perhaps this is simply "not done". It would seem to be a much nicer thing to then do ; name myBrandModel Thanks for reading this far, feel free to complain about my design :D Regards S.

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