July 2006 - Haskell-Cafe

Still not dead
by Einar Karttunen 21 Jul '06

21 Jul '06

Hello As many of you may have noticed I have been away for some months. This has been due to health problems which have unfortunately kept me unable to work on Haskell projects. I am not dead and will be working on resolving the backlog of messages (will probably take a week). I will be slowly back to hacking things when I get everything fixed. - Einar Karttunen

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seq - RTFM?
by Dusan Kolar 21 Jul '06

21 Jul '06

Hello all, my question is probably dull. So answers to better investigate manual are welcome. Why is this correct? ___ ___ _ / _ \ /\ /\/ __(_) / /_\// /_/ / / | | GHC Interactive, version 6.4.1, for Haskell 98. / /_\\/ __ / /___| | http://www.haskell.org/ghc/ \____/\/ /_/\____/|_| Type :? for help. Loading package base-1.0 ... linking ... done. Prelude> putStr "Ahoj\n" Ahoj Prelude> putStr "Ahoj\n" `seq` 3+3 6 Prelude> :q Leaving GHCi. And not Prelude> putStr "Ahoj\n" Ahoj Prelude> putStr "Ahoj\n" `seq` 3+3 Ahoj 6 ??? Does it have something common with monads or is it a behavior of seq? Thanks, Dusan

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Re: [Haskell-cafe] REALLY simple STRef examples
by Chad Scherrer 21 Jul '06

21 Jul '06

Hi, > The short answer: use > runST (long expression) > rather than > runST $ long expression > > when it comes to higher-ranked functions such as runST. I suppose the same holds for runSTUArray, right? But this still gives me that same error, about being less polymorphic than expected. (+=) x y = let updateX i = do xi <- readArray x i writeArray x i (xi + y!i) in sequence_ . map updateX $ indices x sumArrays [] = error "Can't apply sumArrays to an empty list" sumArrays (x:xs) = runSTUArray result where result = do x0 <- thaw x mapM_ (x0 +=) xs x0 -- Chad Scherrer "Time flies like an arrow; fruit flies like a banana" -- Groucho Marx

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Re: Why is there no splitBy in the list module?
by dons＠cse.unsw.edu.au 21 Jul '06

21 Jul '06

marco-oweber: > There is already lines. > Why not generalise it to take an additional parameter '\n' and call it > split or splitBy? > There are some cases where you want to split a list not on '\n'. This comes up a lot. I thought at some time last year there'd been a long discussion about all the various flavours of 'split' that we could come up with. But then, what happened to the code? Did we not agree on a version to go into Data.List? Hacking up your own custom split (or a tokens/splitOnGlue) must be one of the most common questions from beginners on the irc channel. Even fps has a split/splitBy: http://www.cse.unsw.edu.au/~dons/fps/Data-ByteString.html#v%3Asplit http://www.cse.unsw.edu.au/~dons/fps/Data-ByteString.html#v%3AsplitWith http://www.cse.unsw.edu.au/~dons/fps/Data-ByteString.html#v%3Ajoin Anyone rememeber what the result of the "let's get split into the base library" movement's work was? -- Don (Moved to haskell-cafe@)

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Re: rigid variables
by Rodney D Price 20 Jul '06

20 Jul '06

Thanks for the responses. The semantics of check as described the "Data Invariants" paper on STM are really close to what I am looking for. Unfortunately, that work is not in GHC yet, so I'm attempting my own poor substitute (below). I have a store that is to be accessed by other threads, and I want to throw an error under certain conditions. (The names below are borrowed from the "Scheme in 48 Hours" tutorial.) In reality I'm looking for an inconsistent store, but for simplicity's sake I've just put in a standard read operation below. Is there an easier way? Thanks, -Rod -- module Store where import Control.Monad.Error import Control.Concurrent.STM data StoreError = DoesNotExist String | Default String instance Error StoreError where noMsg = Default "Store error" strMsg = Default type ThrowsError = Either StoreError type IOThrowsError = ErrorT StoreError IO type Store a = TVar [ (String, TVar a) ] -- read from the store getSTM :: String -> Store a -> STM (ThrowsError a) getSTM name store = do st <- readTVar store case lookup name st of Nothing -> return (Left $ DoesNotExist name) Just rd -> readTVar rd >>= return . Right get :: String -> Store a -> IOThrowsError a get name store = do d <- liftIO $ atomically $ getSTM name store case d of Left err -> throwError err Right d' -> return d'

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doubt on gui
by Alvaro Galan 20 Jul '06

20 Jul '06

Hi, im almost new in haskell world, but im trying to do a simple graphical interface for a small program, i developed the program under winhugs, and now i want to develop the gui also with it, but all the libraries and kits that i download from the haskell page is either for ghc or need to be make or stuff, my question, is or isnt a library or something that i can download, then import and use it on winhugs without compile, download a ton of things, etc etc? is there some way of developing a very easy gui (based on radio buttons) with normal librarys on winhugs?, or its compulsory get ghc? Kind Regards. Alvaro. -- It's only Rock'n'Roll (..but I like it)

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Re: [Haskell] rigid variables
by Robert Dockins 20 Jul '06

20 Jul '06

[moved to cafe] On Jul 20, 2006, at 12:48 PM, Rodney D Price wrote: > I've gotten this sort of error several times, which mysteriously > disappears > when I add more functions to the code: > > storeError.hs:13:38: > Couldn't match expected type `a' (a rigid variable) > against inferred type `String' > `a' is bound by the type signature for `throwError' > at <no location info> > Expected type: a > Inferred type: String > In the first argument of `return', namely `msg' > In the call (return msg) > > (This is GHCi.) The code is below. The type variable a can't be > bound to > String, obviously, but a relative novice like myself has no idea > why. Can > someone tell me? > > Thanks, > > -Rod > > -- > module Store where > > import Control.Monad.Error > import Control.Concurrent.STM > > data StoreError = Default String > > instance Error StoreError where > noMsg = Default "Store error" > strMsg = Default > > instance MonadError StoreError STM where > throwError (Default msg) = return msg Lets take a look here at the definition of MonadError from Control.Monad.Error: class Monad m => MonadError e m | m -> e where throwError :: e -> m a catchError :: m a -> (e -> m a) -> m a In the signature for 'throwError' there are three type variables: e, m and a. e and m are bound by the instance declaration, but a is free. In Haskell the rule is that free variables are implicitly bound with a universal quantifier. So, the type for throwError can be regarded as, throwError :: forall a. e -> m a, for some concrete choices of e and m which are determined by the instance. The forall means that a user of this function can put any type there that he likes. In other words, the monadic action created by 'throwError', when executed, evaluates to _a value of any type at all_ (yes, I know the terminology is a little loose here). That means you can't just 'return msg', which has type 'm String' because a user might have used throwError to create an action of type 'm Int', for example. In fact, you won't really be able to return anything at all, because there isn't any way to write a program that can generate a value of any unknown type. This should hopefully correspond to your intuition about what throwing an exception does. The error generated by the typechecker basically tells you that the function you have written is not polymorphic enough. It has type 'StoreError -> STM String' rather than 'forall a. StoreError -> STM a' as it ought. I think perhaps you have misunderstood how MonadError is used. The idea is to expose to users a particular non-local control flow construct (throw/catch style exceptions) by hiding all the stuff necessary for that inside the monad plumbing. Usually, whoever writes the monad itself will provide the necessary instances. It's often not possible to write instances like this by using the external API of the monad. This is particularly the case for the abstract monads available in GHC (IO, ST, and STM). In short, I don't think you'll be successful in writing a 'MonadError' instance for STM that has the customary semantics. What you may be looking for is the ErrorT monad transformer, which will let you layer error handling over STM. It's hard to know with what info you've provided here. If you give a few more details on what you're trying to accomplish, someone may be able to give you a push in the correct direction. Rob Dockins Speak softly and drive a Sherman tank. Laugh hard; it's a long way to the bank. -- TMBG

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Re: [Haskell-cafe] REALLY simple STRef examples
by Chad Scherrer 20 Jul '06

20 Jul '06

Whoa. That changes everything I thought I knew about ($). Come to think of it, one of the examples that does work it written main = print $ runST f where f is defined separtely. So that's consistent. I'll take a look at the references. Thanks! > > Indeed. The short answer: use > runST (long expression) > rather than > runST $ long expression > > when it comes to higher-ranked functions such as runST. > A longer answer: > http://www.haskell.org/pipermail/haskell-cafe/2004-December/008062.html > > > let me know what i need to read to fix it myself > MLF (see Daan Leijen, A. Loeh, `Qualified types for MLF', ICFP05) > >

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trace function
by Alexander Vodomerov 20 Jul '06

20 Jul '06

Hello! The function trace is supposed to write debug messages to console. However, when I trying to run the following program import Debug.Trace main = do putStrLn "xxx" return (trace "yyy" ()) putStrLn "zzz" only xxx and zzz is displayed. yyy is missing. Why trace is not working? PS. GHC-6.4.2, Debian Linux on x86. With best regards, Alexander.

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Opening a file that another process is writing
by Maurício 20 Jul '06

20 Jul '06

Hi, I want to open for reading a log file that another process is locking for write. I know it's possible, since 'cat' and 'vim' can read that file (but not edit it, of course). How can I do that in Haskell? 'openFile' says "permission denied". Thanks, Maurício

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