September 2010 - Beginners

Debugging overlapping instances (mtl vs transformers)
by Paolo Losi 10 Sep '10

10 Sep '10

Hi all, I'm definitely stuck with the migration of my project from mtl to transformers. The use of transformer is mandated by a new dependency that I need to add. Overlapping instances for Monad (Either String) arising from a use of `tstampToForecasts' at ThreeLab/Gfs.hs:94:32-55 Matching instances: instance (Control.Monad.Trans.Error.Error e) => Monad (Either e) -- Defined in Control.Monad.Trans.Error instance Monad (Either e) -- Defined in mtl-1.1.1.0:Control.Monad.Error I really have no clue of who and why is bringing mtl instance in the scope. If I compile the file directly with: ghc -package monads-tf -package transformers -hide-package mtl ThreeLab/Gfs.hs -c I can successfully compile the module, so it seems the problem is not specific to that module. But the compilation fails if I do ghc -package monads-tf -package transformers -hide-package mtl --make Main.hs What's the best practice to debug this problem? What are the constraints of indirectly depending on both mtl and transformers? I'm really stuck :-/ Thanks Paolo

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specify type for a function in GHCi
by Henry Olders 10 Sep '10

10 Sep '10

Is there a way to specify the type for a function inside GHCi, that is, without creating a separate file and loading it in? Henry

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List Manipulation
by Lorenzo Isella 10 Sep '10

10 Sep '10

Dear All, Thanks to the help I got on the mailing list, I was able to load a table of integers as a list where every entry is a list of integers. To fix the ideas consider main :: IO () main = do txt <- readFile "mydata.dat" let dat :: [[Integer]] dat = convert txt print dat -- this prints out my chunk of data convert x = (map (map read . words) . lines) x and the file mydata.txt is given by 1246191122 1336 1337 1246191142 1336 1337 1246191162 1336 1337 1246191182 1336 1337 1246191202 1336 1337 1246191222 1336 1337 1246191242 1336 1337 1246191262 1336 1337 1246191282 1336 1337 1246191302 1336 1337 1246191322 1336 1337 1246191342 1336 1337 1246191362 1336 1337 1246191382 1336 1337 1246191402 1336 1337 1246191422 1336 1337 Up to some extent, I am done (later on I found out that reading this into a matrix for hmatrix is a one-liner), but I would like to achieve more just performing standard list manipulations. For instance, since my data files are always this simple (integers/real arrays of numbers) (1) How can I tell the length of every sublist in my list? (2) How can I e.g. stitch together the 1st element in every sublist in order to get the 1st column of the original data file? Many thanks Lorenzo

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Convert String to List/Array of Numbers
by Lorenzo Isella 09 Sep '10

09 Sep '10

Dear All, I must be stuck on something pretty basic (I am struggling badly with I/O). Let us assume you have a rather simple file mydata.dat (3 columns of integer numbers), see below. 1246191122 1336 1337 1246191142 1336 1337 1246191162 1336 1337 1246191182 1336 1337 1246191202 1336 1337 1246191222 1336 1337 1246191242 1336 1337 1246191262 1336 1337 1246191282 1336 1337 1246191302 1336 1337 1246191322 1336 1337 1246191342 1336 1337 1246191362 1336 1337 1246191382 1336 1337 1246191402 1336 1337 1246191422 1336 1337 Now, my intended pipeline could be read file as string--> convert to list of integers-->pass it to hmatrix (or try to convert it into a matrix/array). Leaving aside the last step, I can easily do something like let dat=readFile "mydata.dat" in the interactive shell and get a string, but I am having problems in converting this to a list or anything more manageable (where every entry is an integer number i.e. something which can be summed, subtracted etc...). Ideally even a list where every entry is a row (a list in itself) would do. I found online this suggestion http://bit.ly/9jv1WG but I am not sure if it really applies to this case. Many thanks Lorenzo

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Convert String to List/Array of Numbers
by Rafael Gustavo da Cunha Pereira Pinto 09 Sep '10

09 Sep '10

You should use: convert = map (map read . words) . lines Or: convert x = (map (map read . words) . lines) x Or, yet: convert x = map (map read . words) . lines $ x The function application occurs before the dot composition! On Wed, Sep 8, 2010 at 14:24, Lorenzo Isella <lorenzo.isella(a)gmail.com>wrote: > Hi Daniel, > Thanks for your help. > I have a couple of questions left > (1) The first one is quite down to earth. > The snippet below > > --------------------------------------------------- > main :: IO () > > main = do > txt <- readFile "mydata.dat" > > let dat = convert txt > > print dat -- this prints out my chunk of data > > return () > > convert x = lines x > > ----------------------------------------------- > > pretty much does what it is supposed to do, but if I use this definition of > convert x > > convert x = map (map read . words) . lines x > > I bump into compilation errors. Is that the way I am supposed to deal with > your function? > > (2) This is a bit more about I/O in general. I start an action with "do" to > read some files and I define outside the action some functions which are > supposed to operate (within the do action) on the read data. > Is this the way it always has to be? I read something about monads but did > not get very far (and hope that they are not badly needed for simple I/O). > Is there a way in Haskell to have the action return to the outside world > e.g. the value of dat and then work with it elsewhere? > That is what I would do in Python or R, but I think I understood that > Haskell's philosophy is different... > Am I on the right track here? And what is the benefit of this? > > Cheers > > Lorenzo > > > On 09/08/2010 04:06 PM, Daniel Fischer wrote: > >> On Wednesday 08 September 2010 15:31:19, Lorenzo Isella wrote: >> >>> Dear All, >>> I must be stuck on something pretty basic (I am struggling badly with >>> I/O). Let us assume you have a rather simple file mydata.dat (3 columns >>> of integer numbers), see below. >>> >>> >>> >>> 1246191122 1336 1337 >>> 1246191142 1336 1337 >>> 1246191162 1336 1337 >>> 1246191182 1336 1337 >>> 1246191202 1336 1337 >>> 1246191222 1336 1337 >>> 1246191242 1336 1337 >>> 1246191262 1336 1337 >>> 1246191282 1336 1337 >>> 1246191302 1336 1337 >>> 1246191322 1336 1337 >>> 1246191342 1336 1337 >>> 1246191362 1336 1337 >>> 1246191382 1336 1337 >>> 1246191402 1336 1337 >>> 1246191422 1336 1337 >>> >>> Now, my intended pipeline could be >>> >>> read file as string--> convert to list of integers-->pass it to hmatrix >>> (or try to convert it into a matrix/array). >>> Leaving aside the last step, I can easily do something like >>> >>> let dat=readFile "mydata.dat" >>> >>> >>> in the interactive shell and get a string, >>> >> >> Not quite. `dat' is the IO-action that reads the file, of type (IO String) >> and not a String. >> In a programme, you'd do something like >> >> main = do >> ... -- argument parsing perhaps >> txt<- readFile "mydata.dat" >> let dat = convert txt >> doSomething with dat >> >> but I am having problems in >>> converting this to a list or anything more manageable (where every entry >>> is an integer number i.e. something which can be summed, subtracted >>> etc...). Ideally even a list where every entry is a row (a list in >>> itself) would do. >>> >> >> Depending on what the reult type should be, different solutions are >> required. >> The simplest solutions for such a file format are built from >> >> read -- to convert e.g. "135" to 135 >> lines :: String -> [String] >> words :: String -> [String] >> map :: (a -> b) -> [a] -> [b] >> >> If you want a flat list of Integers from that file, >> >> convert = map read . words >> >> will do. First, `words' splits the String on whitespace (spaces and >> newlines), producing a list of digit-strings, those are then read as >> Integers. >> >> If you want a list of lists, each line its own list inside the top level >> list, >> >> convert = map (map read . words) . lines >> >> is what you want. >> >> If you want to convert each line into a different data structure, say >> (Integer, Double, Int64), the general form would still be >> >> convert = map parseLine . lines >> >> and parseLine would depend on the structure you want. For the above, >> >> parseLine str >> = case words str of >> (a : b : c : _) -> (read a, read b, read c) >> _ -> error "Bad line format" >> >> would be a solution. >> >> For any but the simplest formats, you should write a real parser to deal >> with possible bad formatting though (writing parsers is fun in Haskell). >> >> I found online this suggestion >>> http://bit.ly/9jv1WG >>> but I am not sure if it really applies to this case. >>> Many thanks >>> >>> Lorenzo >>> >> >> > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://www.haskell.org/mailman/listinfo/beginners > -- Rafael Gustavo da Cunha Pereira Pinto -- Rafael Gustavo da Cunha Pereira Pinto

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[OT] To Mock A Mockingbird
by Patrick LeBoutillier 08 Sep '10

08 Sep '10

Hi all, I've been reading the book "To Mock A Mockingbird" by Robert Smullyan recently and I don't get the part about the birds at all. I think I understand that a bird is a function, but what would it's type be in Haskell? It says that birds hear a call and answer by another call, so I thought String -> String or something like that. But if you have a mockingbird that can answer what a bird would answer to itself, that means that it's input must contain the other bird... Do all the birds have the same type? Thanks, Patrick -- ===================== Patrick LeBoutillier Rosemère, Québec, Canada

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Setting Default Integer and Real Types
by Lorenzo Isella 08 Sep '10

08 Sep '10

Dear All, I am quite new to Haskell and planning to see where it will lead me in scientific computing (I will be looking into hmatrix soon). Unless there are real memory problems, I would like to make sure that all real numbers are Double and all integer numbers are Integer types. Now, I understand that Haskell in most of the cases is smart enough to infer the type of a variable (integer, real, etc...), but can I also set its default precision once for all? I.e. if I say a=5.2 I want a to be used as a double precision real everywhere in the code. Cheers Lorenzo

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Mixing IO and other monads
by Brian Victor 08 Sep '10

08 Sep '10

I've managed to create the sort of nested case code that all the monad tutorials warn you against. I haven't found a way to work myself out of it, though. If I've understood monad transformers conceptually, this seems like the sort of thing they'd be good for, but I don't know how to actually make what I need. Here's the code I have. getDefaultInputDeviceID and getDefaultOutputDeviceID are from PortMidi, and return IO (Maybe Int). openInput and openOutput return IO (Either PMStream PMError). At this point, I'd be happy to get the second half of the function refactored into monads. main :: IO () main = do initialize ideviceId <- getDefaultInputDeviceID case ideviceId of Nothing -> putStrLn "No default input device" Just inputDeviceId -> do odeviceId <- getDefaultOutputDeviceID case odeviceId of Nothing -> putStrLn "No default output device" Just outputDeviceId -> do openInputResult <- openInput inputDeviceId case openInputResult of Right err -> putStrLn $ show err Left inputStream -> do openOutputResult <- openOutput outputDeviceId 0 case openOutputResult of Right err -> putStrLn $ show err Left outputStream -> runTranslationLoop inputStream outputStream It seems like I ought to be able to change the second half of the function to something like this: openStreamsAndLoop :: (Num a) => a -> a -> IO (Maybe err) openStreamsAndLoop inputDeviceId outputDeviceId = do inputStream <- openInput inputDeviceId outputStream <- openOutput outputDeviceId 0 runTranslationLoop inputStream outputStream return Nothing But how do I create an IO (Maybe err) monad? -- Brian

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More De/Serialisation Woes
by Tom Hobbs 07 Sep '10

07 Sep '10

Hi all, Can anyone help me with some Haskell de/serialisation woes I'm encountering? What I'm trying to do it implement a proprietary network protocol which will enable me to talk to a network Java/Tuple Space. Specifically, I'm trying to serialise a "TypeStructurePreamble" and send that down the Handle (my network stream), I'm then expecting the Space to send me a TypeStructurePreamble back again, but with different data. So I need to de/serialise this type. I've not got all the code together yet, because I'm trying to figure out the concepts rather than any particular coding problem. Here's some code I have so far; import System.IO (hGetLine,hClose,hPutStrLn,hSetBuffering,BufferMode(..),Handle,stdout) import Data.Binary.Get import qualified Data.ByteString.Lazy as L type TypeStructurePreamble = (String,Int,[String]) -- This function I know works because it was spoon-fed to me by someone on this list! :-) readStrings :: Int -> Get [String] readStrings n = replicateM n $ do len <- getWord32be name <- getByteString $ fromIntegral len return $ UTF.toString name deserialisePreamble :: Get(String, Int, [String]) deserialisePreamble = do len <- getWord32be typename <- getByteString $ fromIntegral len c <- getWord32be numFields <- getWord32be fields <- readStrings (fromIntegral numfields) return (typename,c,numFields) -- not sure what this type should be serialisePreamble :: Handle -> TypeStructurePreamble -> ?? serialisePreamble h (n,c,fs) = do L.hPut h (encode (0xFAB1000A :: Word32)) L.hPut h (encode (length n)) L.hPut h (encode n) L.hPut h (encode (c :: Word32)) L.hPut h (encode (length fs)) -- not sure how I would L.hPut out every String in fs It is my (maybe wrong) understanding that these de/serialise functions don't actually do the work, they basically just form a description of what must be done in order to get the work done, so I'm slightly confused on how I should use it. For example; main = do -- set up Handle called h here, I know how to do this serialisePreamble h ("MyType",0,["field1","field2"]) --now I'm expecting to be able to read a modified TypeStructurePreamble from h (n,c,fs) <- deserialisePreamble -- but according to the type of deserialisePreamble, how does the h get into it? print "c was "+c -- can I just slot sout style statements into here? evaluate (n,c,fs) hClose h return (n,c,fs) Does this make sense? Can someone help me fill in the blanks of serialisePreamble and main? I've read the tutorials on the Get monad which google turns up, am I missing some really good ones? Is there some other resource which I'm missing? I'm been thrown a lot of the Hackage documentation on these things but (assuming I'm looking in the right place) they seem to work more like JavaDocs and give the "what it does", rather than the "how it should be used". Many thanks, Tom

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containers and extensibility, typeclasses vs. multiple value constructors
by Greg 07 Sep '10

07 Sep '10

I think I'm starting to get a sense for this, but want to check my understanding... I'm writing a program that plots data. I want to be able to have multiple plots simultaneously-- some are scatter plots, some are lines, some are functions, etc. I think there are two choices in how to define these different plot styles: I can have one type with constructors for scatter, line, function, etc; or I can have independent types for each plot style and a typeclass to define the shared functionality. I want to be able to treat these plots abstractly within the program: I don't care what style of plot it is at certain levels of processing, I just know I want to render it. This suggests a typeclass handling, where the typeclass has a render method. However, I also want to maintain a list of these plots so I can 'mapM_ render plotlist'. I believe I can only create a list of plots if they're all the same type, therefore they must be multiple constructors for the same type. 'render' then will need to be handled by pattern matching. The last piece of this, is that it would be nice to be able to add new plot styles later. Ideally this will all become a library for me, and I'd like to be able to add a new plot style without having to modify the existing code. I think the only way to do this is through the typeclass mechanism, where I can create the new type and create a class instance in my local code. So, am I stuck with this tradeoff? I can either put my plots in a container using one type and multiple constructors, or I can make the system extensible using typeclasses? Thanks-- Greg

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