November 2011 - Beginners

guards
by kolli kolli 11 Nov '11

11 Nov '11

Hi,, I am new to use ..plz bare with me.. How do we write two conditions using guards suppose if we have a if condition if t1 t2 t3 typing :: TypingContext -> Term -> Maybe Type --the typing relation typing capGamma (Var x) = contextLookup x capGamma typing capGama Tru = Just(TypeBool) typing capGama Fls = Just(TypeBool) typing capGama Zero = Just(TypeBool) typing capGama (Succ t) = typing capGama t typing capGama (Pred t) = typing capGama t typing capGama (IsZero t) = typing capGama t typing capGama (If t1 t2 t3) | typing capGama t1 == TypeBool && typing capGama t2 == x = Just(x) | otherwise = Nothing where x = typing capGama t3 its giving me error Couldn't match expected type `Maybe Type' against inferred type `Type' In the second argument of `(==)', namely `TypeBool' In the first argument of `(&&)', namely `typing capGama t1 == TypeBool' In the expression: typing capGama t1 == TypeBool && typing capGama t2 == TypeBool how can i give two conditions in a single guard...

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Show a function?
by Costello, Roger L. 10 Nov '11

10 Nov '11

Hi Folks, Consider this function: true = (\x y -> x) When I type this at WinGHCi: true I would like WinGHCi to show this string: TRUE Is there a way to "show" a function? /Roger

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Search for docs on operator '<+>'.
by Allen S. Rout 10 Nov '11

10 Nov '11

Google seems to barf on '<+>'. I've found a haskell "list of operators", and it's not mentioned. Could some kind soul point me at documentation for this thing? - Allen S. Rout

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My Bitcoin miner
by Zhi-Qiang Lei 10 Nov '11

10 Nov '11

Hi, Learning Haskell, I write a Bitcoin miner for practice. Currently it's very basic(single thread, non-long-polling support). You are free and welcome to modify it. Best regards, Zhi-Qiang Lei zhiqiang.lei(a)gmail.com

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Help : foldl skipping one value in list
by Lev Broido 09 Nov '11

09 Nov '11

Hi I am implementing toy loop search algorithm in graph . I encountered strange problem -- it seems , that fold doesn't run on the whole list . Code is at http://hpaste.org/53776 Section in question is updVis function Problem occurs when _newStateMap_2 is being calculated Please advise. Thanks, Lev

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problem with lazy IO
by Dennis Raddle 09 Nov '11

09 Nov '11

I have the following code, which reads several thousand text files and creates some Map data. On Windows XP, it crapped out with the error "resource exhausted: too many open files." So I'm sure I need to make it stricter. I've never tried to do that before, so I'm not sure where the strictness is necessary. Maybe all I need is to switch to strict ByteStrings? -- Read a bunch of text files. The files are organized in pairs, where the file names in each pair are -- "a<number>.txt" and "f<fnumber>.txt". Example: "a00001.txt" and "f00001.txt". -- Each file is a list of float values specified in a certain text format (which happens to be -- an output format of the audio-related program Csound) -- -- Input: -- [(String,String)] : a list of pairs of file names -- String : directory in which the files exist readPvsFiles :: [(String,String)] -> String -> IO (Map Int (Map Float Float)) readPvsFiles filenames dir = do -- contents :: [(Int,Map Float Float)] -- where Int is the number in the file name. -- and Map Float Float is a map of numbers in the -- in the first file mapped to numbers in the second file contents <- mapM (oneFilePair dir) filenames return $ M.fromList contents -- Read one file pair. -- -- Input: -- String : directory of the files -- (String,String) : the two file names in the pair -- (Int,Map Float Float) : -- Int is the number in the file name. -- and Map Float Float is a map of numbers in the -- in the first file mapped to numbers in the second file oneFilePair :: String -> (String,String) -> IO (Int,Map Float Float) oneFilePair dir (ffile,afile) = do -- Read the float values from each file (which is a list of floats in -- a text format. fvalues <- readTableValues (dir ++ "/" ++ ffile) avalues <- readTableValues (dir ++ "/" ++ afile) -- t is the number in the filename. let t = read . take 6 . drop 1 $ ffile return (t, M.fromList $ zip fvalues avalues) -- Open the file via readFile, and parse all the text values in it. readTableValues :: String -> IO [Float] readTableValues s = do b <- readFile s let bs = lines b lenLine = head . drop 1 $ bs n = read (drop 6 lenLine) :: Int -- valueLines :: [String]. This is a list of the lines -- in the file that have the float values. One float -- value in text format per line. valueLines = take n . drop 25 $ bs return $ map read valueLines

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Problem with an if statement
by Karol Samborski 09 Nov '11

09 Nov '11

Hi, With a function defined like this (colors is type of Array Int [Int] and edges is [Int]): saturation (_,edges,_) = foldr (\ed ac ->if null (colors!ed) then ac else ac+1) 0 edges I'm getting an infinite loop. But when I remove the if statement like this: saturation (_,edges,_) = foldr (\ed ac -> ac+1) 0 edges everything's working. How should I define my function with the if statement to not getting an infinite loop? Best Regards, Karol Samborski

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Laziness and XML parsing
by Sean Hess 09 Nov '11

09 Nov '11

I want to parse a large xml file (2GB), without putting the whole thing into memory. It's pretty simple with a sax parser in most languages, you just stream bytes to the sax parser, and wait for sax events. Here's what I think the equivalent is in Haskell - https://gist.github.com/1346854 Is the xml file being read lazily? It seems lazy, but it also seems like all the sax events would be loaded into memory. If not, how is that possible? In order to be lazy, it seems like parse would have to be an impure function, so that it could back to the disk to get more stuff. ~sean

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Noisy Linking
by Zhi-Qiang Lei 08 Nov '11

08 Nov '11

Hi, The console comes out lots of warning message and performs slowly when I compile my code. Google does not help. Does anybody know what it refers to? I'm using Mac OS X 10.7.2. bogon% make test ghc --make -isrc -fhpc Test.hs [1 of 2] Compiling Miner ( src/Miner.hs, src/Miner.o ) [2 of 2] Compiling Main ( Test.hs, Test.o ) Linking Test ... ld: warning: could not create compact unwind for .LFB3: non-standard register 5 being saved in prolog ld: warning: text reloc in _r15W_info to _base_GHCziWord_zdfIntegralWord32_closure ld: warning: text reloc in _r15W_info to _Cryptozm4zi2zi4_DataziDigestziSHA2_zdfShowHash8_closure ld: warning: text reloc in _r15W_info to non_lazy_ptr ld: warning: text reloc in _r15W_info to non_lazy_ptr ld: warning: text reloc in _r15W_info to non_lazy_ptr ld: warning: text reloc in _r15Y_info to _base_GHCziWord_zdfEqWord32_closure ld: warning: text reloc in _r15Y_info to _Cryptozm4zi2zi4_DataziDigestziSHA2_zdfEqHash8_closure ld: warning: text reloc in _r15Y_info to non_lazy_ptr ld: warning: text reloc in _r15Y_info to non_lazy_ptr ld: warning: text reloc in _r15Y_info to non_lazy_ptr ld: warning: text reloc in _r160_info to _r15Y_closure ld: warning: text reloc in _r160_info to _base_GHCziWord_zdfOrdWord32_closure ld: warning: text reloc in _r160_info to _Cryptozm4zi2zi4_DataziDigestziSHA2_zdfOrdHash8_closure ld: warning: text reloc in _r160_info to non_lazy_ptr ld: warning: text reloc in _r160_info to non_lazy_ptr ld: warning: text reloc in _r160_info to non_lazy_ptr ld: warning: text reloc in _r162_info to _base_GHCziWord_zdfEqWord8_closure ld: warning: text reloc in _r162_info to _base_GHCziClasses_zdfEqZMZN_closure ld: warning: text reloc in _r162_info to non_lazy_ptr ld: warning: text reloc in _r162_info to non_lazy_ptr ld: warning: text reloc in _r162_info to non_lazy_ptr ld: warning: text reloc in _r164_info to _r162_closure ld: warning: text reloc in _r164_info to _base_GHCziWord_zdfOrdWord8_closure ld: warning: text reloc in _r164_info to _base_GHCziClasses_zdfOrdZMZN_closure ld: warning: text reloc in _r164_info to non_lazy_ptr ld: warning: text reloc in _r164_info to non_lazy_ptr ld: warning: text reloc in _r164_info to non_lazy_ptr …... Best regards, Zhi-Qiang Lei zhiqiang.lei(a)gmail.com

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Issue with lazy eval + trace
by Hugo Ferreira 08 Nov '11

08 Nov '11

Hello, I am using "import Debug.Trace" to debug some code but it is not working as expected. I believe this is due to lazy evaluation. But I cannot see how I can circumvent it. To exemplify I have the following function: tagFun :: (POSTags -> Tag) -> POSTags -> POSTags tagFun f corpus = Z.foldlz' tag Z.empty corpus where tag acc z = Z.insert newTag acc where (token, correctTag, _proposdTag) = trace "cursor " Z.cursor z proposed = trace "proposed tag" (f z) newTag = ( token, correctTag, proposed ) If I place tracing at: tag acc z = trace ("tag "++token++" = "++ show newTag) Z.insert newTag acc then it works ok. However the lines (token, correctTag, _proposdTag) = trace "cursor " Z.cursor z proposed = trace "proposed tag" (f z) don't work. They only appear once. I assume they should appear with every element in the zipper. Can anyone explain why? TIA, Hugo F.

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