- Beginners - Haskell.org

Averaging a string of numbers
by goodman.m.w＠gmail.com 12 Dec '11

12 Dec '11

Hi beginners list, I want to take a string of numbers and return the average. It is a string because I expect the input to be from STDIN or a file. I would like it to handle infinite lists. Further, I create the Stats data structure because later I will add support for other statistics, like max, min, sum, prod, etc. The following code works, but I'm not sure it's the proper way to do things in Haskell. For instance, maybe I could use a fold method instead of the explicit recursion in getStats? Could you please let me know if anything looks clunky or could be improved (efficiency, aesthetics, etc)? Thank you -- avg.hs -- sm is sum, len is number of elements data Stats = Stats { sm :: Double, len :: Int } deriving Show getAverage :: String -> Double getAverage str = (sm s / fromIntegral (len s)) where s = (getStats . readNumbers . lines) str getStats :: [Double] -> Stats getStats [] = Stats 0.0 0 getStats [x] = Stats x 1 getStats (x:xs) = Stats (x + (sm s)) ((len s) + 1) where s = getStats xs readNumbers :: [String] -> [Double] readNumbers = map read -- end avg.hs And here's the results in ghci: *Main> :load avg.hs [1 of 1] Compiling Main ( avg.hs, interpreted ) Ok, modules loaded: Main. *Main> getAverage "0" 0.0 *Main> getAverage "1.4\n0.5\n85.2\n30.2\n-10" 21.46 *Main> getAverage "" NaN -- -Michael Wayne Goodman

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correction: hGetLine, not getLine
by MJ Williams 12 Dec '11

12 Dec '11

Apologies guys, I meant, hGetLine, not getLine. Sorry about that. Regards Matthew

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checking file type (without getLine)
by MJ Williams 12 Dec '11

12 Dec '11

Hi guys, When the hSetEncoding is set to utf8, getLine will attempt to read each line as belonging to a UTF-8 and when it encounters an error in the encoding it throws an exception. I wonder if there's a way of checking for errors without getLine. I didn't find any function or functions in System.IO that might do the job. is there another library that I should be looking at? Regards Matthew

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let in GHCI
by Henry Lockyer 12 Dec '11

12 Dec '11

Hello Haskellers Why is it that in GHCI I can do, for example, > replicate 6 5 [5,5,5,5,5,5] > let my6 = replicate 6 > my6 5 [5,5,5,5,5,5] but if I do > sort "bav" "abv" this is ok, but > let mySort = sort > mySort "bav" <interactive>:1:8: Couldn't match expected type `()' with actual type `Char' Expected type: [()] Actual type: [Char] In the first argument of `mySort', namely `"bav"' In the expression: mySort "bav" and/or > mySort [6,5,9] <interactive>:1:13: No instance for (Num ()) arising from the literal `9' Possible fix: add an instance declaration for (Num ()) In the expression: 9 In the first argument of `mySort', namely `[6, 5, 9]' In the expression: mySort [6, 5, 9] This is eluding me at the moment..! ;-) / Henry

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Installing regex-pcre (revisited)
by MJ Williams 11 Dec '11

11 Dec '11

Hi guys, I understand there's a UNIX-like environment that one can install on Windows and it may already include clibpcre that GHC requires for installing regex-pcre. for the life of me I can't remember its name. Does anyone know what I'm referring to? Regards Matthew

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Parallel Matrix Multiplication
by mukesh tiwari 11 Dec '11

11 Dec '11

Hello all I am trying to learn parallel Haskell and I have gone through couple of resources ( Real world Haskell and http://research.microsoft.com/en-us/um/people/simonpj/papers/parallel/AFP08… ). I understood par and pseq and I wrote matrix multiplication using these two function but it does not look. Although I did not get all these details from ( ./Matpar +RTS -N2 -s ) but none of the sparks converted which is really important for parallel programming. Could some one please tell me how to improve this. Also kindly recommend me some literature for parallel programming in Haskell. Regards Mukesh Tiwari import Data.List import Control.Parallel parHelp :: ( Num a ) => [ a ] -> [ a ] -> a parHelp [] [] = 0 parHelp ( x : xs ) ( y : ys ) = ret where ret = par a ( pseq a ( a + parHelp xs ys ) ) where a = x * y helpMult :: ( Num a ) => [ a ] -> [ [ a ] ] -> [ a ] helpMult _ [] = [] helpMult x ( y : ys ) = ret where ret = par a ( pseq a ( a : helpMult x ys ) ) where a = parHelp x y mult :: ( Num a ) => [ [ a ] ] -> [ [ a ] ] -> [ [ a ] ] mult [] _ = [] mult ( x : xs ) ys = ret where ret = par a ( pseq a ( a : mult xs ys ) ) where a = helpMult x ys main = print $ mult [[1 .. 4 ] , [ 1 .. 4 ] , [ 1 .. 4 ] , [ 1 .. 4] ] ( transpose [[1 .. 4 ] , [ 1 .. 4 ] , [ 1 .. 4 ] , [ 1 .. 4] ]) [user@haskell Programming]$ ghc -O2 -threaded -rtsopts Matpar.hs [1 of 1] Compiling Main ( Matpar.hs, Matpar.o ) Linking Matpar ... [user@haskell Programming]$ ./Matpar +RTS -N2 -s ./Matpar +RTS -N2 -s [[10,20,30,40],[10,20,30,40],[10,20,30,40],[10,20,30,40]] 85,480 bytes allocated in the heap 5,216 bytes copied during GC 47,328 bytes maximum residency (1 sample(s)) 22,304 bytes maximum slop 2 MB total memory in use (0 MB lost due to fragmentation) Generation 0: 0 collections, 0 parallel, 0.00s, 0.00s elapsed Generation 1: 1 collections, 0 parallel, 0.00s, 0.00s elapsed Parallel GC work balance: -nan (0 / 0, ideal 2) MUT time (elapsed) GC time (elapsed) Task 0 (worker) : 0.00s ( 0.00s) 0.00s ( 0.00s) Task 1 (worker) : 0.00s ( 0.00s) 0.00s ( 0.00s) Task 2 (bound) : 0.00s ( 0.00s) 0.00s ( 0.00s) Task 3 (worker) : 0.00s ( 0.00s) 0.00s ( 0.00s) * SPARKS: 84 (0 converted, 0 pruned)* INIT time 0.00s ( 0.00s elapsed) MUT time 0.00s ( 0.00s elapsed) GC time 0.00s ( 0.00s elapsed) EXIT time 0.00s ( 0.00s elapsed) Total time 0.00s ( 0.00s elapsed) %GC time 33.3% (13.5% elapsed) Alloc rate 42,761,380 bytes per MUT second Productivity 33.3% of total user, 45.0% of total elapsed gc_alloc_block_sync: 0 whitehole_spin: 0 gen[0].sync_large_objects: 0 gen[1].sync_large_objects: 0 [user@haskell Programming]$

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comments requested
by rickmurphy 10 Dec '11

10 Dec '11

Hey All: A few months ago I got some help here starting up some RDF and JSON work. Special thanks to David McBride for getting me on the right path. http://osdir.com/ml/beginners@haskell.org/2011-10/msg00348.html Since then I've been exercising some skill building. I've moved this work to github with the hope that folks here can comment and offer suggestions for improvement. You can find the files here: https://github.com/rickmurphy/hafr and a bit of a write up here. http://phaneron.rickmurphy.org/ I welcome your feedback on refactoring, style, performance tuning, etc. -- Rick

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Installing regex-pcre
by MJ Williams 09 Dec '11

09 Dec '11

Dear all, I'm trying to install regex-pcre on my XP Pro machine. I have the latest version of GHC. Do any of you recognise and understand the following error message? install regex-pcre Resolving dependencies... Configuring regex-pcre-0.94.2... cabal: Missing dependency on a foreign library: * Missing C library: pcre This problem can usually be solved by installing the system package that provides this library (you may need the "-dev" version). If the library is already installed but in a non-standard location then you can use the flags --extra-include-dirs= and --extra-lib-dirs= to specify where it is. cabal: Error: some packages failed to install: regex-pcre-0.94.2 failed during the configure step. The exception was: ExitFailure 1 Apologies if the query is a little offtopic. Regards Matthew

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Matrix / Matrix Addition
by Paul Monday 09 Dec '11

09 Dec '11

I'm working on simple matrix / matrix addition for a short project. Doing this with lists (not worrying about matrix sizes and such) is relatively easy and makes quite a bit of sense to me: basically: madd :: Num a => [[a]] -> [[a]] -> [[a]] I could read this as: - with a as a Numeric type, take two lists of lists of Numerics and return a list of lists of Numerics And then to do the job simple madd m n = zipWith (zipWith (+)) m n And I can read that easily enough: -- madd has two inputs, m and n and it will do the function "zipWith (+)" across its inputs of the inner lists from m and n (the rows, basically) Ok, so madd [[1,2,3], [4,5,6], [7,8,9]] [[10,11,12], [13,14,5], [16,17,18]] yields [[11,13,15],[17,19,11],[23,25,27]] Cool I can get a little nicer (possibly) if I use a Matrix type: data Matrix a = Matrix [[a]] deriving (Show, Eq) madd :: Num a => [[a]] -> [[a]] -> [[a]] madd m n = zipWith (zipWith (+)) m n madd2 :: (Num a) => Matrix a -> Matrix a -> Matrix a madd2 (Matrix m) (Matrix n) = Matrix $ zipWith (zipWith (+)) m n The only real change in terms of the code is in the madd2 definition where I have to construct a Matrix to return from the results of the zipWiths … and some better type safety of course. *Main> madd2 (Matrix [[1,2,3], [4,5,6], [7,8,9]]) (Matrix [[10,11,12], [13,14,5], [16,17,18]]) Matrix [[11,13,15],[17,19,11],[23,25,27]] Cool' Now, I really want to use Unboxed Vectors. As I look around the web and poke around through various resources, I believe that I need the rows to be unboxed but the columns can be boxed … the actual operations on the rows will give me the speed up I desire. So, here it is: import qualified Data.Vector as V import qualified Data.Vector.Unboxed as U data UMatrix a = UMatrix (V.Vector (U.Vector a)) deriving (Show, Eq) madd3 :: (U.Unbox a, Num a) => UMatrix a -> UMatrix a -> UMatrix a madd3 (UMatrix m) (UMatrix n) = UMatrix $ V.zipWith (U.zipWith (+)) m n And it works … (really, when I started writing this note, I was completely lost … welcome to the power of sitting and walking through Haskell) madd3 (UMatrix (Data.Vector.fromList [Data.Vector.Unboxed.fromList [1,2,3], Data.Vector.Unboxed.fromList [4,5,6], Data.Vector.Unboxed.fromList [7,8,9]])) (UMatrix (Data.Vector.fromList [Data.Vector.Unboxed.fromList [10,11,12], Data.Vector.Unboxed.fromList [13,14,5], Data.Vector.Unboxed.fromList [16,17,18]])) I have ONE thing that I can't really explain … Why does the declaration of madd3 REQUIRE the use of U.Unbox a as a data type? madd3 :: (U.Unbox a, Num a) => UMatrix a -> UMatrix a -> UMatrix a I would have thought that my declaration of UMatrix would have done this? Cheers. Paul Monday Parallel Scientific, LLC. paul.monday(a)parsci.com

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Compiling shared (dll) library
by Alexander.Vladislav.Popov 06 Dec '11

06 Dec '11

Hi, Haskellers. I'm trying to compile following program (where Regex.Genex is a package what I need to produce all possible expresions by the given pattern and `adder' is just FFI sample): -- genexlib.hs {-# LANGUAGE BangPatterns, ForeignFunctionInterface #-} module GenexLib where import Regex.Genex import System.IO import System.Environment adder :: Int -> Int -> IO Int -- gratuitous use of IO adder x y = return (x+y) foreign export stdcall adder :: Int -> Int -> IO Int -- genexlib.hs end // start.c #include <Rts.h> void HsStart() { int argc = 1; char* argv[] = {"ghcDll", NULL}; // argv must end with NULL // Initialize Haskell runtime char** args = argv; hs_init(&argc, &args); } void HsEnd() { hs_exit(); } // start.c end I'm using ghc >ghc --version The Glorious Glasgow Haskell Compilation System, version 7.0.2 compiling: >ghc -c genexlib.hs >ghc -c start.c >ghc -shared -o genexlib.dll genexlib.o genexlib_stub.o start.o genexlib.o:fake:(.text+0xd1): undefined reference to `__stginit_regexzmgenexzm0zi3zi2_RegexziGenex_' Creating library file: genexlib.dll.a collect2: ld returned 1 exit status and get undefined reference. But If I try to compile the executable from similar code: -- genexlib.hs {-# LANGUAGE BangPatterns, ForeignFunctionInterface #-} -- module GenexLib where import Regex.Genex import System.IO import System.Environment defaultRegex :: String defaultRegex = "a(b|c)d{2,3}e*" main :: IO () main = do hSetBuffering stdout NoBuffering args <- getArgs case args of [] -> do prog <- getProgName if prog == "<interactive>" then run [defaultRegex] else do fail $ "Usage: " ++ prog ++ " regex [regex...]" rx -> run rx run :: [String] -> IO () run regex = do let s = genexPure regex mapM_ print s -- genexlib.hs end >ghc --make genexlib.hs -O2 it's ok, no errors, and you can see in GHCi: *Main> :main "abdd" "acdd" "abddd" "acddd" "abddeee" "acddeee" "abdddeee" "acdddeee" "abddee" "acddee" "abdddee" "acdddee" "abdde" "acdde" "abddde" "acddde" Where is my mistake? What am I doing wrong? In first case, when compiling shared dll, I tried to link libraries what I've found in `cabal' directory (like `libHSregex-genex-0.3.2.a') to work around errors but all in vain. -- Alexander Popov

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