- Beginners - Haskell.org

Fwd: Averaging a string of numbers
by Ben Kolera 12 Dec '11

12 Dec '11

That is just because you are calling min and max against the Maybe rather than on the values inside of your maybes. Max is working because there is an instance of Ord for Maybe and Nothing > Just n > Just ( n + 1 ) This is certainly not the most elegant solution ( I am a beginner, too ) but here is what I would do: instance Monoid Stats where mempty = Stats 0 Nothing Nothing 0 mappend (Stats sm1 mn1 mx1 len1) (Stats sm2 mn2 mx2 len2) = Stats (sm1 + sm2) (chooseMaybe min mn1 mn2) (chooseMaybe max mx1 mx2) (len1 + len2) chooseMaybe _ Nothing Nothing = Nothing chooseMaybe _ (Just a) Nothing = Just a chooseMaybe _ Nothing (Just b) = Just b chooseMaybe f (Just a) (Just b) = Just $ f a b Hopefully this quick answer can get you on your way to solving your problem and we can both learn a better way of doing it when someone optimises my solution. ;) On Mon, Dec 12, 2011 at 7:37 AM, goodman.m.w(a)gmail.com <goodman.m.w(a)gmail.com> wrote: > Thanks for the reply, Brent! > > I have a question about the "later" I referred to: > > On Sun, Dec 11, 2011 at 7:19 AM, Brent Yorgey <byorgey(a)seas.upenn.edu> wrote: >> On Sat, Dec 10, 2011 at 07:21:58PM -0800, goodman.m.w(a)gmail.com wrote: >>> ... later I will add support for other statistics, like >>> max, min, sum, prod, etc. > > Specifically, how max and min work with the Monoid implementation. > >> Let's add a Monoid instance for Stats, which specifies how two Stats >> objects should be combined: >> >> instance Monoid Stats where >> mempty = Stats 0 0 >> mappend (Stats sm1 len1) (Stats sm2 len2) = Stats (sm1 + sm2) (len1 + len2) > > Monoids are new to me, but I just read a bit about them in Real World > Haskell, and what you have makes sense. mempty is the identity > function of Stats, and mappend is the associative binary operator. And > thus far these properties hold true for the definition of Stats. If I > add entries for min and max, it seems like it could still be valid. Of > course, for an empty list of numbers, whereas sum can be 0.0 and len > can be 0, there is no corresponding value for max and min, so I make > them Maybe Double with an initial value of Nothing, and modify your > code as follows: > > data Stats = Stats { sm :: Double, mn, mx :: Maybe Double, len :: Int } > deriving Show > > instance Monoid Stats where > mempty = Stats 0.0 Nothing Nothing 0 > mappend (Stats sm1 mn1 mx1 len1) (Stats sm2 mn2 mx2 len2) > = Stats (sm1 + sm2) (min mn1 mn2) (max mx1 mx2) (len1 + len2) > > mkStats x = Stats x (Just x) (Just x) 1 > > This compiles and works for max, but min does not work. I tested in ghci, and: > > Prelude> min (Just 0.1) (Nothing) > Nothing > Prelude> max (Just 0.1) (Nothing) > Just 0.1 > > Which explains the problem. Any idea for the solution? > > -- > -Michael Wayne Goodman > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://www.haskell.org/mailman/listinfo/beginners

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

11 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 11 Dec '11

11 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 11 Dec '11

11 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 11 Dec '11

11 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 10 Dec '11

10 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 09 Dec '11

09 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 08 Dec '11

08 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 08 Dec '11

08 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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