February 2010 - Beginners

defining functions in ghci
by Joe Van Dyk 20 Feb '10

20 Feb '10

This messes me up: $ ghci GHCi, version 6.8.2: http://www.haskell.org/ghc/ :? for help double x =Loading package base ... linking ... done. Prelude> double x = x + x <interactive>:1:9: parse error on input `=' Why does that happen? -- Joe Van Dyk http://fixieconsulting.com

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[OT] Haskell-inspired functions for BASH
by Patrick LeBoutillier 19 Feb '10

19 Feb '10

Hi all, I've been studying Haskell for about a year now, and I've really come to like it. In my daily work I write a lot of BASH shell scripts and I thought I'd try add some of the haskell features and constructs to BASH to make my scripting life a bit easier. So I've been working on a small BASH function library that implements some basic functional programming building blocks. Note: There is no actual Haskell code involved here. All this is very prototypical, but here is an example of some of the stuff I've got so far (map, filter, foldr): $ ls data 1.txt 2.txt # basic map, argument goes on the command line $ ls -d data/* | map basename 1.txt 2.txt # map with lambda expression $ ls -d data/* | map '\f -> basename $f .txt' 1 2 # simple filter, also works with lambda $ ls -d data/* | map basename | filter 'test 1.txt =' 1.txt # sum $ ls -d data/* | map '\f -> basename $f .txt' | foldr '\x acc -> echo $(($x + $acc))' 0 3 Before I continue with this project, I'm looking for a bit of feedback/info: - Does anyone know if there are already similar projets out there? - Does anyone find this interesting? - Any other comment/suggestion/feedback Thanks a lot, Patrick LeBoutillier -- ===================== Patrick LeBoutillier Rosemère, Québec, Canada

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Loading QuickCheck in GHCi.
by Kwanghoon Choi 16 Feb '10

16 Feb '10

Dear All, Whenever I tried to load QuickCheck in GHCi, I met the following problem: Prelude> :l Test.QuickCheck <no location info>: module `Test.QuickCheck' is a package module Failed, modules loaded: none. Test.QuickCheck seems to be a module anyway, bit it couldn't be loaded directly by issuing the command. I don't understand why. Thanks in advance. Kwanghoon PS. Of course, I installed QuickCheck-2.1.0.3, and I can use it when I load it by loading some haskell program having "import Test.QuickCheck".

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Equivalent of FOOLANGUAGEPATH environment variable?
by Tom Tobin 16 Feb '10

16 Feb '10

Is there an easy way to declare a particular directory as a location for finding Haskell modules, similar to PYTHONPATH and similar environment variables for other languages? The GHC_PACKAGE_PATH doesn't seem to be quite the same thing. There's the option of setting -iSOMEPATH in one's .ghci file, but that doesn't seem to allow declaring a relative path via ~ or $HOME, making it a bit of a pain to maintain separate versions of one's .ghci across different machines.

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space leak
by Uchida Yasuo 16 Feb '10

16 Feb '10

Hello, I came across the following space leak problem today. How can I fix this? (Tested on Mac OS X 10.5.8, GHC 6.10.3) -- test.hs module Main where import System import qualified Data.ByteString.Lazy.Char8 as L main = do args <- getArgs let n = read $ args !! 0 cs <- L.getContents let !a = L.take n cs mapM_ (print . L.length) $ L.lines cs print a -- gen.hs module Main where main = do putStrLn $ take 1000000 $ cycle "foo" main These are compiled with the following options: $ ghc --make -O2 test $ ghc --make -O2 gen The memory usage seems to depend on the argument(=17000) passed. On my MacBook(Core2 Duo 2.0GHz), 16000 works fine. $ ./gen | head -1000 | ./test 17000 +RTS -sstderr ... 3,793,673,564 bytes allocated in the heap 9,901,516 bytes copied during GC 635,576,092 bytes maximum residency (11 sample(s)) 248,725,136 bytes maximum slop 1759 MB total memory in use (562 MB lost due to fragmentation) Generation 0: 6941 collections, 0 parallel, 16.91s, 18.15s elapsed Generation 1: 11 collections, 0 parallel, 0.03s, 0.03s elapsed INIT time 0.00s ( 0.00s elapsed) MUT time 3.04s ( 27.64s elapsed) GC time 16.94s ( 18.18s elapsed) EXIT time 0.00s ( 0.01s elapsed) Total time 19.99s ( 45.82s elapsed) %GC time 84.8% (39.7% elapsed) Alloc rate 1,245,766,300 bytes per MUT second Productivity 15.2% of total user, 6.6% of total elapsed $ ./gen | head -1000 | ./test 16000 +RTS -sstderr ... 4,000,652,128 bytes allocated in the heap 7,428,180 bytes copied during GC 1,057,588 bytes maximum residency (1001 sample(s)) 525,092 bytes maximum slop 5 MB total memory in use (1 MB lost due to fragmentation) Generation 0: 6362 collections, 0 parallel, 0.10s, 0.12s elapsed Generation 1: 1001 collections, 0 parallel, 0.09s, 0.09s elapsed INIT time 0.00s ( 0.00s elapsed) MUT time 2.59s ( 23.26s elapsed) GC time 0.18s ( 0.22s elapsed) EXIT time 0.00s ( 0.00s elapsed) Total time 2.77s ( 23.47s elapsed) %GC time 6.6% (0.9% elapsed) Alloc rate 1,545,246,968 bytes per MUT second Productivity 93.4% of total user, 11.0% of total elapsed -- Regards, Yasuo Uchida

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indentation
by Gabi 15 Feb '10

15 Feb '10

BTW, is there some indent tool for Haskell. Something like "indent" command line tool for c? -- Regards, Gabi http://bugspy.net

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indentation
by John Moore 15 Feb '10

15 Feb '10

Hi all, I must be doing something wrong with my tabs. Because this keeps telling me parse error. Can some one see if it is indentation. I'm sending a file so as you'll be able to see what going on. If I copy it here it can appear wrong the difference is with the Var expression. John

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Calculating SHA1 hashes?
by Tom Tobin 15 Feb '10

15 Feb '10

I'm trying to figure out the best way to calculate SHA1 hashes for files using Haskell. There are several libraries I tried from Hackage, but they all seem to end up computing the hash far too slowly vs. OpenSSL. Some of the libraries seem to be FFI bindings to OpenSSL, but I haven't figured out how to efficiently feed these. (For instance, hopenssl consumes a [Word8], but I haven't figured out how to lazily produce that from a file; using "unpack" on a ByteString seems to end up much too slow.) I also tried mimicking nano-md5 by writing a "nano-sha1" that was largely a "replace the name of the digest function" hack job, but I couldn't manage to get that working. Is there a particular accepted way to generate hashes that I'm missing?

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getting memory usage to be constant when mixing wav files
by Renick Bell 14 Feb '10

14 Feb '10

I've written a program that mixes wav files read in by hsndfile, and it does so with reasonably satisfactory performance using RTS options -A and -K. Unfortunately, it does not have constant memory usage. Profiling the program shows that almost all of the memory allocation comes down to the following two functions: the first which calculates and writes the values of individual indexes to the target array (longer), and the second which runs the first for all the indexes in the shorter array. addInBase :: IOCArray Int Double -> IOCArray Int Double -> (Int, Int) -> IO () addInBase longer shorter (!li,!si) = do x <- readArray longer li y <- readArray shorter si let result = x + y writeArray longer li result addIn :: IOCArray Int Double -> IOCArray Int Double -> Int -> Int -> Int -> IO () addIn longer shorter shorterBounds !li !si | si > shorterBounds = return () | si <= shorterBounds = addInBase longer shorter (li,si) >> addIn longer shorter shorterBounds (li+1) (si+1) Basically, the addIn function is called by mapM_ across a list of all of the occurrences of the input wav files. I've tried ! and seq in many different places and combinations, but I cannot get the memory usage constant. How can I achieve that? What technique or principle will show me more specifically what is not being evaluated so that I can avoid this trouble in the future? I have read the wiki pages on Strictness, Laziness, and Performance, but maybe I have not fully understood how to apply what is written there to this code. Any help is greatly appreciated. If you notice any other glaring mistakes or bad ideas in the code above, I'm interested in that as well. -- Renick Bell http://the3rd2nd.com

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running HGL on OSX
by Vojtěch Knyttl 14 Feb '10

14 Feb '10

Hello, I am having difficulties running HGL on Mac OSX, Snow Leopard. Here is what I've done: –– 1. Installed X11 for OSX ("About" dialog says: The X Window System, XQuartz 2.3.4 (xorg-server 1.4.2-apple45)) –– 2. Got X11 from Hackage – configured, built and installed – however, there where quite a lot of warnings: … [ 8 of 25] Compiling Graphics.X11.Xlib.Color ( Graphics/X11/Xlib/Color.hs, dist/build/Graphics/X11/Xlib/Color.o ) Graphics/X11/Xlib/Color.hs:38:0: Warning: The import of `Foreign.C.Types' is redundant except perhaps to import instances from `Foreign.C.Types' To import instances alone, use: import Foreign.C.Types() [ 9 of 25] Compiling Graphics.X11.Xlib.Font ( dist/build/Graphics/X11/Xlib/Font.hs, dist/build/Graphics/X11/Xlib/Font.o ) Graphics/X11/Xlib/Font.hsc:167:1: Warning: A do-notation statement discarded a result of type CInt. Suppress this warning by saying "_ <- xTextExtents font_struct c_string (fromIntegral nchars) direction_return font_ascent_return font_descent_return overall_return", or by using the flag -fno-warn-unused-do-bind … and so on. –– 3. Got HGL package, built and installed with some warnings like: … [ 1 of 27] Compiling Graphics.HGL.Internals.Flag ( Graphics/HGL/Internals/Flag.hs, dist/build/Graphics/HGL/Internals/Flag.o ) Graphics/HGL/Internals/Flag.hs:1:0: Warning: Module `Prelude' is deprecated: You are using the old package `base' version 3.x. Future GHC versions will not support base version 3.x. You should update your code to use the new base version 4.x. … –– Now, when I try to run simple program like: main = runGraphics $ withWindow_ "Hello World Window" (300, 200) $ \ w -> do drawInWindow w $ text (100, 100) "Hello World" drawInWindow w $ ellipse (100, 80) (200, 180) getKey w it says: $ ghci graphics.hs GHCi, version 6.12.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Loading package ffi-1.0 ... linking ... done. Ok, modules loaded: Main. Prelude Main> main Loading package syb-0.1.0.2 ... linking ... done. Loading package X11-1.5.0.0 ... can't load .so/.DLL for: X11 (dlopen(libX11.dylib, 9): image not found) Prelude Main> Just to precise, I have X11 running (it can be seen in the OSX dock) Thanks

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