August 2010 - Beginners

very detailed monad tutorials
by Michael Vanier 16 Aug '10

16 Aug '10

Hi everyone, I've been writing a bunch of in-depth monad tutorials that I thought some of you might find useful. So far there are six, with at least three more planned. Here are the links: Part 1 (Basics): http://mvanier.livejournal.com/3917.html Part 2 (>>= and return): http://mvanier.livejournal.com/4305.html Part 3 (The monad laws): http://mvanier.livejournal.com/4586.html Part 4 (The Maybe and list monads): http://mvanier.livejournal.com/4647.html Part 5 (Error-handling monads, part 1): http://mvanier.livejournal.com/5103.html Part 6 (Error-handling monads, part 2): http://mvanier.livejournal.com/5343.html The next ones will deal with state monads. I would love to get some feedback on these. Cheers, Mike

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global variables
by prad 15 Aug '10

15 Aug '10

i'm curious about the best way to deal with conn to make db connections. i define it in main like this, then when i want to use it in a function i have to pass it as a parameter: main = do conn <- connectPostgreSQL ... fn conn something fn conn x = ... now conn is a global variable i guess because it is defined in main, but perhaps main isn't considered a global namespace? i recall in python a global variable could be accessed by functions, but in php they can't unless you use "global" within the function. so is this the correct way to handle such things? right now it seems to be the only way in haskell. -- In friendship, prad ... with you on your journey Towards Freedom http://www.towardsfreedom.com (website) Information, Inspiration, Imagination - truly a site for soaring I's

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= vs <-
by prad 15 Aug '10

15 Aug '10

i'm using them right i think because it works, but my understanding is fuzzy. it seems <- is used when you do things like load a file or get arguments from outside or if you are return something from a function these seem to take the form of something other than an internal type like Int or String and have to be 'massaged' into use with things like show or fromSql etc the = is used with let and seems to work for any assignments that are internal. are these assessments correct? -- In friendship, prad ... with you on your journey Towards Freedom http://www.towardsfreedom.com (website) Information, Inspiration, Imagination - truly a site for soaring I's

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leaky folding
by Travis Erdman 15 Aug '10

15 Aug '10

in the code below, finalmap seems fast enough ... but it has a space leak. otoh, finalmap'rnf runs in constant space, but its performance is terrible, at least 4x slower than finalmap. this is a common problem i'm having ... foldl' isn't strict enough, but foldl'rnf kills performance. and not only with IntMap as the cumulating data structure, but others as well. any ideas on this one? how can i get a fast fold in constant space? thanks again, travis {-# LANGUAGE BangPatterns #-} import System.Environment import Foreign (unsafePerformIO) import System.Random.Mersenne import Data.List import Control.DeepSeq import Control.Parallel.Strategies import qualified Data.IntMap as IntMap mersennegen = unsafePerformIO $ newMTGen Nothing infrandoms = unfoldr ( Just . splitAt 3) $ map (\x -> abs (x `mod` n)) (unsafePerformIO $ (randoms mersennegen)::[Int]) n = 200 foldl'rnf :: NFData a => (a -> b -> a) -> a -> [b] -> a foldl'rnf f z xs = lgo z xs where lgo z [] = z lgo !z (x:xs) = lgo (runEval (rdeepseq (f z x))) xs startmap = IntMap.fromDistinctAscList $ zip [0..] [1..n] finalmap x = foldl' g startmap (take x infrandoms) finalmap'rnf x = foldl'rnf g startmap (take x infrandoms) g:: IntMap.IntMap Int -> [Int] -> IntMap.IntMap Int g !a [x,y,z] = IntMap.adjust (const $ y + (a IntMap.! z) `mod` n) x a main = do args <- getArgs print $ finalmap (read $ head args)

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how to access command line arguments
by prad 14 Aug '10

14 Aug '10

here's one way: import System (getArgs) args <- getArgs first = head args last = last args and so on. i can also do this (first:last:z) <- getArgs and avoid using head and tail. in fact, this seems nicer because i can pattern match for a specific series of inputs. however, if the inputs aren't there (eg say program arg1 only instead of program arg1 arg2), then the runtime pattern match failure error: user error (Pattern match failure in do expression at Tests.hs:14:4-14) is understandably generated. this of course doesn't happen if i just use args because there is no pattern matching. is the former the better way to do it then? or are there other alternatives? -- In friendship, prad ... with you on your journey Towards Freedom http://www.towardsfreedom.com (website) Information, Inspiration, Imagination - truly a site for soaring I's

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Spacewars! clone needs code review
by Aur Saraf 13 Aug '10

13 Aug '10

Hello All, I've been spending all my nights lately on my final self-given Haskell "learning exercise" - a Spacewar! [http://en.wikipedia.org/wiki/Spacewar!] clone. It is playable but lacks the features that perfect a game - score system, menu etc' - and there's some hacks in areas where I didn't feel that implementing it prettily would teach me anything. The game is available under the BSD license in my github [ http://github.com/SonOfLilit/purewars] I would be very happy if anyone would bother to review my code and suggest ways that I could improve it, since this is a learning exercise. Thanks, -- Aur

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Profiling introduces a space leak where there was none before?
by Travis Erdman 13 Aug '10

13 Aug '10

In Ch 25 of Real World Haskell, the authors introduce some naive code for finding the average of a big list; it has a space leak, and they present several solutions. Below are two of the solutions that successfully eliminate the space leak (though, the first one -- the one that uses foldl'rnf -- is quite a bit faster). However, if compiled with profiling, the first one (using foldl'rnf) NOW has a leak. The second solution (foldl') does not have a leak even when profiling is enabled. I have used this foldl'rnf function in my own code, as it is the only solution I have found for a space leak in my own code. But, since it leaks when profiled, it is making analysis difficult. Is this a feature, bug, or user error? If a known issue, is there a workaround? The code and some documenting output follows. thanks, Travis ------------------------------------ {-# LANGUAGE BangPatterns #-} import System.Environment import Text.Printf import Control.Parallel.Strategies import Control.DeepSeq import Data.List (foldl') main = do [d] <- map read `fmap` getArgs printf "%f\n" (mean [1..d]) foldl'rnf :: NFData a => (a -> b -> a) -> a -> [b] -> a foldl'rnf f z xs = lgo z xs where lgo z [] = z lgo z (x:xs) = lgo z' xs where z' = f z x `using` rdeepseq -- first mean fn aka foldl'rnf mean :: [Double] -> Double mean xs = s / fromIntegral n where (n, s) = foldl'rnf k (0, 0) xs k (n, s) x = (n+1, s+x) :: (Int, Double) -- second mean fn aka foldl' -- mean :: [Double] -> Double -- mean xs = s / fromIntegral n -- where -- (n, s) = foldl' k (0, 0) xs -- k (!n, !s) x = (n+1, s+x) ------------------------------------------ [NO PROFILING, NO SPACE LEAK] C:\Documents and Settings\Travis\My Documents\Haskell Code>ghc --make temp5 -O2 -fasm [1 of 1] Compiling Main ( temp5.hs, temp5.o ) Linking temp5.exe ... C:\Documents and Settings\Travis\My Documents\Haskell Code>temp5 1e7 +RTS -sstderr temp5 1e7 +RTS -sstderr 5000000.5 1,170,230,652 bytes allocated in the heap 128,876 bytes copied during GC 3,372 bytes maximum residency (1 sample(s)) 13,012 bytes maximum slop 1 MB total memory in use (0 MB lost due to fragmentation) Generation 0: 2232 collections, 0 parallel, 0.05s, 0.05s elapsed Generation 1: 1 collections, 0 parallel, 0.00s, 0.00s elapsed INIT time 0.02s ( 0.03s elapsed) MUT time 1.52s ( 1.55s elapsed) GC time 0.05s ( 0.05s elapsed) EXIT time 0.00s ( 0.00s elapsed) Total time 1.58s ( 1.63s elapsed) %GC time 3.0% (2.9% elapsed) Alloc rate 764,232,262 bytes per MUT second Productivity 96.0% of total user, 93.3% of total elapsed C:\Documents and Settings\Travis\My Documents\Haskell Code>temp5 1e8 +RTS -sstderr temp5 1e8 +RTS -sstderr 50000000.5 11,702,079,228 bytes allocated in the heap 1,253,872 bytes copied during GC 3,372 bytes maximum residency (1 sample(s)) 13,012 bytes maximum slop 1 MB total memory in use (0 MB lost due to fragmentation) Generation 0: 22321 collections, 0 parallel, 0.38s, 0.39s elapsed Generation 1: 1 collections, 0 parallel, 0.00s, 0.00s elapsed INIT time 0.02s ( 0.00s elapsed) MUT time 15.47s ( 15.72s elapsed) GC time 0.38s ( 0.39s elapsed) EXIT time 0.00s ( 0.00s elapsed) Total time 15.86s ( 16.11s elapsed) %GC time 2.4% (2.4% elapsed) Alloc rate 755,734,682 bytes per MUT second Productivity 97.5% of total user, 96.0% of total elapsed [NOW TURN ON PROFILING, GET SPACE LEAK] C:\Documents and Settings\Travis\My Documents\Haskell Code>ghc --make temp5 -O2 -fasm -prof -auto-all [1 of 1] Compiling Main ( temp5.hs, temp5.o ) Linking temp5.exe ... C:\Documents and Settings\Travis\My Documents\Haskell Code>temp5 1e6 +RTS -sstderr -p -K128M temp5 1e6 +RTS -sstderr -p -K128M 500000.5 395,774,976 bytes allocated in the heap 238,684,620 bytes copied during GC 102,906,760 bytes maximum residency (7 sample(s)) 66,283,900 bytes maximum slop 179 MB total memory in use (4 MB lost due to fragmentation) Generation 0: 493 collections, 0 parallel, 4.83s, 4.84s elapsed Generation 1: 7 collections, 0 parallel, 0.23s, 0.30s elapsed INIT time 0.02s ( 0.03s elapsed) MUT time 0.81s ( 0.91s elapsed) GC time 5.06s ( 5.14s elapsed) RP time 0.00s ( 0.00s elapsed) PROF time 0.00s ( 0.00s elapsed) EXIT time 0.00s ( 0.00s elapsed) Total time 5.89s ( 6.08s elapsed) %GC time 85.9% (84.6% elapsed) Alloc rate 477,916,952 bytes per MUT second Productivity 13.8% of total user, 13.4% of total elapsed

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ghci access to .hs functions
by prad 12 Aug '10

12 Aug '10

in an effort to figure out type declarations i thought i'd let ghci do the work. so i wrote a program: import ... main = do ... func1 func2 now when i load this into ghci i can't to :t func1 etc and get a not in scope error however, if i put a return () onto the end of main: import ... main = do ... return () func1 func2 the functions are in scope of ghci and i can find out the types. so what is happening here? i understand that return is different in haskell than in other languages, but i don't understand just what it is doing. :( -- In friendship, prad ... with you on your journey Towards Freedom http://www.towardsfreedom.com (website) Information, Inspiration, Imagination - truly a site for soaring I's

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FastCGI error
by Wayne R 12 Aug '10

12 Aug '10

I'm trying to use Network.FastCGI with Apache2, but running into an error. I'd like to believe that the web server is configured correctly, because I can successfully view the following, compiled to <web root>/fastcgi/test.fcgi: #include <fcgi_stdio.h> int main(void){ int count = 0; while(FCGI_Accept()>=0){ printf("Content-Type: text/html\r\n"); printf("\r\n"); printf("Hello, World: %d", count++); } return 0; } Now I compile Fcgi.hs to <web root>/fastcgi/test2.fcgi: import Control.Concurrent import Network.FastCGI action :: CGI CGIResult action = do setHeader "Content-type" "text/plain" tid <- liftIO myThreadId output $ unlines [ "I am a FastCGI process!" , "Hear me roar!" , "" , show tid ] main = runFastCGIConcurrent' forkIO 10 action When I view localhost/fastcgi/test.fcgi, I get the hello world page, but attempting to view test2.fcgi shows an 'Internal Server Error" in firefox. The apache error log says: [Sun Aug 08 07:44:17 2010] [notice] Apache/2.2.14 (Ubuntu) mod_fastcgi/2.4.6 mod_lisp2/1.3.1 PHP/5.3.2-1ubuntu4.2 with Suhosin-Patch mod_ssl/2.2.14 OpenSSL/0.9.8k configured -- resuming normal operations [Mon Aug 09 11:11:23 2010] [warn] FastCGI: (dynamic) server "/var/www/fastcgi/test.fcgi" started (pid 20354) [Mon Aug 09 11:11:27 2010] [warn] FastCGI: (dynamic) server "/var/www/fastcgi/test2.fcgi" started (pid 20360) [Mon Aug 09 11:11:31 2010] [warn] FastCGI: (dynamic) server "/var/www/fastcgi/test2.fcgi" started (pid 20365) [Mon Aug 09 11:11:34 2010] [warn] FastCGI: (dynamic) server "/var/www/fastcgi/test2.fcgi" started (pid 20371) [Mon Aug 09 11:11:37 2010] [warn] FastCGI: (dynamic) server "/var/www/fastcgi/test2.fcgi" started (pid 20375) [Mon Aug 09 11:11:40 2010] [warn] FastCGI: (dynamic) server "/var/www/fastcgi/test2.fcgi" started (pid 20379) [Mon Aug 09 11:11:43 2010] [warn] FastCGI: (dynamic) server "/var/www/fastcgi/test2.fcgi" started (pid 20383) [Mon Aug 09 11:11:46 2010] [warn] FastCGI: (dynamic) server "/var/www/fastcgi/test2.fcgi" started (pid 20387) [Mon Aug 09 11:11:49 2010] [warn] FastCGI: (dynamic) server "/var/www/fastcgi/test2.fcgi" started (pid 20391) [Mon Aug 09 11:11:52 2010] [warn] FastCGI: (dynamic) server "/var/www/fastcgi/test2.fcgi" started (pid 20395) [Mon Aug 09 11:11:55 2010] [warn] FastCGI: scheduled the start of the last (dynamic) server "/var/www/fastcgi/test2.fcgi" process: reached dynamicMaxClassProcs (10) [Mon Aug 09 11:11:55 2010] [warn] FastCGI: (dynamic) server "/var/www/fastcgi/test2.fcgi" started (pid 20399) [Mon Aug 09 11:12:01 2010] [error] [client 127.0.0.1] FastCGI: comm with (dynamic) server "/var/www/fastcgi/test2.fcgi" aborted: (first read) idle timeout (30 sec) [Mon Aug 09 11:12:01 2010] [error] [client 127.0.0.1] FastCGI: incomplete headers (0 bytes) received from server "/var/www/fastcgi/test2.fcgi" I've tried Fcgi.hs with two different sample FastCGI programs with the same results. Any suggestions? Wayne

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Trying to statically link Gtk2Hs application (Windows XP, network drive)
by Peter Schmitz 11 Aug '10

11 Aug '10

I am trying to statically link Gtk2Hs applications. E.g., the Gtk2Hs demo ...\demo\hello\World.hs from the SourceForge tarball: http://sourceforge.net/projects/gtk2hs/files/gtk2hs/0.10.1/gtk2hs-0.10.1.ta… My failed link attempts are listed at the end of this mail, but first some notes about my configuration. I need to develop in a Windows XP environment, where the toolset (Haskell Platform, Cabal, gtk, Gtk2hs, etc.) is located on a network drive (H:) instead of C:. (I know this may be unusual, but I need to be able to use any one of several PCs for development, with one common tools tree.) It is okay for temp files to reside on C:, but otherwise I have: H:\proc\tools\Haskell Platform\2010.1.0.0\ H:\proc\tools\gtk\ H:\proc\tools\cabal\ etc. (Fyi, I set up Cabal for this environment with some kind help: http://www.haskell.org/pipermail/haskell-cafe/2010-July/thread.html#80930 ). The _dynamically_ linked World.exe runs okay: PATH is: PATH= H:\proc\tools\Haskell Platform\2010.1.0.0\bin; H:\proc\tools\cabal\bin; H:\proc\tools\gtk\bin; H:\proc\tools\Haskell Platform\2010.1.0.0\lib\extralibs\bin; H:\proc\tools\Haskell Platform\2010.1.0.0\mingw\bin H:\proc\dev\Gtk2Hs\demo\hello>ghc --make World.hs [1 of 1] Compiling Main ( World.hs, World.o ) Linking World.exe ... Process completed, Exit Code 0. I had to add the mingw\bin dir to PATH to get cpp (needed during "cabal install gtk"), and the extralibs dir to get alex (for "cabal install gtk2hs-buildtools"). I would like users to be able to run my statically linked apps without having to worry about .dll issues at runtime, and just by double clicking on the app in a Windows explorer (file system display) window. When I try that for this dynamically linked World.exe, I get the error dialog: World.exe - Unable To Locate Component The application has failed to start because libcairo-2.dll was not found. Reinstalling the application may fix this problem. I consulted the ghc user guide, and did some Googling. I am not sure if the static linking difficulty is due to needing to link with the gtk (non Haskell) code (i.e., http://www.gtk.org/download-windows.html and http://ftp.gnome.org/pub/gnome/binaries/win32/gtk+/2.20/gtk+_2.20.0-1_win32… ), as perhaps alluded to in section "3. Static linking vs dynamic linking" in http://www.cs.virginia.edu/~wh5a/blog/Some%20GHC%20notes%20for%20myself.html (... The C libraries are not statically linked by default. To produce a completely static binary ...) or if it is just due in some way to my non-standard network drive configuration (ghc not finding some ".a" files library dirs to complete the static link, and so it defaults to dynamic linking?). Fyi, here are some of my dirs that have various *.a files: H:\proc\tools\cabal\cairo-0.11.0\ghc-6.12.1 H:\proc\tools\cabal\gio-0.11.0\ghc-6.12.1 H:\proc\tools\gtk\lib H:\proc\tools\Haskell Platform\2010.1.0.0\lib H:\proc\tools\Haskell Platform\2010.1.0.0\mingw\lib Other than PATH, I don't have any environment shell variable defined for searching a library path. Finally, here are my attempts to statically link: PATH is: PATH=H:\proc\tools\Haskell Platform\2010.1.0.0\bin;H:\proc\tools\cabal\bin;H:\proc\tools\gtk\bin;H:\proc\tools\Haskell Platform\2010.1.0.0\lib\extralibs\bin;H:\proc\tools\Haskell Platform\2010.1.0.0\mingw\bin H:\proc\dev\Gtk2Hs\demo\hello>ghc --make -static -optl-static World.hs [1 of 1] Compiling Main ( World.hs, World.o ) Linking World.exe ... H:\proc\tools\Haskell Platform\2010.1.0.0\mingw\bin\ld.exe: cannot find -lz collect2: ld returned 1 exit status Process completed, Exit Code 1. PATH is: PATH=H:\proc\tools\Haskell Platform\2010.1.0.0\bin;H:\proc\tools\cabal\bin;H:\proc\tools\gtk\bin;H:\proc\tools\Haskell Platform\2010.1.0.0\lib\extralibs\bin;H:\proc\tools\Haskell Platform\2010.1.0.0\mingw\bin H:\proc\dev\Gtk2Hs\demo\hello>ghc --make -optl-static -optl-pthread World.hs [1 of 1] Compiling Main ( World.hs, World.o ) Linking World.exe ... H:\proc\tools\Haskell Platform\2010.1.0.0\mingw\bin\ld.exe: cannot find -lz collect2: ld returned 1 exit status realgcc.exe: unrecognized option `-pthread' Process completed, Exit Code 1. Any advice, pointers, etc. would be appreciated. Thanks very much, -- Peter

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