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hClose: illegal operation (handle is finalized) and/or <<loop>>
by Travis Erdman 18 Aug '10

18 Aug '10

> > putStrLn $ show result > > > > <<loop>> > > That is the *real* error message. > > It means that where you define result, you refer to result > also on the right-hand side in a way that is an endless > loop instead of well-behaved recursion. > > We would have to see more of your code to be able to > say much more. Thank you very much ... now I have isolated the offending "loop" code: I am folding with a function that takes a tree and the input data, and delivers a "processed"/"updated" tree. The function that is used in the fold is "calcTree". calcTree is also recursive ... it updates the root node, then recursively calls itself to update each of its children, which is all working fine. Here are simplified versions of my structure: data Tree a = Node a [Tree a] calcTree (Node oldData oldsubtree) input = Node newData recurseChildren where {-calculate newData from oldData and input-} recurseChildren = if {BOOLEANS} then oldsubtree else map (\old -> calcTree old input) oldsubtree And, in words: if certain conditions become true, there will be no change in any decedent node from the current node (ie all newData = oldData, and tree structure never changes), so rather than waste exponential time traversing all the decedent nodes, I'd like to just re-use the ones I already have, as they are identical. This is "pruning" the tree. If I remove this pruning, the "<<LOOP>>" objection ceases and the code executes as expected ... except it takes lots longer than it should, because there are usually lots of oppt'ys for pruning, and I'd rather not explicitly traverse all of them. How can I make this work? Thanks again for all the A's to my Q's !

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Man/dir/help pages for haskell?
by Kevin Burke 17 Aug '10

17 Aug '10

Hello, I've been learning Haskell by working through the O'Reilly book, but a lot of the coding I do is offline. One thing that's been really helpful in other languages are the terminal instructions and detail for functions. Is there something similar for Haskell? I know I can type :help at the prompt but that only gives you info on the broad commands. Thanks for your help, Kevin

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hClose: illegal operation (handle is finalized) and/or <<loop>>
by Travis Erdman 17 Aug '10

17 Aug '10

I read in some data from a file (via decodeFile), process it, and write the result back to a file. My last line of code is .... encodeFile (filename++".sol") (gametype,gstree,newiterlist,newstamplist,result) and it is all of the sudden now causing this run-time error: mincfr: kclv5.sol: hClose: illegal operation (handle is finalized) It has been working fine for quite a while, but now quite mysteriously gives this error. (But obviously I have been heavily tinkering with the processing part of the program). If I remove the encodeFile line and instead ... putStrLn $ show result I get this run-time error: <<loop>> Help? thanks, Travis

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very detailed monad tutorials
by Michael Vanier 17 Aug '10

17 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 15 Aug '10

15 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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