April 2010 - Beginners

cabal haddock problem
by Ashish Agarwal 13 May '10

13 May '10

I've setup a cabal file for a project. Building works fine but cabal haddock gives the following error: src/AFRP/AFRP.hs:1:0: File name does not match module name: Saw: `Main' Expected: `AFRP.AFRP' However, the module name specified in AFRP.hs is in fact AFRP.AFRP so why is haddock seeing Main, especially since cabal build completes successfully?

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Problem installing Haskell platform on Ubuntu Karmic
by Kim Stebel 10 May '10

10 May '10

Hello List, I can't compile the haskell platform. I followed this guide http://davidsiegel.org/haskell-platform-in-karmic-koala/ and downloaded http://hackage.haskell.org/platform/2010.1.0.0/haskell-platform-2010.1.0.0.… as suggested in comment 5. configure runs fine, but when I try to "make" it, it fails to compile the network package. Preprocessing library network-2.2.1.7... Building network-2.2.1.7... [1 of 5] Compiling Network.URI ( Network/URI.hs, dist/build/Network/URI.o ) [2 of 5] Compiling Network.Socket.Internal ( dist/build/Network/Socket/Internal.hs, dist/build/Network/Socket/Internal.o ) [3 of 5] Compiling Network.Socket ( dist/build/Network/Socket.hs, dist/build/Network/Socket.o ) [4 of 5] Compiling Network.BSD ( dist/build/Network/BSD.hs, dist/build/Network/BSD.o ) [5 of 5] Compiling Network ( Network.hs, dist/build/Network.o ) Network.hs:274:15: Warning: Pattern match(es) are overlapped In a case alternative: a -> ... Network/URI.hs:108:7: Could not find module `Text.ParserCombinators.Parsec': Perhaps you haven't installed the profiling libraries for package `parsec-2.1.0.1'? Use -v to see a list of the files searched for. Error: Building the network-2.2.1.7 package failed make: *** [build.stamp] Error 2 I'd really appreciate any help.

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Data.Binary.Get for large files
by Philip Scott 30 Apr '10

30 Apr '10

Hello again folks, Sorry to keep troubling you - I'm very appreciative of the help you've given so far. I've got one more for you that has got me totally stumped. I'm writing a program which deals with largish-files, the one I am using as a test case is not stupidly large at about 200mb. After three evenings, I have finally gotten rid of all the stack overflows, but I am unfortunately left with something that is rather unfeasably slow. I was hoping someone with some keener skills than I could take a look, I've tried to distill it to the simplest case. This program just reads in a file, interpreting each value as a double, and does a sort of running average on them. The actual function doesn't matter too much, I think it is the reading it in that is the problem. Here's the code: import Control.Exception import qualified Data.ByteString.Lazy as BL import Data.Binary.Get import System.IO import Data.Binary.IEEE754 myGetter acc = do e <- isEmpty if e == True then return acc else do t <- getFloat64le myGetter $! ((t+acc)/2) myReader file = do h <- openBinaryFile file ReadMode bs <- BL.hGetContents h return $ runGet (myGetter 0) bs main = do d <- myReader "data.bin" evaluate d This takes about three minutes to run on my (fairly modern) laptop.. The equivilant C program takes about 5 seconds. I'm sure I am doing something daft, but I can't for the life of me see what. Any hints about how to get the profiler to show me useful stuff would be much appreciated! All the best, Philip PS: If, instead of computing a single value I try and build a list of the values, the program ends up using over 2gb of memory to read a 200mb file.. any ideas on that one?

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Re: [Haskell-beginners] Re: Iterating through a list of char...
by Daniel Fischer 29 Apr '10

29 Apr '10

Am Friday 30 April 2010 03:13:45 schrieben Sie: > Sorry to bug you again :\ > I'm getting the following when trying to run it this time...Absolutely > no idea why! > > Not in scope: type constructor or class `Word8' I forgot import Data.Word (Word8) :( > > (I replied to you directly since I didn't want to make the already-large > thread on the mailing list even larger ;) I do :) > > Jean-Nicolas > > On 2010-04-29, at 8:29 PM, Daniel Fischer wrote: > > Am Freitag 30 April 2010 02:03:44 schrieb Jean-Nicolas Jolivet: > >> I tried to run your code, however, I'm getting a: > >> "Module `Data.ByteString.Unsafe' does not export `unsafeAt'" > > > > Yes, of course. It's "unsafeIndex" (unsafeAt is for arrays), d'oh. > > > >> (Using the latest ghc... > >> http://www.haskell.org/ghc/docs/latest/html/libraries/bytestring-0.9. > >>1.6 /Data-ByteString-Unsafe.html ) > >> > >> Jean-Nicolas Jolivet

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Converting an imperative program to haskell
by Hein Hundal 29 Apr '10

29 Apr '10

Hi, I am new to Haskell. I programmed in imperative languages for many years and I have been using Mathematica for the last 15 years which mixes functional and imperative programming. I started learning Haskell about 2 months ago mostly by reading the first 13.5 chapters of Real World Haskell and doing about 50 of the Euler Problems. "The Evolution of a Haskell Programmer", the Haskell Wiki, and this listserv have also been helpful. I figured I should try a larger program in Haskell, so I am converting one of my Mathematica programs, a simulator for the card game Dominion, over to Haskell. Most of that is going well except for one patch of imperative code. My Haskell version of this code is ugly. I was hoping someone could recommend a better way to do it. I will paste a simplified version of the code below. If necessary, I can provide all the other code used for this simplified example (about 30 more lines of Haskell code, or an equivalent program in C++ (130 lines of code). Cheers, Hein -----------------Ugly Haskell Code------------------------- data LoopState = LSt GameState Strat Int Int Int [Int] Int Int Bool proc :: GameState -> Strat -> (Int, Int, GameState) proc gs' strat = let iA = 1 iTh = 0 i = 0 aA = replicate 20 0 iB = 1 iC = 0 bDone = False gs = apps ("<<<"++show(gs')++">>>") gs' lst = LSt gs strat iA iTh i aA iB iC bDone lst2 = until isDone procloop lst isDone (LSt gs strat iA iTh i aA iB iC bDone) = bDone output (LSt gs strat iA iTh i aA iB iC bDone) = (iB, iC, appd aA gs) in output lst2 procloop :: LoopState -> LoopState procloop (LSt gs' strat iA iTh i aA iB iC bDone) = do let iCd = stratchoose strat gs' iA iTh iB aA let gs = apps ("iB "++show iB++ "\n" ) gs' if iA<=0 || i>=20 || actcd gs <=0 || iCd == -1 then LSt gs strat iA iTh (i+1) aA iB iC True else if iCd == 1 then LSt gs strat iA (iTh+1) (i+1) aA iB iC False else let gs2 = delh iCd gs aA2 = append aA iCd (iA3, iC3, iB3, gs3, aA3) = doAct iA iC iB gs2 aA2 strat iCd (iA4, iC4, iB4, gs4, aA4) = doAct iA3 iC3 iB3 gs3 aA3 strat iCd in if iTh>0 then LSt gs4 strat iA4 (iTh-1) (i+1) aA4 iB4 iC4 False else LSt gs3 strat iA3 (iTh-1) (i+1) aA3 iB3 iC3 False ---------------end of Haskell code-------------------------------

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Iterating through a list of char...
by Jean-Nicolas Jolivet 29 Apr '10

29 Apr '10

Hi there! I'm trying to iterate through each character of a string (that part I can do!) however, I need to apply a transformation to each character...based on the previous character in the string! This is the part I have no clue how to do! I'm totally new to Haskell so I'm pretty sure I'm missing something obvious... I tried with list comprehensions...map... etc... but I can't figure out how I can access the previous character in my string in each "iteration".... to use simple pseudo code, what i need to do is: while i < my_string length: if my_string[i-1] == some_char: do something with my_string[i] else do something else with my_string[i] I'm using imperative programming here obviously since it's what I am familiar with...but any help as to how I could "translate" this to functional programming would be really appreciated! Jean-Nicolas Jolivet

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[Int] oddness with summing large lists
by Philip Scott 29 Apr '10

29 Apr '10

Hi all, I kind of stumbled into this while doing something else - but I thought it was worth posting since I have never actually managed to crash ghci before :) Prelude> let d = [1..1000000000000] :: [Int] Prelude> sum d 0 0? I mean, I might have expected an integer overflow, but 0? And now the really odd part... take away one zero Prelude> let d = [1..100000000000] :: [Int] Prelude> sum d < ghci crashes and quits > A slightly more reasonable number.. Prelude> let d = [1..10000000] :: [Int] Prelude> sum d *** Exception: stack overflow At least I can appreciate what is going on in this one :) I'm aware that this is a silly way to sum from 1 to n, but I am at a loss to explain the above behavior (Except the stack overflow, that is fair enough). I'm also aware that Int is a machine Int, not a clever infinite haskell Int. Any ideas? All the best, Phil

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Re: [Haskell-beginners] Re: Iterating through a list of char...
by Hein Hundal 29 Apr '10

29 Apr '10

Jean-Nicolas, Here is a variation on Corentin's solution: -- This function replaces every character that -- follows an 'a' with 'A' repA :: String -> String repA s = zipWith f (' ':s) s where f 'a' y = 'A' f x y = y Cheers, Hein > From: Jean-Nicolas Jolivet <jeannicolascocoa(a)gmail.com> > > I'm trying to iterate through each character of a string > (that part I > can do!) however, I need to apply a transformation to each > character...based on the previous character in the string! > This is the part I have no clue how to do! > [snip] > while i < my_string length: > if my_string[i-1] == some_char: > do something with > my_string[i] > else > do something else > with my_string[i]

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let (x',seed') = x seed
by Amy de Buitléir 29 Apr '10

29 Apr '10

I'm confused by the syntax this function definition I found in a tutorial: bind :: (a → StdGen → (b,StdGen)) → (StdGen → (a,StdGen)) → (StdGen → (b,StdGen)) bind f x seed = let (x',seed') = x seed in f x' seed' The part that confuses me is "let (x',seed') = x seed". The left side is a tuple, and the right side is two separate values, so how does the binding work? I gather from the context of the example (a tutorial on monads) that the value of x is bound to x', and the value of seed is bound to seed', so that "bind f" is a function that acts on a tuple and produces a new tuple. Is that correct? Thank you in advance to anyone who can de-confuse me.

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Feedback on google code jam practice
by Aur Saraf 29 Apr '10

29 Apr '10

Hello, New to this list, so hello all :-). I've uploaded practice solutions for google code jam questions here: http://github.com/SonOfLilit/codejam . As I'm still learning idiomatic Haskell, I'd love to get feedback on my code. How would /you/ do it? Thanks a lot, -- Aur

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