March 2010 - Haskell-Cafe

Occurs check error, help!
by boblettoj 22 Mar '10

22 Mar '10

Hi i am writing a shuffle function which takes in a random number i and a list and then produces a jumbled up version of the list at the end. Here is what i have at the moment (i'm not sure if it works yet as it won't compile! --function used to shuffle cards --list equals random member of array plus the rest of the array --i is randomly generated from range of length equal to that of cards. shuffle :: int -> [a] -> [a] shuffle i [] = [] shuffle i [cards] = (cards!!i) + (shuffle (randomR (0, ((length cards)-2))) (delete (cards!!i) cards)) I get this error message when i try to compile: Occurs check: cannot construct the infinite type: a = [[a]] I'm very new to haskell so you'll have to excuse any ignorance! thanks -- View this message in context: http://old.nabble.com/Occurs-check-error%2C-help%21-tp27966341p27966341.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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RE: No warning in GHC
by Simon Peyton-Jones 22 Mar '10

22 Mar '10

[Redirecting to haskell-cafe] Try http://www.haskell.org/ghc/docs/latest/html/users_guide/flag-reference.html… With -Wall I get bash-3.2$ ghc -c -Wall Foo.hs Foo.hs:3:0: Warning: Definition but no type signature for `func' Inferred type: func :: forall t t1. (Num t1) => t -> t1 Foo.hs:3:5: Warning: Defined but not used: `x' Foo.hs:4:10: Warning: This binding for `x' shadows the existing binding bound at Foo.hs:3:5 | -----Original Message----- | From: cvs-ghc-bounces(a)haskell.org [mailto:cvs-ghc-bounces@haskell.org] On Behalf Of | Grigory Sarnitskiy | Sent: 21 March 2010 19:37 | To: cvs-ghc(a)haskell.org | Subject: No warning in GHC | | Hello! I wonder, why GHC shows no warning in situations like | | func x = x | where x = 5 | | Sometimes I forget about argument names used, and use the same names in let or where. | And it can take me quite a lot time to understand what is wrong. I think it would be | nice to warn users if the argument is "redefined" in let/where clause. Shall I report | a bug/feature request? I'm using ghc 6.10.4. | | btw, I couldn't post to IRC: #ghc :Cannot send to channel | | _______________________________________________ | Cvs-ghc mailing list | Cvs-ghc(a)haskell.org | http://www.haskell.org/mailman/listinfo/cvs-ghc

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Bubble sort algorithm implementations (Haskell vs. C)
by Yasir Arsanukaev 22 Mar '10

22 Mar '10

Hello. I have written 2 implementation of bubble sort algorithm in C and Haskell. Haskell implementation: module Main where main = do contents <- readFile "./data" print "Data loaded. Sorting.." let newcontents = bubblesort contents writeFile "./data_new_ghc" newcontents print "Sorting done" bubblesort list = sort list [] False rev = reverse -- separated. To see rev2 = reverse -- who calls the routine sort (x1:x2:xs) acc _ | x1 > x2 = sort (x1:xs) (x2:acc) True sort (x1:xs) acc flag = sort xs (x1:acc) flag sort [] acc True = sort (rev acc) [] False sort _ acc _ = rev2 acc I've compared these two implementations having run both on file with size of 20 KiB. C implementation took about a second, Haskell — about 1 min 10 sec. I have also profiled the Haskell application: Compile for profiling: C:\Temp> ghc -prof -auto-all -O --make Main Profile: C:\Temp> Main.exe +RTS -p and got these http://hpaste.org/fastcgi/hpaste.fcgi/view?id=24190#a24190 This is a pseudocode of the algorithm: procedure bubbleSort( A : list of sortable items ) defined as: do swapped := false for each i in 0 to length(A) - 2 inclusive do: if A[i] > A[i+1] then swap( A[i], A[i+1] ) swapped := true end if end for while swapped end procedure The performance may suffer from the memory allocation for the list. I wonder if it's possible to make Haskell implementation work faster without changing the algorithm (there's are actually a few tricks to make it work faster, but neither implementations have these optimizations). I'm interested not in particular algorithm performance but rather in performance of its implementations in various languages. I compiled the Haskell implementation in GHC (Haskell Platform 2009.2.0.2), which is the latest available from the site.

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Google summer of code idea.
by Mujtaba Boori 21 Mar '10

21 Mar '10

Hello I would like to discuss my idea for google summer of code. I'm an active Haskell programmer . I have read the idea list . I think Improve Cabal's test support really suit my ability of haskell. I would like to give me advice about my idea . and how to improve my proposal . Thank you. -- Mujtaba Ali Alboori

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Data.Number.BigFloat/Fixed
by H. 21 Mar '10

21 Mar '10

Hello, With ghc-6.10.4 the following results are produced with numbers 2009.8.9: (7e-3 :: BigFloat Prec50) < (6e-4 :: BigFloat Prec50) False (7e-3 :: BigFloat Prec50) < (8e-4 :: BigFloat Prec50) True where in the sourecode is written: data BigFloat e = BF (Fixed e) Integer deriving (Eq, Ord) and import Data.Ratio newtype Fixed e = F Rational deriving (Eq, Ord, Enum, Real, RealFrac) H.

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How to set a breakpoint in GHCi
by Gracjan Polak 21 Mar '10

21 Mar '10

Hi all, Tried to use :break today, without success: guestbook-session-bug>ghci -DMIN_VERSION_template_haskell(a,b,c)=1 -isrc Main -i../happstack/happstack-ixset/src GHCi, version 6.12.1: http://www.haskell.org/ghc/ :? for help ... ... [10 of 13] Compiling Happstack.Data.IxSet ( ..\happstack\happstack-ixset\src\Happstack\Data\IxSet.hs, interpreted ) ... ... *Main> :m +Happstack.Data.IxSet *Main Happstack.Data.IxSet> :bre getOrd No breakpoints found at that location. *Main Happstack.Data.IxSet> What now? -- Gracjan

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Re: [Haskell-cafe] ANN: fixed-list -- A fixed length list library
by Casey McCann 21 Mar '10

21 Mar '10

Job Vranish <job.vranish(a)gmail.com> wrote: > Its main advantages are: > Very easy to use. > Almost entirely Haskell98 (the non Haskell98 pieces are not critical, just > nice) > The datatype is a member of Foldable, Traverable, Applicative, Monad, > etc... > Then length of the list is encoded in the type in a natural way. Unfortunately, it's very easy to get a context reduction stack overflow from GHC this way, which makes using such datatypes awkward for anything but very short lists. Explicit type annotations will often make things work, but at that point the type class isn't helping you anyway. For example, assuming the default stack size: import Data.FixedList fixed18 :: FixedList18 Int fixed18 = fromFoldable' [1..] -- this works fixed20 = 20 :. 20 :. fixed18 -- this works fixed22 :: FixedList22 Int fixed22 = 22 :. 22 :. fixed20 -- this only works with a type annotation show18 = show fixed18 -- this works -- this doesn't work: -- show20 = show fixed20 show20' :: FixedList20 Int -> String show20' list20 = show list20 show20 = show20' fixed20 -- this does work Using head and tail on longer lists fails likewise. I expect there's some way to make it work without simply increasing the stack depth, but I'm not sure how. Any thoughts? - C.

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Parallel Pi
by Artyom Kazak 21 Mar '10

21 Mar '10

Hello! I tried to implement the parallel Monte-Carlo method of computing Pi number, using two cores: --PROGRAM module Main where import Random import Data.Ratio import Data.List import System.IO import GHC.Conc main = do putStrLn "pi 1" putStr "n: " hFlush stdout t <- getLine piMonte (read t) >>= (putStrLn . show) piMonte n = do (g1, g2) <- split `fmap` getStdGen let r1 = r (n `div` 2) g1 r2 = r (n `div` 2 + n `mod` 2) g2 in return (ratio (r1 `par` (r2 `pseq` (merge r1 r2)))) where r n g = (length (filter id lAll), n) where l = take n . randomRs (0, 1) inCircle :: Double -> Double -> Bool inCircle a b = a*a + b*b <= 0.25 lAll = zipWith inCircle (l g1) (l g2) (g1, g2) = split g ratio :: (Int, Int) -> Double ratio (a, b) = fromRational (toInteger a % toInteger b * 16) merge (a, b) (c, d) = (a + c, b + d) --END But it uses only on core: C:\>ghc --make -threaded Monte.hs -fforce-recomp [1 of 1] Compiling Main ( Monte.hs, Monte.o ) Linking Monte.exe ... C:\>monte +RTS -N2 -s monte +RTS -N2 -s pi 1 n: 1000000 3.143616 2,766,670,536 bytes allocated in the heap 1,841,300 bytes copied during GC 5,872 bytes maximum residency (1 sample(s)) 23,548 bytes maximum slop 2 MB total memory in use (0 MB lost due to fragmentation) Generation 0: 5285 collections, 5284 parallel, 0.64s, 0.31s elapsed Generation 1: 1 collections, 1 parallel, 0.00s, 0.00s elapsed Parallel GC work balance: 1.00 (454838 / 454676, ideal 2) MUT time (elapsed) GC time (elapsed) Task 0 (worker) : 0.00s ( 9.33s) 0.00s ( 0.00s) Task 1 (worker) : 0.63s ( 9.33s) 0.00s ( 0.00s) Task 2 (worker) : 6.00s ( 9.34s) 0.64s ( 0.31s) Task 3 (worker) : 0.00s ( 9.34s) 0.00s ( 0.00s) SPARKS: 1 (0 converted, 1 pruned) INIT time 0.02s ( 0.00s elapsed) MUT time 6.63s ( 9.34s elapsed) GC time 0.64s ( 0.31s elapsed) EXIT time 0.00s ( 0.00s elapsed) Total time 7.28s ( 9.66s elapsed) %GC time 8.8% (3.2% elapsed) Alloc rate 416,628,033 bytes per MUT second Productivity 91.0% of total user, 68.6% of total elapsed We see that our one spark is pruned. Why? And another question. I compiled it also with -O: C:\>ghc --make -threaded Monte.hs -O -fforce-recomp [1 of 1] Compiling Main ( Monte.hs, Monte.o ) Linking Monte.exe ... C:\>monte +RTS -N2 -s monte +RTS -N2 -s pi 1 n: 1000000 3.148096 2,642,947,868 bytes allocated in the heap 1,801,952 bytes copied during GC 5,864 bytes maximum residency (1 sample(s)) 18,876 bytes maximum slop 2 MB total memory in use (0 MB lost due to fragmentation) Generation 0: 5077 collections, 5076 parallel, 0.08s, 0.05s elapsed Generation 1: 1 collections, 1 parallel, 0.00s, 0.00s elapsed Parallel GC work balance: 1.00 (445245 / 444651, ideal 2) MUT time (elapsed) GC time (elapsed) Task 0 (worker) : 3.94s ( 14.02s) 0.00s ( 0.00s) Task 1 (worker) : 0.00s ( 14.02s) 0.00s ( 0.00s) Task 2 (worker) : 5.61s ( 14.03s) 0.08s ( 0.05s) Task 3 (worker) : 0.00s ( 14.05s) 0.00s ( 0.00s) SPARKS: 1 (0 converted, 0 pruned) INIT time 0.02s ( 0.02s elapsed) MUT time 9.55s ( 14.03s elapsed) GC time 0.08s ( 0.05s elapsed) EXIT time 0.00s ( 0.00s elapsed) Total time 9.64s ( 14.09s elapsed) %GC time 0.8% (0.3% elapsed) Alloc rate 276,386,705 bytes per MUT second Productivity 99.0% of total user, 67.7% of total elapsed We see, that with -O, 2 worker threads were doing some job, but overall performance is not better. >From one spark, zero - converted, zero - pruned. Is it a bug?

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ANN: regular-web 0.1 | generic web programming in Haskell
by Chris Eidhof 21 Mar '10

21 Mar '10

From the ZuriHac hackathon, I am happy to announce the first release of the regular-web package [1]. The package contains functions for generic web programming: generating HTML, JSON and Formlets. It is based on the regular generic programming library [2]. Generic HTML and forms are often not sufficient for real purposes, so we encourage the use of regular-web in combination with the fclabels package [3]. There is an example in the documentation of the Formlets module. Furthermore, there is a slightly larger example [4] on github that shows off some more features of the package. This is a preliminary release and the API will probably change. I am interested to see what other people think of this. So please send me your comments or fork the project on github. [1]: http://hackage.haskell.org/package/regular-web [2]: http://hackage.haskell.org/package/regular [3]: http://hackage.haskell.org/package/fclabels [4]: http://github.com/chriseidhof/regular-web/blob/master/Example.lhs

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ANN: fixed-list -- A fixed length list library
by Job Vranish 20 Mar '10

20 Mar '10

I uploaded a new fixed length list library to hackage: http://hackage.haskell.org/package/fixed-list Its main advantages are: Very easy to use. Almost entirely Haskell98 (the non Haskell98 pieces are not critical, just nice) The datatype is a member of Foldable, Traverable, Applicative, Monad, etc... Then length of the list is encoded in the type in a natural way. Comments/critiques/suggestions welcome :) - Job

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