- Beginners - Haskell.org

Primes calculation
by legajid 13 Mar '10

13 Mar '10

Hi, i'm trying to write a function to find all primes from 2 to N. My idea is : take the first number (2) try to find whether it's a multiple of one of all existing primes ([] at first) add 2 to the list of primes take the following number (3) find if multiple of existing primes ([2]) add 3 to the list of primes take the following number (4) find if multiple of existing primes ([2, 3]) do not add 4 to the list of primes ... take the following number (8) find if multiple of existing primes ([2, 3, 5, 7]) do not add 8 to the list of primes take the following number (9) find if multiple of existing primes ([2, 3, 5, 7]) do not add 9 to the list of primes (multiple of 3) and so on... So, i would like a function like : f (x : xs) = g x : f xs g would return x if x is prime, [] otherwise But g would use the preceding value of f (list of primes before the calculation for x) that is a result of g itself. f needs g that needs f : what's wrong with my mind ? Perhaps i am still under control of imperative programming ? Thanks for your help, Didier.

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Issues With Install Script in Haskell Platform 2009.2.0.2
by aditya siram 12 Mar '10

12 Mar '10

Hi all, I am running Ubuntu Karmic Kameleon and GHC 6.10.4 and am having issues with the Haskell Platform 2009.2.0.2 source installer. The 'configure' and 'make' step work fine, but 'make install' errors with: scripts/install.sh Installing mtl-1.1.0.2... Error: The mtl-1.1.0.2/Setup script does not exist or cannot be run And indeed mtl-1.1.0.2/Setup does not exist. It appears that the 'make' step builds some packages and not others. The 'Setup' executable is only in the following package directories: ./network-2.2.1.4/Setup ./editline-0.2.1.0/Setup ./alex-2.3.1/Setup ./happy-1.18.4/Setup ./haskell-platform-2009.2.0.2/Setup ./fgl-5.4.2.2/Setup ./cgi-3001.1.7.1/Setup ./cabal-install-0.6.2/Setup ./HTTP-4000.0.6/Setup ./GLUT-2.1.1.2/Setup ./OpenGL-2.2.1.1/Setup ./zlib-0.5.0.0/Setup ./time-1.1.2.4/Setup Here is the current contents of my global and local packages: Global Packages: Cabal-1.6.0.3, HUnit-1.2.0.3, QuickCheck-1.2.0.0, array-0.2.0.0, base-3.0.3.1, base-4.1.0.0, bytestring-0.9.1.4, containers-0.2.0.1, directory-1.0.0.3, (dph-base-0.3), (dph-par-0.3), (dph-prim-interface-0.3), (dph-prim-par-0.3), (dph-prim-seq-0.3), (dph-seq-0.3), extensible-exceptions-0.1.1.0, filepath-1.1.0.2, (ghc-6.10.4), ghc-prim-0.1.0.0, haddock-2.4.2, haskell-src-1.0.1.3, haskell98-1.0.1.0, hpc-0.5.0.3, html-1.0.1.2, integer-0.1.0.1, mtl-1.1.0.2, network-2.2.1.2, old-locale-1.0.0.1, old-time-1.0.0.2, packedstring-0.1.0.1, parallel-1.1.0.1, parsec-2.1.0.1, pretty-1.0.1.0, process-1.0.1.1, random-1.0.0.1, regex-base-0.72.0.2, regex-compat-0.71.0.1, regex-posix-0.72.0.3, rts-1.0, stm-2.1.1.2, syb-0.1.0.1, template-haskell-2.3.0.1, time-1.1.4, unix-2.3.2.0, xhtml-3000.2.0.1 Local Packages: haskelldb-0.13, haskelldb-hsql-0.10, haskelldb-hsql-sqlite3-0.10, hsql-1.8.1, hsql-sqlite3-1.8.1

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Issues Installing haskelldb-hsql-sqlite3
by aditya siram 12 Mar '10

12 Mar '10

Hi all, I am having an issue installing haskelldb-hsql-sqlite3. When I do a 'cabal-install install haskelldb-hsql-sqlite3', it tries to rebuild 'directory-1.0.0.3' even though it is available and gives the following error: Building directory-1.0.0.3... System/Directory.hs:81:7: Could not find module `Control.Exception.Base': it is a member of the hidden package `base' Use -v to see a list of the files searched for. Any feedback is appreciated. -deech

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error with cabal install readline
by Ashish Agarwal 11 Mar '10

11 Mar '10

I'm on a Mac, and trying to install readline through cabal. I installed the readline library through macports and thus tried the command below. However, readline is still not found. Are there other options I can try? Thank you. $ cabal install readline --extra-include-dirs=/opt/local/include/ --extra-lib-dirs=/opt/local/lib/ Resolving dependencies... Configuring readline-1.0.1.0... checking for gcc... gcc checking for C compiler default output file name... a.out checking whether the C compiler works... yes checking whether we are cross compiling... no checking for suffix of executables... checking for suffix of object files... o checking whether we are using the GNU C compiler... yes checking whether gcc accepts -g... yes checking for gcc option to accept ISO C89... none needed checking for GNUreadline.framework... checking for readline... no checking for tputs in -lncurses... yes checking for readline in -lreadline... yes checking for rl_readline_version... yes checking for rl_begin_undo_group... no configure: error: readline not found, so this package cannot be built See `config.log' for more details. cabal: Error: some packages failed to install: readline-1.0.1.0 failed during the configure step. The exception was: exit: ExitFailure 1

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Library source
by Nicolas Couture-Grenier 10 Mar '10

10 Mar '10

Where can I find the haskell sources of the standard library? I searched for all files ending in .hs in my haskell folder and found none.

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Better way to write this?
by Tim Perry 09 Mar '10

09 Mar '10

I'm working my way through the 99 sample programs for haskell and I'm on #4: find the number of elements in a list. http://haskell.org/haskellwiki/99_questions/1_to_10 I wrote the obvious recursion. Then I rewrote it using foldl myLength :: [a] -> Int myLength [] = 0 myLength xs = foldl addOne 0 xs where addOne lhs rhs = lhs + 1 However, the solution given uses notation that confuses my little mind. Can someone point me to a good resource on this notation? myLength :: [a] -> Int myLength = foldr (\x n -> n + 1) 0 Thanks, Tim

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Re: Printing the result of a function evaluation
by Travis Erdman 09 Mar '10

09 Mar '10

Steve, I should have provided the following details ... The code I pasted was a timing function I've come across in various Haskell literature, and it "works" as is ... at least, the timing results it prints are plausible. My sole "contribution" to the code was the "print v" line, in an attempt to have it print BOTH the result of the function evaluation AND the time it took to evaluate it (and still only evaluating the function once). As noted, my "print v" prints only "()" regardless of the function -- not what I wanted. But the timing part still works. There has got to be an easy fix here, but I have yet to discover it ...

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Re: [Haskell-beginners] Help in pattern matching
by Stephen Tetley 08 Mar '10

08 Mar '10

Hi Joe - I've cc'ed back to list... I'm supposing from what you've said that only DataConst1 & DataConst2 have ids - so I've made a type synonym VId type VId = Int data TestData = DataConst1 VId String | DataConst2 VId Int | DataConst3 Sting String | DataConst4 String Int If both DataConst1 & DataConst2 have VId's the you right - you have to pattern match for both: setVid_to_4 :: TestData -> TestData setVid_to_4 (DataConst1 _ y) = DataConst1 4 y setVid_to_4 (DataConst2 _ y) = DataConst2 4 y setVid_to_4 var = var If all the cases of TestData had a VId e.g.: data TestData = DataConst1 VId String | DataConst2 VId Int | DataConst3 VId Sting String | DataConst4 Vid String Int ... then you could factor out VId to get these two data types data FactoredTD = FactoredTD VId TD data TD = DC1 String | DC2 Int | DC3 String String | DC4 String Int setVid_to_4 :: FactoredTD -> FactoredTD setVid_to_4 (FactoredTD _ y) = FactoredTD 4 y This factoring transformation is quite common. For your particular data type you could factor the other way: data TestDataX = DataConst1_or_2 VId (Either String Int) | DataConst3 Sting String | DataConst4 String Int setVid_to_4 :: TestDataX -> TestDataX setVid_to_4 (DataConst1_or_2 _ y) = DataConst1_or_2 4 y setVid_to_4 y = y However this factoring is quite horrible - I can't think of anywhere where I've seen it. Generally I wouldn't be too concerned about redundancy in pattern matching. If its easy to consistently name all the constructors in an algebraic data type, then that's a strong indication that you've put it into a good form. If you're still taxed by duplicate pattern matching you can then write projection and modification functions to do it once and once only: -- Has to return a Maybe as DC3 and DC4 have no ID getVId :: TestData -> Maybe VId getVid (DataConst1 iden _) = Just iden getVid (DataConst2 iden _) = Just iden getVid _ = Nothing -- note - uses an update function that accesses the original value -- rather than a simple replace -- updateVid :: (Vid -> Vid) -> TestData -> TestData updateVid f (DataConst1 iden y) = DataConst1 (f iden) y updateVid f (DataConst2 iden y) = DataConst2 (f iden) y updateVid f var = var incrementVid :: TestData -> TestData incrementVid = updateVid (\x -> x+1) Best wishes Stephen On 8 March 2010 12:19, Joe Fox <fox.joe87(a)gmail.com> wrote: > Hi Guys, > > First thanks for all yours inputs. > > Actually test is just a simple function , actually my actual function is > pretty complex and huge ... > > let me redefine Test function , i guess i took a bad example which created > lot of confusion > > the type of the test functions will be something like this , (i don't have > any ambiguity on type of the function, ) > > test:: TestData -> TestData > > My question is like is there any way i can match multiple pattern in a > function statement . > > like this (I know this doesnt work , but just trying to explain whats on my > mind) > > > test var@(DataConst1 x y) == var@(DataConst2 x y) = var{id=4} -- let's say > id is the record holder for first record > test var@(_) = var --- any for other two its returns the same > > the working code would be like > test var@(DataConst1 x y) = var{id=4} > test var@(DataConst2 x y) = var{id=4} --- I want these two lines in one > line, because the logic is same > test var@(_) = var > > some thing of this sort. > I hope i didnt add to the confusion > > > Thanks > Joe >

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Printing the result of a function evaluation
by Travis Erdman 08 Mar '10

08 Mar '10

Why oh why doesn't this work as I expect, and what is the simplest fix? speedTest 4 = 44 time :: (Show t) => IO t -> IO t time a = do start <- getCPUTime v <- a end <- getCPUTime let diff = (fromIntegral (end - start)) / (10^12) print v -- WHY DOESNT THIS WORK? ALWAYS PRINTS () .... printf "Computation time: %0.3f sec\n" (diff :: Double) return v main :: IO () main = do args <- getArgs time $ speedTest (read $ head args) `seq` return ()

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Call stack
by Nicolas Couture-Grenier 08 Mar '10

08 Mar '10

Is there an easy way to print the call stack of the evaluation of an expression in ghci without setting breakpoints or changing the code? That would be very useful.

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