November 2011 - Haskell-Cafe

Re: [Haskell-cafe] memorize function with number parameterized types in GHC
by oleg＠okmij.org 09 Nov '11

09 Nov '11

Bin Jin wrote: > Here is a function that will be called everytime by `(*)` in `Num` typeclass > > montgKeys :: (PostiveN p, Integral a, Bits a) => p -> a > > as you can imagine, I always pass (undefined :: p) as parameter to > `montgKeys`, so if it's handled well, it should be memorized for > future usage. But tracing shows that both `p2num` and `montgKeys` are > evaluated every time being called. First of all, let us get rid of the argument p. Let's define > newtype W p a = W{unW:: a} then we can easily re-write montgKeys to give it the following signature: > > montgKeys :: (PostiveN p, Integral a, Bits a) => W p a You can use ScopedTypevariables to set the needed 'p' from the context. So, montgKeys becomes a polymorphic constant, quite like minBound. Now, the hope is that when the types p and a are determined, GHC could specialize montgKeys and turn it into a real constant. Perhaps some RULE or specialize pragmas may help...

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capri cabal-dev virtualenv cab
by Rustom Mody 08 Nov '11

08 Nov '11

Does anyone give me a little comparison of these? What would all my requirements be? Not sure... these seem important for me 1. Need to sandbox not just haskell-projects but ghc (different compilations/versions) itself 2. Stability of the (sandboxing) tool 3. Correcting the deficiencies of cabal (eg inability to uninstall, quarrels between cabal and apt etc) OS: Debian/testing System haskell: 7.0.4 My question on beginners for a little more context: http://www.haskell.org/pipermail/beginners/2011-November/008943.html capri: http://www.haskell.org/haskellwiki/Capri virtualenv: https://github.com/Paczesiowa/virthualenv cabal-dev: http://corp.galois.com/blog/2010/12/20/cabal-dev-sandboxed-development-buil… cab: http://www.mew.org/~kazu/proj/cab/en/

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Send SMTP email with plain text authentication
by Justin Greene 08 Nov '11

08 Nov '11

Hi All, I am attempting to send email to an SMTP server that uses plain text authentication. So far I have tried both the SMTPClient ( http://hackage.haskell.org/package/SMTPClient-1.0.4) and HaskellNet ( http://hackage.haskell.org/package/HaskellNet) The SMTPClient library works fine for connecting to a non-restricted local SMTP server, but doesn't seem to have any ability to authenticate, so unless it gets updated I believe it's out of the picture (unless I'm wrong about it not being able to authenticate). The HaskellNet SMTP bindings are having problems connecting to my local postfix server even without authentication involved. I get a "connect: does not exist (Connection refused)" error. This seems odd to me since the SMTPClient code connects to my local server just fine, so I don't believe this is a postfix configuration issue (I could be wrong here of course). Here is the code I am currently using: http://hpaste.org/53764 Any help is appreciated. Thanks,

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Putting constraints on "internal" type variables in GADTs
by Anupam Jain 08 Nov '11

08 Nov '11

Hi all, I was trying to do something very simple with GADTs when I ran into this problem - -- My datatype data T o where Only ∷ o → T o TT ∷ T o1 → (o1 → o2) → T o2 -- Show instance for debugging instance Show o ⇒ Show (T o) where show (Only o) = "Only " ⊕ (show o) show (TT t1 f) = "TT (" ⊕ (show t1) ⊕ ")" When I try to compile this I get the following - Could not deduce (Show o1) arising from a use of `show' from the context (Show o) While I understand why I get this error, I have no idea how to fix it! I cannot put a Show constraint on o1 because that variable is not exposed in the type of the expression. I can work around this by changing my data type declaration to include Show constraints but I don't want to restrict my data type to only Showable things just so I could have a "Show" instance for debugging - Only ∷ Show o ⇒ o → T o TT ∷ (Show o1, Show o2) ⇒ T o1 → (o1 → o2) → T o2 What else can I do to declare a Show instance for my datatype? Thanks, Anupam Jain

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Project Euler Problem 357 in Haskell
by mukesh tiwari 08 Nov '11

08 Nov '11

Hello all Being a Haskell enthusiastic , first I tried to solve this problem in Haskell but it running for almost 10 minutes on my computer but not getting the answer. A similar C++ program outputs the answer almost instant so could some one please tell me how to improve this Haskell program. import Control.Monad.ST import Data.Array.ST import Data.Array.Unboxed import Control.Monad prime :: Int -> UArray Int Bool prime n = runSTUArray $ do arr <- newArray ( 2 , n ) True :: ST s ( STUArray s Int Bool ) forM_ ( takeWhile ( \x -> x*x <= n ) [ 2 .. n ] ) $ \i -> do ai <- readArray arr i when ( ai ) $ forM_ [ i^2 , i^2 + i .. n ] $ \j -> do writeArray arr j False return arr pList :: UArray Int Bool pList = prime $ 10 ^ 8 divPrime :: Int -> Bool divPrime n = all ( \d -> if mod n d == 0 then pList ! ( d + div n d ) else True ) $ [ 1 .. truncate . sqrt . fromIntegral $ n ] main = putStrLn . show . sum $ [ if and [ pList ! i , divPrime . pred $ i ] then pred i else 0 | i <- [ 2 .. 10 ^ 8 ] ] C++ program which outputs the answer almost instant. #include<cstdio> #include<iostream> #include<vector> #define Lim 100000001 using namespace std; bool prime [Lim]; vector<int> v ; void isPrime () { for( int i = 2 ; i * i <= Lim ; i++) if ( !prime [i]) for ( int j = i * i ; j <= Lim ; j += i ) prime [j] = 1 ; for( int i = 2 ; i <= Lim ; i++) if ( ! prime[i] ) v.push_back( i ) ; //cout<<v.size()<<endl; //for(int i=0;i<10;i++) cout<<v[i]<<" ";cout<<endl; } int main() { isPrime(); int n = v.size(); long long sum = 0; for(int i = 0 ; i < n ; i ++) { int k = v[i]-1; bool f = 0; for(int i = 1 ; i*i<= k ; i++) if ( k % i == 0 && prime[ i + ( k / i ) ] ) { f=1 ; break ; } if ( !f ) sum += k; } cout<<sum<<endl; } Regards Mukesh Tiwari

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Re: [Haskell-cafe] cabal install: Could not find module `Text.XML.HXT.Arrow'
by dokondr 08 Nov '11

08 Nov '11

On Tue, Nov 8, 2011 at 2:31 PM, Audun Skaugen <audunskaugen(a)gmail.com>wrote: > Erik Hesselink wrote: > >> On Tue, Nov 8, 2011 at 12:16, Ivan Lazar Miljenovic >> <ivan.miljenovic(a)gmail.com> wrote: >> >> >>> On 8 November 2011 22:10, Erik Hesselink <hesselink(a)gmail.com> wrote: >>> >>> >>>> This is because hSimpleDB doesn't specify version ranges on its >>>> dependencies, when it should. Since hxt changed its module structure >>>> going from 9.0 to 9.1, hSimpleDB doesn't build against 9.0. >>>> >>>> You can try to build it by adding '--constraint=hxt==9.0.\*' after >>>> your cabal-install command. You can also ask the author to add version >>>> ranges to the package. >>>> >>>> >>> Is the escape needed if you're using single quotes? >>> >>> >> >> I don't know. I always escape *s in shell commands to be sure the >> shell doesn't expand them, but in most of the cases, it probably works >> without them. It will likely even work without quotes and without the >> escape, unless you have files matching the pattern. >> >> > The single quote doesn't expand anything, so you don't need to escape the > *. Actually, this will result in the backslash also being passed to cabal, > so I would be surprised if your command works at all. > No matter how you escape *, you get the same error: Preprocessing library hSimpleDB-0.3... Building hSimpleDB-0.3... src/Network/AWS/Authentication.hs:47:7: Could not find module `Text.XML.HXT.Arrow':

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cabal install: Could not find module `Text.XML.HXT.Arrow'
by dokondr 08 Nov '11

08 Nov '11

Hi, On Mac OSX, ghc-6.12.3, I have successfully installed the 'hxt' package: http://hackage.haskell.org/package/hxt-8.5.2 Registering hxt-9.1.4... Installing library in /Users/user/.cabal/lib/hxt-9.1.4/ghc-6.12.3 Now when I try to install hSimpleDB ( http://hackage.haskell.org/package/hSimpleDB) I get the following error: cabal install hSimpleDB ... Registering HTTP-4000.0.9... Installing library in /Users/user/.cabal/lib/HTTP-4000.0.9/ghc-6.12.3 Registering HTTP-4000.0.9... Configuring hSimpleDB-0.3... Preprocessing library hSimpleDB-0.3... Building hSimpleDB-0.3... src/Network/AWS/Authentication.hs:47:7: Could not find module `Text.XML.HXT.Arrow': Use -v to see a list of the files searched for. cabal: Error: some packages failed to install: hSimpleDB-0.3 failed during the building phase. The exception was: ExitFailure 1 Any ideas how to solve this? Thanks! Dmitri

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Quicksort in Haskell
by yrazes 07 Nov '11

07 Nov '11

module Main (main) where quicksort [] = [] quicksort (s:xs) = quicksort [x|x <- xs,x < s] ++ [s] ++ quicksort [x|x <- xs,x >= s] main = putStr "Ingrese la lista\n" [xs] <- getLine

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zlib build failure on recent GHC
by Ben Gamari 07 Nov '11

07 Nov '11

With GHC 1ece7b27a11c6947f0ae3a11703e22b7065a6b6c zlib fails to build, apparently due to Safe Haskell (bug 5610 [1]). The error is specifically, $ cabal install zlib Resolving dependencies... Configuring zlib-0.5.3.1... Preprocessing library zlib-0.5.3.1... Building zlib-0.5.3.1... [1 of 5] Compiling Codec.Compression.Zlib.Stream ( dist/build/Codec/Compression/Zlib/Stream.hs, dist/build/Codec/Compression/Zlib/Stream.o ) Codec/Compression/Zlib/Stream.hsc:857:1: Unacceptable argument type in foreign declaration: CInt When checking declaration: foreign import ccall unsafe "static zlib.h inflateInit2_" c_inflateInit2_ :: StreamState -> CInt -> Ptr CChar -> CInt -> IO CInt Codec/Compression/Zlib/Stream.hsc:857:1: Unacceptable argument type in foreign declaration: CInt When checking declaration: foreign import ccall unsafe "static zlib.h inflateInit2_" c_inflateInit2_ :: StreamState -> CInt -> Ptr CChar -> CInt -> IO CInt Codec/Compression/Zlib/Stream.hsc:857:1: Unacceptable result type in foreign declaration: IO CInt Safe Haskell is on, all FFI imports must be in the IO monad When checking declaration: foreign import ccall unsafe "static zlib.h inflateInit2_" c_inflateInit2_ :: StreamState -> CInt -> Ptr CChar -> CInt -> IO CInt ... This is a little strange since, a) It's not clear why Safe Haskell is enabled b) The declarations in question seem to be valid Does this seem like a compiler issue to you? Cheers, - Ben [1] http://hackage.haskell.org/trac/ghc/ticket/5610

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FFI / enums
by Johannes Waldmann 07 Nov '11

07 Nov '11

Does this work with "ghc -XForeignFunctionInterface" ? http://en.wikibooks.org/wiki/Haskell/FFI#Enumerations I am getting a syntax error right after "#{". But then, the description is for "hsc2hs" (not ghc). And it mentions macros, while C meanwhile has "real" enums. Well, at leat one can write "enum match { foo, bar }". The FFI Addendum does not mention enums at all. So - what's the agreeable way to do this? - Thanks, J.W.

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