- Haskell-Cafe - Haskell.org

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

4 3

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

6 12

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':

1 1

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

3 5

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

3 3

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

4 4

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.

2 1

Message
by Goutam Tmv 07 Nov '11

07 Nov '11

Would you ever see yourself write a web application like Twitter or Facebook in Haskell?

16 20

[ANNOUNCE] CoreErlang-0.0.2 + maintainer change
by Alex Kropivny 07 Nov '11

07 Nov '11

Hello, With the authors blessing, I'm taking over maintenance for http://hackage.haskell.org/package/CoreErlang, a parser/printer for an intermediate language for the Erlang VM. Changes in 0.0.2: - fixed float parse bug - added Data,Typeable instances to AST Package is rather esoteric, but it's nice for static source analysis - and when someone decides they want to compile a new language to the ERTS, the package is ready... Regards, Alex Kropivny

1 0

Time zones and IO
by Ben Gamari 07 Nov '11

07 Nov '11

Recently I've been working on an iCalendar parser implementation with support for recurrence rules. For the recurrence logic, I've been relying on Chris Heller's excellent time-recurrence package. Unfortunately, it seems we have both encountered great difficulty in cleanly handling time zones. For example, as seen in the case of the CalendarTime LocalTime instance[1], Chris does the following, instance CalendarTimeConvertible LocalTime where toCalendarTime (LocalTime day t) = CalendarTime (fromEnum $ todSec t) (todMin t) (todHour t) d (toEnum m) y weekDay yearDay tz where (y, m, d, weekDay, yearDay) = dayInfo day tz = unsafePerformIO getCurrentTimeZone Ouch. Unfortunately, I have been entirely unable to come up with any better way to deal with timezones in a referentially transparent way. Passing the current timezone as an argument seems heavy-handed and reveals the leakiness of the CalendarTimeConvertible abstraction. Short of making toCalendarTime an action, can anyone see a suitable way of refactoring CalendarTimeConvertible to avoid unsafe IO? Perhaps a TimeZoneInfo monad is in order, exposing lookup of arbitrary (through timezone-olson) as well the current timezone? It seems like this would be inconvenient and overkill, however. Ideas? - Ben [1] https://github.com/hellertime/time-recurrence/blob/master/src/Data/Time/Cal…

6 6