December 2009 - Haskell-Cafe

Announce: data-clist-0.0.3 -- A RingBuffer/CircularList/Ring data type
by John Van Enk 31 Dec '09

31 Dec '09

Hi all, data-clist-0.0.3 is up on hackage: http://hackage.haskell.org/package/data-clist-0.0.3 Any feedback is appreciated. The docs are getting there, but not quite complete. Enjoy! /jve

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Lifting monad syntax sugar
by Maciej Piechotka 31 Dec '09

31 Dec '09

I'm not quite sure if it exists but with working with lifted monads/lifted arrows it would be useful. Let f be function from m a to n a where m and n are monad (arrows respectivly) for any a then: > do v <- something 0 > lift (f) g <- somethingElse > h <- somethingInLift > i <- somethingCompletlyDifferent v > doSomething g h is syntaxt sugar of > do v <- something 0 > (g, h) <- f $ do g <- somethingElse > h <- something > i <- somethingCompletlyDifferent v > return (g, h) > doSomething g h and > proc (x) -> do v <- something -< x > lift (f) g <- somethingElse -< () > h <- somethingInLift -< () > i <- doSomething -< v > somethingCompletlyDifferent -< (g, h) is syntax sugar of > proc (x) -> do v <- something -< g > (g, h) <- f (proc (v) -> do g <- somethingElse -< () > h <- somethingInLift -< () > i <- doSomething -< v > returnA -< (g, h)) -< v > somethingCompletlyDifferent -< (g, h) I am aware that syntatic sugar is evil (http://www.haskell.org/haskellwiki/Syntactic_sugar/Cons) and therefore I'd prefere to post it here before submitting feature request to ghc (I'll also threat no resoponses as nobody care so why pollute syntax). Regards PS. I'm aware that lift is existing function (Control.Monad.Trans) so possibly other name will be choosen. PPS. Syntax is similar to rec as last line can be binding

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Dynamically Linking Foreign Functions
by Richard Warburton 31 Dec '09

31 Dec '09

Apologies in advance for the length of this email, but I've tried to be as clear as possible. Any help on the matter most appreciated. Goal: I have a simple interpreter for a language that I've written, and I wish to implement an FFI. My ideal api would be like 'ctypes' in python. Here's an example: from ctypes import * libc = CDLL("libc.so.6") libc.printf("An int %d, a double %f\n", 1234, c_double(3.14)) Prints out: An int 1234, a double 3.140000 Implementation: I am currently trying to use the dynamic linker in order to specify the library string, for example the following seems to work: import Foreign import Foreign.C.Types import Foreign.C.String import Monad (liftM) import System.Posix.DynamicLinker type Fun = CString -> IO CInt foreign import ccall unsafe "dynamic" c_printf :: FunPtr Fun -> Fun printf :: String -> IO Int printf str = do withDL "libc.so.6" [RTLD_NOW] $ \dl -> do printf_ptr <- dlsym dl "printf" let fun = c_printf printf_ptr liftM fromIntegral $ withCString str fun main = printf "hello world" Problem: I don't understand how I can generate the foreign import statements at runtime. I am currently considering generating strings of such expressions, and using hs-plugins in order to compile them. This seems like a complete hack though - I'm dynamically loading a haskell library in order to dynamically load a foreign library. Another option is to write a dynamic loader in C, and then call this from haskell, but how would I type the loading function - obviously I can give it a CString for the library, and another for the function in question - but what would the return type be? regards, Richard Warburton

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Is cabal option --extra-lib-dirs working?
by Ahn, Ki Yung 31 Dec '09

31 Dec '09

I had some problem with --extra-lib-dirs option in cabal-install. I've been trying installing bindings-yices package on hackage. Since yices may be installed in non-standard location, such as under your own home directory, one may have to use --extra-include-dirs and --extra-lib-dirs option. --extra-include-dirs work well and locates the header for the yices C binding, but --extra-lib-dirs option does not seem to work well. Would this be a cabal-install bug, or some package misconfiguration? ==================================================== kyagrd@kyavaio:~/tmp/bindings-yices-0.1$ cabal configure Resolving dependencies... Configuring bindings-yices-0.1... cabal: Missing dependency on a foreign library: * Missing header file: yices_c.h * Missing C library: yices This problem can usually be solved by installing the system package that provides this library (you may need the "-dev" version). If the library is already installed but in a non-standard location then you can use the flags --extra-include-dirs= and --extra-lib-dirs= to specify where it is. kyagrd@kyavaio:~/tmp/bindings-yices-0.1$ cabal configure --extra-include-dirs=/home/kyagrd/yices-1.0.21/include Resolving dependencies... Configuring bindings-yices-0.1... cabal: Missing dependency on a foreign library: * Missing C library: yices This problem can usually be solved by installing the system package that provides this library (you may need the "-dev" version). If the library is already installed but in a non-standard location then you can use the flags --extra-include-dirs= and --extra-lib-dirs= to specify where it is. kyagrd@kyavaio:~/tmp/bindings-yices-0.1$ cabal configure --extra-include-dirs=/home/kyagrd/yices-1.0.21/include --extra-lib-dirs=/home/kyagrd/yices-1.0.21/lib Resolving dependencies... Configuring bindings-yices-0.1... cabal: Missing dependency on a foreign library: * Missing C library: yices This problem can usually be solved by installing the system package that provides this library (you may need the "-dev" version). If the library is already installed but in a non-standard location then you can use the flags --extra-include-dirs= and --extra-lib-dirs= to specify where it is. kyagrd@kyavaio:~/tmp/bindings-yices-0.1$ kyagrd@kyavaio:~/tmp/bindings-yices-0.1$ ls /home/kyagrd/yices-1.0.21/include/ yices_c.h yicesl_c.h kyagrd@kyavaio:~/tmp/bindings-yices-0.1$ ls /home/kyagrd/yices-1.0.21/lib/ libyices.a libyices.so kyagrd@kyavaio:~/tmp/bindings-yices-0.1$ ls -al /home/kyagrd/yices-1.0.21/include/ 합계 40 drwxr-xr-x 2 kyagrd kyagrd 4096 2009-07-17 20:58 . drwxr-xr-x 6 kyagrd kyagrd 4096 2009-07-07 23:19 .. -rw-r--r-- 1 kyagrd kyagrd 27948 2009-05-05 19:03 yices_c.h -rw-r--r-- 1 kyagrd kyagrd 1786 2009-05-05 19:03 yicesl_c.h kyagrd@kyavaio:~/tmp/bindings-yices-0.1$ ls -al /home/kyagrd/yices-1.0.21/lib/ 합계 8936 drwxr-xr-x 2 kyagrd kyagrd 4096 2009-12-30 19:29 . drwxr-xr-x 6 kyagrd kyagrd 4096 2009-07-07 23:19 .. -rw-r--r-- 1 kyagrd kyagrd 5167710 2009-05-05 19:03 libyices.a -rwxr-xr-x 1 kyagrd kyagrd 3955648 2009-05-05 19:03 libyices.so

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Re: [Haskell-cafe] Space Efficiency When Sorting a List of Many Lists
by Patai Gergely 31 Dec '09

31 Dec '09

Hi, > explode [[[1,2],[3],[4,5,6]], [[1, 2], [14,15], [16]]] --> [[1,3,4], > [1,3,5],[1,3,6],[2,3,4],[2,3,5],[2,3,6],[1,14,16],[1,15,16], > [2,14,16],[2,15,16]] I don't think the following will solve your problem, but explode can be rewritten with existing functions thanks to the list monad: explode = concatMap sequence Gergely -- http://www.fastmail.fm - A no graphics, no pop-ups email service

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Re: FASTER primes
by Steve 30 Dec '09

30 Dec '09

On Wed, 2009-12-30 at 11:09 -0500, haskell-cafe-request(a)haskell.org wrote: > Am Mittwoch 30 Dezember 2009 01:23:32 schrieb Will Ness: > > Daniel Fischer <daniel.is.fischer <at> web.de> writes: .... > No, it's my own code. Nothing elaborate, just sieving numbers 6k1, > twice as fast as the > haskellwiki code (here) and uses only 1/3 the memory. For the record: > > ---------------------------------------------------------------------- > {-# LANGUAGE BangPatterns #-} > module SievePrimes where > > import Data.Array.Base (unsafeRead, unsafeWrite, unsafeAt) > import Data.Array.ST > import Data.Array.Unboxed > import Control.Monad (when) > import Data.Bits > > primesUpTo :: Integer -> [Integer] > primesUpTo bound > = 2:3:[fromIntegral (3*i + (if testBit i 0 then 4 else 5)) > | i <- [0 .. bd], par `unsafeAt` i] > where > bd0 = 2*(bound `div` 6) - case bound `mod` 6 of { 0 -> 2; 5 -> > 0; _ -> 1 } > bd = fromInteger bd0 > sr = floor (sqrt (fromIntegral bound)) > sbd = 2*(sr `div` 6) - case sr `mod` 6 of { 0 -> 2; 5 -> 0; _ > -> 1 } > par :: UArray Int Bool > par = runSTUArray $ do > ar <- newArray (0,bd) True > let tick i s1 s2 > | bd < i = return () > | otherwise = do > p <- unsafeRead ar i > when p (unsafeWrite ar i False) > tick (i+s1) s2 s1 > sift i > | i > sbd = return ar > | otherwise = do > p <- unsafeRead ar i > when p (let (!start,!s1,!s2) = if even i > then > (i*(3*i+10)+7,2*i+3,4*i+7) else (i*(3*i+8)+4,4*i+5,2*i+3) in tick > start s1 s2) > sift (i+1) > sift 0 > ---------------------------------------------------------------------- > > The index magic is admittedly not obvious, but if you take that on > faith, the rest is > pretty clear. To find the n-th prime, > > ---------------------------------------------------------------------- > module Main (main) where > > import SievePrimes (primesUpTo) > import System.Environment (getArgs) > > printNthPrime :: Int -> IO () > printNthPrime n = print (n, primesUpTo (estim n) !! (n-1)) > > main = do > args <- getArgs > printNthPrime $ read $ head args > > estim :: Int -> Integer > estim k > | n < 13 = 37 > | n < 70 = 349 > | otherwise = round x > where > n = fromIntegral k :: Double > x = n * (log n + log (log n) - 1 + 1.05 * log (log n) / log n) > ---------------------------------------------------------------------- > > If I don't construct the prime list but count directly on the array, > it's ~6% faster. The speed difference varies with the value of 'bound' - the difference between counting directly on the array and creating a list from the array increases as 'bound' increases. I tested it for 10^8 and 10^9 (using Int not Integer) and using the array without creating a list is 50+% faster. It looks like a very fast method for creating the 'set' of primes up to bound. Steve

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ANN: TxtSushi 0.5.0
by Keith Sheppard 30 Dec '09

30 Dec '09

Hello Cafe, I've uploaded a new version of TxtSushi to http://hackage.haskell.org/package/txt-sushi and announced the details here http://blog.keithsheppard.name/2009/12/txtsushi-050.html. TxtSushi is a set of command-line utilities for transforming CSV and tab-delimited files including an SQL based utility. Best Regards Keith -- keithsheppard.name

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on HaskellDBs switch to HList
by Günther Schmidt 30 Dec '09

30 Dec '09

Hello Justin, since you are about to migrate HaskellDB to using HList, do we still need singleton types for the "columns" as before? Günther

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How to get a list of constructors for a type?
by Gregory Propf 30 Dec '09

30 Dec '09

Say I have something like data DT a = Foo a | Bar a | Boo a I want something like a list of the constructors of DT, perhaps as [TypeRep]. I'm using Data.Typeable but can't seem to find what I need in there. Everything there operates over constructors, not types.

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ANN: HPath-0.0.0
by Jason Dusek 30 Dec '09

30 Dec '09

HPath is a command line utility to grab the Haskell source for a given identifier: :; dist/build/hpath/hpath HPath.Path.parse 2>/dev/null parse :: String -> Either ParseError Path parse s = Text.ParserCombinators.Parsec.parse (qualified []) s s This is an alpha release. There are many things HPath can't retrieve -- like constructors -- and there seem to be some complications with the underlying parser/pretty printer (some times you get far too many newlines). There will be another release when the newline issue is resolved. The hope is to release HPath in conjunction with LaTeX macros to allow easy inclusion of Haskell in elaborate documents. -- Jason Dusek

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