July 2008 - Haskell-Cafe

Alternatives to convoluted record syntax
by Dougal Stanton 04 Jul '08

04 Jul '08

Hi Haskellers, This is a style question, as my program works fine but looks really ugly and can be confusing to maintain. I take options from the command line using GetOpt and hand back this structure for later use. > data Opts = Opts > { query :: Query > , queryLimit :: Maybe Limit > , disabledOnly :: Bool > } deriving Show Here's a snippet from the parser for one option (others omitted for clarity): > options :: [OptDescr (Opts -> Opts)] > options = > [ Option "b" ["bus"] (ReqArg busNum "NUM") "Bus number" > , ... > ] > where busNum n os = let b = (query os) { queryBusNumber = Just n } > in if isBusId n then os { query = b } else os Variations on that ugliness are repeated four times for other fields. Is there an alternative way to change the value of nested fields? Thanks, Dougal. -- Dougal Stanton dougal(a)dougalstanton.net // http://www.dougalstanton.net

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Integer = infinite precision integer? How?
by leledumbo 04 Jul '08

04 Jul '08

Does anyone have an explanation how Haskell implement this? Or a pointer to a article describing this? -- View this message in context: http://www.nabble.com/Integer-%3D-infinite-precision-integer--How--tp182738… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Existential Data Types
by Richard G. 04 Jul '08

04 Jul '08

Hello I've been doing some stuff with existential data types and came across a curious pattern. If anyone has thoughts, comments, or feedback, it would be appreciated. --- I'm trying to port and extend the Be messaging scheme to Haskell for use in distributed, heterogenous environments. BeOS was object oriented and pervasively threaded. That, and my background as an OOP programmer, has resulted in a very OOP approach. Because OOP allows for subclassing and the Be API was designed for it, I've used a wrapper approach. class HandlerC h where . . . data Handler = forall h . HandlerC h => Handler h instance HandlerC Handler where . . . data DefaultHandler = DefaultHandler ... instance HandlerC DefaultHandler where . . . newDefaultHandler = Handler (DefaultHandler ...) newHandler h = Handler h The HandlerC defines the interface. When Handler implements the interface, it mostly passes the functions to the inner Handler that actually has the data and the real functions. The advantage is that I can wrap anything that implements the HandlerC interface and have it behave in a known and simple way from the outside. ---- One area where this might have some application is in unit testing to make mock objects easier to use. If I had a Looper ADT that depended on the Handler, I would want test the Looper without having to worry about the complexity of the Handler interfering with the tests. I could do this with the following: data MockHandler = MockHandler ... instance HandlerC MockHandler where . . . h = newHandler (MockHandler ...) where h is passed to the Looper some how. ---- Overall, this seems very similar to data D = D1 | D2 | D3 | ... The big difference is that all the Dn constructors don't need to be known at compile time, which is very useful when writing libraries. Also, the wrapped data can include functions and state data, making it very easy to create applets. This is handy when doing message passing or request handling, such as in a web server, especially when using STM. Richard

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Wouter Swierstra style extensible records!
by Ron Alford 03 Jul '08

03 Jul '08

Having made over-use of Wouter's expression idioms, I decided to hack it into extensible records. It's not documented, it's probably got holes in its usability, but I thought I'd post it so people can play with it (and give suggestions before I rewrite my code to use it!). I know there are other extensible record systems, but I wondered how this stacks up. Notably, it looks like in HList the records are only extensible in one direction (tacking fields onto the end), although they have a much more developed syntax (if you're into that sort of thing). Current problems: * Defining a new field is rather bulky - currently it takes lines. Definitions are bulky in Wouter's extensible expressions, too, but I could probably shorten the get/set convenience functions to something more readable. * Construction of new records is only convenient by using the defaulting system. Maybe that makes sense, maybe it doesn't. A side benefit: Record access functions are reusable between record types (as long as they have the same type). Anyway, thanks again, Wouter! -Ron

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Re: [Haskell-cafe] Access to Oracle database from Haskell
by Henning Thielemann 03 Jul '08

03 Jul '08

On Thu, 26 Jun 2008, Alistair Bayley wrote: >> Try this version of configOracle in Setup.hs: >> >> configOracle verbose buildtools = do >> if not (sqlplusProgram `isElem` buildtools) >> then return Nothing >> else do >> path <- getEnv "ORACLE_HOME" >> info verbose ("Using Oracle: " ++ path) >> makeConfig path libDir "oci/include" >> where libDir = if isWindows then "bin" else "lib" >> >> >> You'll also need to add this import: >> >> import System.Environment (getEnv) > > Did you get a chance to try this? I'm quite keen to fix the > non-Windows parts of the installation process, if I can. I have replaced configOracle by configOracle verbose buildtools = do if not (sqlplusProgram `isElem` buildtools) then return Nothing else do path <- getEnv "ORACLE_HOME" info verbose ("Using Oracle: " ++ path) let (libDir, includeDir) = if isWindows then ("bin", "oci/include") else ("lib", "/usr/include/oracle/10.2.0.4/client") makeConfig path libDir includeDir This is obviously a hack. We should get the Oracle include path from the user, maybe via another environment variable or a custom Cabal option. Now I can start GHCi with the example program you gave me. However I have to start with -lclntsh, otherwise symbol OCIEnvCreate cannot by found. I thought I do not need this option, because the installed Takusen package contains the library name: $ grep clntsh dist/installed-pkg-config extra-libraries: clntsh clntsh I wonder why it appears twice, because in dist/setup-config it exists only once: extraLibs = ["clntsh"] Nevertheless I can start GHCi with the example program and I can see 'hello world'! Great - many thanks for the hints that led me to that state! Next step is to fetch real data from the database. But now I encounter new problems. If I write a real 'select' statement I get at best: Unexpected null in row 1, column 1. *** Exception: (unknown) and at worst: 50001752 *** glibc detected *** /usr/lib/ghc-6.8.2/ghc-6.8.2: free(): invalid pointer: 0x08d10065 *** ======= Backtrace: ========= /lib/libc.so.6[0xb7dba4b6] /lib/libc.so.6(cfree+0x89)[0xb7dbc179] /usr/lib/oracle/10.2.0.4/client/lib/libclntsh.so(lfvclose+0x1c)[0xb5df9c1e] /usr/lib/oracle/10.2.0.4/client/lib/libclntsh.so(SlfMunmap+0x3f)[0xb5e70c3b] /usr/lib/oracle/10.2.0.4/client/lib/libclntsh.so(ldiutzd+0x37)[0xb5e49e7b] /usr/lib/oracle/10.2.0.4/client/lib/libclntsh.so(kputerm+0x51)[0xb54c96b9] /usr/lib/oracle/10.2.0.4/client/lib/libclntsh.so(OCITerminate+0x1c)[0xb5596888] [0xb6a33801] /usr/lib/ghc-6.8.2/ghc-6.8.2[0x8a600de] ======= Memory map: ======== ... then GHCi quits. > BTW, the location of your header files is still a puzzle. Oracle's docs: > http://download.oracle.com/docs/cd/B19306_01/appdev.102/b14250/ociabdem.htm… > > state that the header files shoulld be in $ORACLE_HOME/rdbms/public. > But perhaps things are different for the Instant Client. I don't know. I have just installed the RPMs and I expect that others will do so as well.

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ANN: The Disciplined Disciple Compiler - alpha 1.1
by Ben Lippmeier 03 Jul '08

03 Jul '08

Hi All, I'm pleased to announce version 1.1 of the Disciplined Disciple Compiler (DDC) Disciple is an explicitly lazy dialect of Haskell which supports: - first class destructive update of arbitrary data. - computational effects without the need for state monads. - type directed field projections. - allied functional goodness. All this and more through the magic of effect typing. New in this version: - support for x86_64 under linux and darwin, thanks to Jared Putnam. - the -make flag now does a full dependency driven build/rebuild. - constructor classes. - irrefutable patterns. - partial support for monadic do notation. - an unboxed Bool# type with true# and false# literals. - field projection punning. - lots more example code. Project page with full release notes and download at: http://www.haskell.org/haskellwiki/DDC DDC: more than lambdas. Ben.

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parallel_map_reduce, in easy one liner mod
by jinjing 03 Jul '08

03 Jul '08

Hi haskellers, So.. the type says it all p_map_reduce :: ([a] -> b) -> (b -> b -> b) -> [a] -> b so the idea is to write your computation extensive funciton as a map_reduce function, simplest example: sum ( can be seen as map id then reduce (+) ) then instead of calling: sum xs just call: p_map_reduce sum (+) xs the function in the background will split the xs into 16 parts, and use all your cores to process the computation. Enless the computation from the map_reduce function is too trivial or the list is too small, it should bump all your cores to 100% usage, and linearly increase overall performance. You can customize the number of parts by calling the helper function p_map_reduce_to I hope this can be useful to someone besides me :) here is the code warning: i'm using a very annoying coding style, by redefining the (.) operater to be reverse application. Please forgive me and metally transform the order, or just pretend you are reading Java / Python / Ruby or whatever :) module Main where import Data.List import Control.Parallel import Prelude hiding ((.)) -- for my poor oo mind (.) :: a -> (a -> b) -> b a . f = f a infixl 9 . (...) :: (b -> c) -> (a -> b) -> a -> c (...) f g x = f (g x) -- helpers reduce = foldl1 in_group_of n [] = [] in_group_of n xs = xs.take(n) : xs.drop(n).in_group_of(n) split_to n xs = xs.in_group_of(size) where size = if xs.length < n then n else xs.length `div` n -- parallel processing p_eval' xs = xs.pseq(xs.reduce(par)) p_reduce' op xs = xs.p_eval'.reduce(op) p_map_reduce_to n m r xs = xs.split_to(n).map(m).p_reduce'(r) p_map_reduce m r xs = p_map_reduce_to 16 m r xs -- test fib 0 = 0 fib 1 = 1 fib n = fib (n-1) + fib (n-2) fibs xs = xs.map(fib).sum test_list = replicate 50 30 s_fibs = test_list.fibs p_fibs = test_list.p_map_reduce fibs (+) main = p_fibs.show.putStrLn

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database 101 question
by Galchin, Vasili 03 Jul '08

03 Jul '08

Hello, I have installed the mysql server (mysqld) on my Ubuntu machine. What are the Haskell libraries/components that I will need to write a mysql client?? Kind regards, Vasili

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POSIX AIO (asych I/O) ...
by Galchin, Vasili 03 Jul '08

03 Jul '08

Hello, I am also testing my aio support. The aio_write binding seems to work ok .. as well as aio_error, Aio_return is a problem child. I think I wrote a really simple binding. I always receive nbytes as 0. I have been staring at the code hoping to catch a stupid mistake. I put putStrLn's in the code. .....Here is the code ... aioReturn :: AIOCB -> IO (AIOCB, ByteCount) aioReturn aiocb = do allocaBytes (#const sizeof(struct aiocb)) $ \ p_aiocb -> do poke p_aiocb aiocb count <- throwErrnoIfMinus1 "aioReturn" (c_aio_return p_aiocb) aiocb <- peek p_aiocb return (aiocb, fromIntegral count) foreign import ccall safe "aio.h aio_return" c_aio_return :: Ptr AIOCB -> IO CInt Maybe someone can spot something that I haven't. Thanks, Vasili

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GHC ARM Hackathon (Re: Haskell on ARM )
by Jeremy Apthorp 02 Jul '08

02 Jul '08

2008/7/3 Greg Fitzgerald <garious(a)gmail.com>: >> Perhaps we should have some sort of GHC on the ARM hackathon >> when 6.10 comes out > > Would anybody be interested in a "GHC on ARM Hackathon" in San Diego this > year? I'd be interested, but I live in Sydney, Australia. San Diego's a bit of a stretch :) > Jeremy Apthorp said: >> it'll also require that I significantly strip down the runtime system, as >> the current RTS won't fit in 4M > > Do you have a plan for how to do this? Maybe we can invite a Simon to teach > us the innards of the RTS and help guide us? I've discussed it briefly with chak (who'll be supervising the project), but as I said, I haven't looked too deeply into the code yet. Jeremy -- - Jem

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