January 2010 - Haskell-Cafe

HackageDB is not for ... ?
by Andrey Sisoyev 05 Jan '10

05 Jan '10

Hello everybody! I'm planning not a small project, and intuition suggests me, that I should use heavily the rule "separate and rule". So I must separate the project into many maximum independent and general pieces (packages), make them, and finally glue it into my project. The pieces and their dependencies will form an oriented graph without cycles and with overall path finishing into the final project. So far so good I have made two pieces: ** http://hackage.haskell.org/package/PCLT-DB ** http://hackage.haskell.org/package/PriorityChansConverger But I'm coming to understand, that it's not polite to other community members to publish just every package my project will consist of, because we do not want to garbage HackageDB. ------------------------------------- Consider cases: (1) I don't want to publish package A, because I doubt anyone will find it useful. But I develop independent packages B and C, that will be usefull for the community, and want to publish them on Hackage, but they require package A! [Solution] Publish package A, but make it maximum general and complete in itself. (2) I don't want to publish package A, because it's tiny. Well imagine something same popular and useful, like Ord class. 10 lines of code. But I don't want to repeat these lines in every other package. [Solution] Same as for (1) I guess. (3) I don't want to publish package A, because it's about adjacent system. I'm making 3 packages, that all use DBMS (PostgreSQL). All three are thught to be independent. But! They all in their DDL scripts use table inheritance from a common ancestor table "nameables", which is everywhere defined the same: > CREATE TABLE nameables ( -- an ancestor table > name varchar NOT NULL DEFAULT '<name not given>' > , description varchar NULL > , comments varchar NULL > ) TABLESPACE tabsp_pcltcatalogs; > GRANT SELECT, INSERT, UPDATE, DELETE ON TABLE nameables TO > user_someCurrentSchema_data_admin; > GRANT SELECT ON TABLE nameables TO user_someCurrentSchema_data_reader; Now what do I do with that?? The problem gets even more complicated be the fact, that dependent packages may use different DB schemes, so DDL sql script from Nameables will have to be run additionally for them. But that's easy to solve - just decide, whether to add or not by build-depends a comment "-- with DDL init", and maybe additional instruction in README. [Solution] ??? [Solution A] Well, of course, I can pretend that this package may be is Haskellish enough, by inventing something which I don't really need: > data Name = Name { nName :: String, nDescription :: Maybe String, > nComments :: Maybe String } > > class Nameables a where > getName :: a -> Name > > class HasPrimaryKey a => NameableInDBOnly a where > getNameFromDB :: Connection -> a -> IO Name This + sql script and instruction, how to initialize DB. Is it okay to publish this? Looks like garbage... [Solution B] Do my own version control and add same pieces to every package that use Nameable. Don't like this solution. May be I should start using other repository systems like github or darcs, and then this solution will seem more natural to me?.. ---------------------------------- These are my thoughts on the theme. In any case I come to wanting to publish the packages anyway. Probably I miss something in ideology and methodology of using Hackage as community repository. And what do you think about it? Regards, Andrey -- View this message in context: http://old.nabble.com/HackageDB-is-not-for-...---tp27022312p27022312.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Compiling a shared library on MacOS X
by Jean-Denis Koeck 04 Jan '10

04 Jan '10

Hello, I'm using the following cabal file to build a shared library with ghc: Build-Type: Simple Executable libmylibrary.dll If os(windows) CPP-Options: -DWIN32 Extensions: ForeignFunctionInterface Build-Depends: ... Main-Is: libmylibrary.hs Ghc-Options: -W --make -no-hs-main -optl-shared -optl-s -O2 The resulting library is called from a C++ graphical user interface (using Qt), which worked great so far on Windows and Linux. However, the compilation fails on MacOS X: Undefined symbols: "_ZCMain_main_closure", referenced from: _ZCMain_main_closure$non_lazy_ptr in libHSrts.a(Main.o) "___stginit_ZCMain", referenced from: ___stginit_ZCMain$non_lazy_ptr in libHSrts.a(Main.o) ld: symbol(s) not found collect2: ld returned 1 exit status I don't know much about shared libraries, even less about them on MacOS X :( Any idea ? Jean-Denis Koeck

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A question on DSL's
by CK Kashyap 04 Jan '10

04 Jan '10

Hi, I am not sure if I'm using DSL in the right context here but I am referring to those solutions that allow us to write code in Haskell and generate a target code source code of another language or even object code for that matter. I am aware of two ways of achieving this - 1. Implement functions that emit the target code - that is, when the Haskell code is compiled and run, the target code is emitted 2. Modify the Haskell compiler's back end to emit the code - that is when the Haskell code is compiled the target code is emitted I am not sure if there are more ways (hybrid perhaps) ... My question is, when would I chose one approach over the other? Regards, Kashyap

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curl on windows?
by Günther Schmidt 04 Jan '10

04 Jan '10

Hi, how can I install curl on windows (XP)? Günther

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HXT XSLT example?
by Günther Schmidt 04 Jan '10

04 Jan '10

Hi, is there a simple example for XSLT via HXT? You know something like take this xml file, apply this xsl file, dump to stdout. Günther

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Why can't we make an instance declaration on a type synonym?
by jberryman 04 Jan '10

04 Jan '10

This may be a dumb question, but why can we not declare a Monad instance of a type synonym? This question came to me while working with the State monad recently and feeling that the requirement that we wrap our functions in the State constructor is a bit... kludgy. Why can't the State monad be defined this way?: > type State s a = \s -> (a , s) > > instance Monad (State s) where > return a = \s -> (a, s) > m >>= k = \s -> let > (a, s') = m s > in k a s' It seems like Haskell's type system could understand the above, but I'm interested in hearing the technical reason which I'm sure exists for why we can't do this. Thanks for any clues, Brandon Simmons http://coder.bsimmons.name/blog/

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Space Efficiency When Sorting a List of Many Lists
by Peter Green 04 Jan '10

04 Jan '10

I'm a Haskell neophyte, so may be missing something obvious in the problem outlined below. I'm fairly proficient in Python + have some limited experience in OCaml and F#, so know just enough to be be dangerous, but not nearly enough to really know what I'm doing here. OK, I have text files containing 'compressed lists' Compressed lists look like this: 8+12,11,7+13,10 1+2+3,1+9,3+6,4 . . Sublists are comma-delimited, and sublist elements are separated by '+' character. I parse these to look like so: [[[8,12],[11],[7,13],[10]], [[1,2,3],[1,9],[3,6],[4]], . . ] I need to explode these and produce a lex-sorted list of exploded lists: [[1,1,3,4],[1,1,6,4],[1,9,3,4],[1,9,6,4],[2,1,3,4],[2,1,6,4],[2,9,3,4], [2,9,6,4],[3,1,3,4],[3,1,6,4],[3,9,3,4],[3,9,6,4], [8,11,7,10],[8,11,13,10],[12,11,7,10],[12,11,13,10]] I then output this data as comma-delimited lists: 1,1,3,4 1,1,6,4 . . 12,11,13,10 I can do all this in my (no doubt fairly naive) program shown below. In real life, one of my test data files contains compressed lists which all contain 7 sublists. This data (correctly) explodes to a list containing ~3,700,000 exploded lists. All good as far as correctly transforming input to output goes. However, sorting the data uses a lot of memory: Partial output from ./explode +RTS -sstderr: 540 MB total memory in use (4 MB lost due to fragmentation) If I do not do any sorting on the exploded lists, i.e. modify the main function to be main = interact (unlines . toCSV . explode . fromCSV . lines) I then see this partial output from ./explode +RTS --stderr: 2 MB total memory in use (0 MB lost due to fragmentation) I can guess that there might be be less laziness and more instantiation when sorting is introduced, but my questions are: (1) Am I doing anything terribly stupid/naive here? (2) If so, what can I do to improve space efficiency? TIA! import Data.List (sort) import Data.List.Split (splitOn) -- Cartesian Product over a List of Lists -- cp [[1,2],[3],[4,5,6]] --> [[1,3,4],[1,3,5],[1,3,6],[2,3,4],[2,3,5], [2,3,6]] cp :: [[a]] -> [[a]] cp [] = [[]] cp (xs:xss) = [y:ys | y <- xs, ys <- cp xss] -- fromCSV ["8+12,11,7+13,10", "1+2+3,1+9,3+6,4"] --> -- [[[8,12],[11],[7,13],[10]],[[1,2,3],[1,9],[3,6],[4]]] fromCSV :: [String] -> [[[Int]]] fromCSV = map parseOneLine where parseOneLine = map parseGroup . splitOn "," where parseGroup = map read . splitOn "+" -- 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]] explode :: [[[a]]] -> [[a]] explode = concatMap cp -- toCSV [[8,11,7,10,12],[8,11,7,10,12],[8,11,7,10,12]] --> -- ["8,11,7,10,12","8,11,7,10,12","8,11,7,10,12"] toCSV :: (Show a) => [[a]] -> [String] toCSV = map $ tail . init . show --toCSV = map (intercalate "," . map show) main = interact (unlines . toCSV . sort . explode . fromCSV . lines)

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Template Haskell - substitution in pattern in a lambda
by Patrick Caldon 04 Jan '10

04 Jan '10

I'm trying to write some template haskell which will transform: $(buildCP 0) into \(SimpleM d1 d2 d3) (SimpleM _ _ _) -> (SimpleM d1 d2 d3) $(buildCP 1) into \(SimpleM _ d2 d3) (SimpleM d1 _ _) -> (SimpleM d1 d2 d3) $(buildCP 1) into \(SimpleM d1 _ d3) (SimpleM _ d2 _) -> (SimpleM d1 d2 d3) and so on. Ultimately I want to generalize this to more variables. I can't seem to get anything to substitute for the pattern variables in a lambda. Is there a straightforward way of doing this? Below is what I've been playing with to try to make this work. Thanks, Patrick. --- module THTest where import Language.Haskell.TH import qualified Data.Bits type Policy = Int data Management = SimpleM Policy Policy Policy deriving Show -- Compiles - but no substitution for the "aX" and "bX" variables buildCP :: Int -> ExpQ buildCP k = [|\(SimpleM a1 a2 a3) (SimpleM b1 b2 b3) -> (SimpleM $e1 $e2 $e3) |] where (e1,a1,b1) = bitToExprs 0 k (e2,a2,b2) = bitToExprs 1 k (e3,a3,b3) = bitToExprs 2 k -- Won't compile: buildCP2 :: Int -> ExpQ buildCP2 k = [|\(SimpleM $a1 $a2 $a3) (SimpleM $b1 $b2 $b3) -> (SimpleM $e1 $e2 $e3) |] where (e1,a1,b1) = bitToExprs 0 k (e2,a2,b2) = bitToExprs 1 k (e3,a3,b3) = bitToExprs 2 k cp1 0 = \(SimpleM d1 d2 d3) (SimpleM _ _ _) -> (SimpleM d1 d2 d3) {- -- idea is to use in calls like this: cp0 0 = $(buildCP 0) -- should be \(SimpleM d1 d2 d3) (SimpleM _ _ _) -> (SimpleM d1 d2 d3) cp0 1 = $(buildCP 1) -} -- There is also a template haskell [p| ... |] syntax, but not yet implemented ... bitToExprs:: Int -> Int -> (ExpQ,PatQ,PatQ) bitToExprs n k = if Data.Bits.testBit (k::Int) (n::Int) then (e,v1,v2) else (e,v2,v1) where v1 = return WildP v2 = return $ VarP (mkName name) e = return $ VarE (mkName name) name = "d" ++ (show $ n + 1) {- -- ulitmate goal is something like this with 10ish d variables: -- cp0 0 (SimpleM d1 d2 d3 m1) (SimpleM _ _ _ m2) = (SimpleM d1 d2 d3 (me1 m1 m2)) cp0 1 (SimpleM d1 d2 _ m1) (SimpleM _ _ d3 m2) = (SimpleM d1 d2 d3 (me2 m1 m2)) cp0 2 (SimpleM d1 _ d3 m1) (SimpleM _ d2 _ m2) = (SimpleM d1 d2 d3 (me1 m1 m2)) cp0 3 (SimpleM d1 _ _ m1) (SimpleM _ d2 d3 m2) = (SimpleM d1 d2 d3 (me2 m1 m2)) cp0 4 (SimpleM _ d2 d3 m1) (SimpleM d1 _ _ m2) = (SimpleM d1 d2 d3 (me1 m1 m2)) cp0 5 (SimpleM _ d2 _ m1) (SimpleM d1 _ d3 m2) = (SimpleM d1 d2 d3 (me2 m1 m2)) cp0 6 (SimpleM _ _ d3 m1) (SimpleM d1 d2 _ m2) = (SimpleM d1 d2 d3 (me1 m1 m2)) cp0 7 (SimpleM _ _ _ m1) (SimpleM d1 d2 d3 m2) = (SimpleM d1 d2 d3 (me2 m1 m2)) cp0 _ _ _ = (trace "cp0 error" undefined) -}

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setting seed for random numbers
by Larry E. 03 Jan '10

03 Jan '10

I want to set a random seed for a random number generator. I tried this: getSeed = do time <- getClockTime return (ctPicosec (toUTCTime time)) seed <- getSeed randList minval maxval = randomRs (minval,maxval) (mkStdGen seed) But I get the error: parse error on input `<-' which refers to "seed<-getSeed". Using "seed<-getSeed" from the ghci command line works. Why doesn't it work in a script? THe second problem is the seed is of type Integer but mkStdGen wants an Int. How can I get an Int from an Integer? I tried mod to get the seed to within range for an int but the result is still of type Integer how can I coerce Integer to Int when I know result will fit? Thanks

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ANNOUNCE: PriorityChansConverger-0.1
by Andrey Sisoyev 03 Jan '10

03 Jan '10

Hello, cafe! Didn't find it in HackageDB, so made it. http://hackage.haskell.org/package/PriorityChansConverger Category: concurency Converges multiple channels into one. When user reads from the PCC, the choice is made - from which channel to read. System selects a nonempty channel, whose (CurrentPriority, StartPriority) tuple is max. The side effect of the channel selection is it's CurrentPriority decrease by one, if it's value becomes less than one, then the CurrentPriority is set to StartPriority. Based on STM.TChan, extended with capacity control. Version wrapped into is also available. The realization probably isn't very fast and isn't good at memory economy, since it uses fresh high level primitive - STM. It wasn't intended to be used with millions of channels. But it's max throughput comparing to the ordinary Chan throughput is to be estimated (will do it in some future version). Regards, Andrey Sisoyev -- View this message in context: http://old.nabble.com/ANNOUNCE%3A-PriorityChansConverger-0.1-tp26999572p269… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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