December 2006 - Haskell-Cafe

Trivial database access in Haskell
by Paul Moore 13 Dec '06

13 Dec '06

I'm an Oracle DBA, and I have been looking at Haskell with interest for a long while now. But I don't feel particularly that I could use it in my "day job", mainly because I can't seem to find a nice, simple way of doing (Oracle) database access in Haskell. What I'm after is very simple - I want to be able to connect to a database and issue SQL queries and DML (updates, inserts, etc). There's nothing particularly "functional" in this - it's more of a basic building block (like file I/O) as far as I'm concerned, so I'm happy for database access to sit in the IO monad. I've looked at the various database access libraries for Haskell, but they all seem to be missing a couple of key pieces if I want to use them: 1. Simple documentation of how to install the library (starting from a vanilla GHC installation on Windows, plus Oracle software, to the point where I can use the library in my code). All I need is Oracle access, so other database client libraries can be ignored. I'd rather not use ODBC, but will if required. 2. Simple examples on how to actually run a query. For example, how do I code the follwing SQL*Plus code (should be pretty self-explanatory) in Haskell: connect scott/tiger@mydb select * from emp; (Connect to the database "mydb" as user "scott", password "tiger", and issue the query "select * from emp"). Ideally, I'd like the Haskell code to not be *too* much more verbose than this (although I can accept that Haskell, beng a general language, won't be able to be quite as concise as a special tool can manage). Without this basis, I don't really have a way to evaluate the various libraries available... If anyone can offer any assistance, I'd be very grateful. Paul.

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Type problem with simple takusen code (was: Trivial database access in Haskell)
by Paul Moore 13 Dec '06

13 Dec '06

(Wow, it looks like my message has generated quite a bit of traffic! Thanks, guys!) On 12/11/06, Taral <taralx(a)gmail.com> wrote: > The magic commands are: > > runhaskell Setup.lhs configure > runhaskell Setup.lhs build > runhaskell Setup.lhs install Excellent! Now I'm getting somewhere. I even found the sample code in the takusen documentation. I'm trying to modify the sample code to work with a local database I have, and I'm getting some type errors which mean nothing much to me. The code I have, stored in a file called db.hs and run via "runhaskell db.hs" is: import Control.Monad.Trans import Database.Oracle.Enumerator import Database.Enumerator query1Iteratee :: (Monad m) => String -> IterAct m [String] query1Iteratee a accum = result' (a:accum) main :: IO () main = do withSession (connect "USER" "PASSWORD" "DB") $ do -- simple query, returning reversed list of rows. r <- doQuery (sql "select username from all_users") query1Iteratee [] liftIO $ putStrLn $ show r The error I'm getting is db.hs:10:64: Couldn't match expected type `forall mark. DBM mark Session a' against inferred type `a1 b' In the second argument of `($)', namely `do r <- doQuery (sql "select username from all_users") query1Iteratee [] liftIO $ (putStrLn $ (show r))' In the expression: (withSession (connect "USER" "PASSWORD" "DB")) $ (do r <- doQuery (sql "select username from all_users") query1Iteratee [] liftIO $ (putStrLn $ (show r))) In the expression: do (withSession (connect "USER" "PASSWORD" "DB")) $ (do r <- doQuery (sql "select username from all_users") query1Iteratee [] liftIO $ (putStrLn $ (show r))) I'm not sure what might be wrong here - or even, for that matter, how to diagnose the problem. To complicate the matter, it's possible that I'm getting a database connection error which I've not got code in to report. (I've no easy way to test that the database connection from Haskell is working, or rather *this* is the easy way :-)) Can anyone offer any pointers? (I take the suggestions about asking on IRC, but I can't get at IRC from here due to firewall issues, so I'll have to stick to email). Paul.

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Large data structures
by Alex Queiroz 13 Dec '06

13 Dec '06

Hi all, I'm considering the use of Haskell to manipulate large data structures for Computer Graphics (large geometric datasets). I'm wondering what's the best way to do it. As "objects" (not in the OO sense) in Haskell are immutable, how can I add a vertex to a large mesh without using obscene amounts of memory? Making an analogy: > enlarge :: a -> [a] -> [a] > enlarge c cs = c : cs > > enlarge 15 [1..100000] Will this copy the whole list to make a new one with an element more? Cheers, -- -alex http://www.ventonegro.org/

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Micro-contributions to MissingH
by Yang 13 Dec '06

13 Dec '06

Hi, I'm at the moment writing a very simple function, spanM (a la mapM). I realize that this is a rather general-purpose function, so I'm wondering if I should submit this as a patch for MissingH. If so, where in the new hierarchy should this be placed? Thanks in advance. Yang

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Request for improvements
by Chris Kuklewicz 13 Dec '06

13 Dec '06

I have written a thin but useful layer on top of parts of Control.Concurrent and Control.Exception to aid in forking new threads. I think I have written it correctly and understand the guarantees (when running under GHC 6.4 or 6.6). It is useful when * you need to be able to check whether a thread has finished running * you need to be able to wait until a thread has finished running * when you want to run within the scope of 'block' * you need to run in another thread with ** an exception handler (catch) ** a finally clause ** bracket commands But it may need improving, especially if I have missed a subtle blocking or race condition. And suggestions on improved names are welcome. The code is on the wiki at http://haskell.org/haskellwiki/WrapConc -- Chris

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How to combine Error and IO monads?
by Cat Dancer 13 Dec '06

13 Dec '06

I've read Jeff Newbern's tutorial on monad transformers (http://www.nomaware.com/monads/html/index.html) but I don't grok it yet and I can't tell how to get started with this particular requirement, or even if I need monad transformers for this. I have a program that performs a series of IO operations, each which can result in an error or a value. If a step returns a value I usually want to pass that value on to the next step, if I get an error I want to do some error handling but usually want to skip the remaining steps. Thus I have a lot of functions with return types like IO (Either String x), where x might be (), Integer, or some other useful value type, and a lot of case statements like a :: Integer -> IO (Either String String) (a :: Either String Integer) <- some_io_action_returning_integer_or_error case a of -- got to get from (Either String Integer) to (Either String String) Left message -> return $ Left message -- continue on Right i -> do_more i If I was using just Either I know how to get rid of the case statements: import Control.Monad.Error p :: Integer -> Either String Integer q :: Integer -> Either String String r :: Either String String r = do a <- p 3 b <- q a return b which of course is much nicer. But how do I combine the Error and IO monads so that r could have the type IO (Either String String)?

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Why so slow?
by Lyle Kopnicky 13 Dec '06

13 Dec '06

The code below is using way more RAM than it should. It seems to only take so long when I build the 'programs' list - the actual reading/parsing is fast. For a 5MB input file, it's using 50MB of RAM! Any idea how to combat this? Thanks, Lyle {-# OPTIONS_GHC -fglasgow-exts #-} -- linear_importer.hs import Control.Monad (unless) import Data.ByteString.Char8 (ByteString) import qualified Data.ByteString.Char8 as BS import Text.Regex as RE import Data.Time.Calendar import Data.Time.LocalTime import System.Environment -- Consider adding strictness as necessary for performance. -- STB time channel data ChannelChange = ChannelChange Int LocalTime Int deriving Show -- channel program_no start end network_no data ScheduleProgram = ScheduleProgram Int Int LocalTime LocalTime (Maybe Int) deriving Show main = do fileNames <- getArgs ensure (length fileNames == 2) usageMessage let [eventFileName,programFileName] = fileNames putStrLn ("Reading program schedule file from '" ++ programFileName ++ "'...") text <- BS.readFile programFileName programs <- sequence $ map parseScheduleProgram (BS.lines text) print (take 20 programs) return () usageMessage = "Usage: linear_importer <channel change file> <schedule file>" parseScheduleProgram :: ByteString -> IO ScheduleProgram parseScheduleProgram s = do let fields = BS.split '|' s ensure (length fields == 7) ("Wrong number of fields in schedule program: " ++ BS.unpack s) let [_,channelNoText,programNoText,_,startTimeText,endTimeText,networkNoText] = fields let channelNo = read $ BS.unpack channelNoText programNo = read $ BS.unpack programNoText startTime <- parseProgramTime startTimeText endTime <- parseProgramTime endTimeText let networkNo = if BS.null networkNoText then Nothing else Just (read (BS.unpack networkNoText)) return $ ScheduleProgram channelNo programNo startTime endTime networkNo parseProgramTime :: ByteString -> IO LocalTime parseProgramTime s = do let parts = BS.split 'T' s ensure (length parts == 2) ("Expected exactly one T in eventChannelChange time: " ++ BS.unpack s) let [datePart,timePart] = parts ensure (BS.length datePart == 8) ("Expected 8 digits in date part of eventChannelChange time: " ++ BS.unpack s) let (yearPart, monthDayPart) = BS.splitAt 4 datePart (monthPart, dayPart) = BS.splitAt 2 monthDayPart year = read $ BS.unpack yearPart month = read $ BS.unpack monthPart day = read $ BS.unpack dayPart let date = fromGregorian year month day ensure (BS.length timePart == 6) ("Expected 6 digits in time part of eventChannelChange time: " ++ BS.unpack s) let (hoursPart,minutesSecondsPart) = BS.splitAt 2 timePart (minutesPart,secondsPart) = BS.splitAt 2 minutesSecondsPart hours = read $ BS.unpack hoursPart minutes = read $ BS.unpack minutesPart seconds = read $ BS.unpack secondsPart let time = TimeOfDay hours minutes (fromInteger seconds) return (LocalTime date time) ensure :: Bool -> String -> IO () ensure x s = unless x $ ioError (userError s)

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Ninety Nine Haskell Problems
by Andrew Wagner 13 Dec '06

13 Dec '06

Hello, fellow Haskellers! jcreigh, from #haskell, has found a great set of exercises to work through. Check out the new wiki pages indexed at http://haskell.org/haskellwiki/99_Haskell_exercises . These have all been done in lisp, but of COURSE they can be done at least as well in Haskell, if not better. This is a great way for relatively new haskell programmers to contribute something, since most of the problems are relatively simple. So have at them! The template is already there, there's existing lisp code, and examples; all you have to do is write the canonical haskell version. Please follow the instructions for 'reserving' a problem so we're not all duplicating each others' work. We should be able to knock these problems out in a matter of hours! Have fun! Andrew

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Re: [Haskell] Waiting on multiple Chan-nels
by dons＠cse.unsw.edu.au 13 Dec '06

13 Dec '06

sebastian.setzer.ext: > Hi, > How do you wait on multiple channels, but read only from one of them > (the first that gets an entry)? Is there a library-function I missed > which already does this? > > What do you think of this solution? Feels to heavy :) An alternative solution that strikes me, is to use a single chan, with a sum type to tag the elements to be received by each thread. Then lazily generate a list of filtered values for each thread. Then the reading threads need do no actual IO to read. In particular, doing the chan reading IO under a lazy stream greatly simplifies the reader threads. For example: -- -- ghc chantest.hs -o chantest -- Usage: Enter "1", "2" or "0" to exit. -- import System.IO import Control.Concurrent import Control.Concurrent.Chan import Text.Printf -- -- Rather tha wait on multiple channels, it strikes me that you could -- have a single channel, and tag each thread's input. then lazily -- stream the chans contents through a filter, passing the *pure* list -- of filtered elements to a consuming thread -- type Pipe = Chan (Either String String) main :: IO () main = do chan <- newChan :: IO Pipe s <- getChanContents chan -- lazy list of chan elements c1Thread <- forkIO $ reader "c1" (catLeft s) -- read only Lefts c2Thread <- forkIO $ reader "c2" (catRight s) -- read only Rights writer chan where catLeft ls = [x | Left x <- ls] catRight ls = [x | Right x <- ls] writer :: Pipe -> IO () writer chan = loop where loop = getChar >>= command command '0' = print "done" command '1' = writeChan chan (Left "main: 1") >> loop command '2' = writeChan chan (Right "main: 2") >> loop command '\n' = loop -- ignore command c = printf "Illegal: %c\n" c >> loop reader :: String -> [String] -> IO () reader name xs = mapM_ (printf "%s %s\n" name) xs {- $ ghc x.hs -threaded $ ./a.out 1 c1 main: 1 2 c2 main: 2 2 c2 main: 2 1 c1 main: 1 2 c2 main: 2 3 Illegal: 3 1 c1 main: 1 2 c2 main: 2 0 "done" -} -- Don P.s. redirected to haskell-cafe(a)haskell.org, which is more appropriate for design discussion.

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Name that function =)
by Louis J Scoras 13 Dec '06

13 Dec '06

Hi all; I'm new to the list, so I just wanted to say hi and -- of course -- ask a quick question: I have some IO actions that I want to map over a list of pairs -- these are just directory names and their down-cased versions. It wasn't difficult to actually get the right behavior by just doing mapM twice. -- putDirs just outputs something like "mv fst snd" mapM_ putDirs pairs mapM_ (uncurry renameFile) pairs This bothered me though, because I suspected that this could be done in one pass. Naively I proceeded to this. mapM_ (putDirs >> (uncurry renameFile)) pairs Which didn't work. I thought about it a little more before realizing that putDirs wouldn't get any parameters this way: I needed some way to distribute the pair over both operations. Here's the higher-order function I needed: foo h f g i = h (f i) (g i) which could then be curried and we get: mapM_ (foo (>>) putDirs $ uncurry renameFile) pairs Works great. So my question: is there a established name for foo? What about foo partially applied to (>>)? This was a fun exercise, but I'd like to use the standard implementations if they exist. Thanks! -- Lou.

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