May 2007 - Haskell-Cafe

Newbie list question
by junkywunky 27 May '07

27 May '07

type Person = (NI, Age, Balance) type Bank = [Person] credit :: Bank -> [Person] credit [(a,b,c)] = [(a,b,c)] This code works when I type in: credit [(1,2,3)] but doesn't work when I type in: credit [(1,2,3),(4,5,6)] Any help? Thanks in advance. -- View this message in context: http://www.nabble.com/Newbie-list-question-tf3823011.html#a10823175 Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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problem reading /proc/interrupts with ByteString
by Leandro Lisboa Penz 27 May '07

27 May '07

Hi I'm having some problems using ByteString.Char8.readFile to read "/proc/interrupts". I reads nothing. I guess maybe that's because the file is sort of special, but I could read it by opening and using ByteString.Char8.hGetContents. The part of interest is: findInterrupts :: B.ByteString -> Int findInterrupts str = findI where findI = read $ B.unpack $ getNums $ skipSpaces $ skip1 from1 = case B.findSubstring (B.pack "1:") str of Nothing -> error $ "Not found in [" ++ B.unpack str ++ "]" Just a -> a skip1 = B.drop from1 str skipSpaces = B.dropWhile (\c -> c == ' ') getNums = B.takeWhile (\c -> c `elem` "1234567890") getNumInterrupts :: IO Int getNumInterrupts = do h <- openFile "/proc/interrupts" ReadMode c <- B.hGetContents h hClose h -- c <- B.readFile "/proc/interrupts" -- With readFile the contents are emtpy -- B.putStrLn c return $ findInterrupts c Whole code can be found at http://hpaste.org/63 Thanks in advance Leandro Penz

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CYK-style parsing and laziness
by Steffen Mazanek 27 May '07

27 May '07

Hello, I have two questions regarding a Cocke, Younger, Kasami parser. Consider this program: type NT = Char -- Nonterminal type T = Char -- Terminal -- a Chomsky production has either two nonterminals or one terminal on its right-hand side type ChomskyProd = (NT, Either T (NT, NT)) -- a grammar consists of a startsymbol, nonterminal symbols, terminal symbols and productions type Grammar = (NT, [NT], [T], [ChomskyProd]) parse::Grammar->[T]->Bool parse (s, nts, ts, prods) w = s `elem` gs n 1 where n = length w table = [[gs i j|j<-[1..n-i+1]]|i<-[1..n]] gs 1 j = [nt|p<-prods,termProd p, let (nt, Left t)=p, w!!(j-1)==t] gs i j = [nt|k<-[1..i-1],p<-prods, not (termProd p), let (nt, Right (a, b))=p, a `elem` table!!(k-1)!!(j-1), b `elem` table!!(i-k-1)!!(j+k-1)] The sets gs i j contain all nonterminal symbols from which the substring of w starting at index j of length i can be derived. Please have a look at the last line of the algorithm. In my first attempt I just referred to gs k j and gs (i-k) (j+k) what looks a lot more intuitive. However I noted that this way the sets gs i j are evaluated multiple times. Is there a better and more readable way to prevent multiple evaluation of these sets? The second question regards lazy evaluation. Consider the stupid grammar S->S S S->A A A->a that generates a^(2n). The performance of the algorithm drops very fast even for small n, probably because the gs i j are getting very large. Is there a trick to get lazy evaluation into play here? It is sufficient to find only one occurence of the start symbol in gs n 1. Best regards, Steffen

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Haskell Library on CNN
by Yitzchak Gale 27 May '07

27 May '07

Border conditions in the Haskell Library have finally made the mainstream media: http://www.cnn.com/2007/US/05/26/border.quirk.ap/index.html -Yitz

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LUGRadio Live anyone?
by Magnus Therning 27 May '07

27 May '07

Just wondering if anyone else who frequents this list is planning on going to Wolverhampton for LRL in July? /M -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus(a)therning.org Jabber: magnus.therning(a)gmail.com http://therning.org/magnus Software is not manufactured, it is something you write and publish. Keep Europe free from software patents, we do not want censorship by patent law on written works. Failure is not an option. It comes bundled with the software.

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Cannot compile Network.CGI programs
by Adrian Neumann 26 May '07

26 May '07

-----BEGIN PGP SIGNED MESSAGE----- Hash: RIPEMD160 Hi, I installed the Network.CGI package and tried to compile the Hello World example on my Ubuntu machine. >ghc cgi.hs -o cgi gives me > cgi.o: In function `r1hk_info': > (.text+0x56): undefined reference to `mtlzm1zi0_ControlziMonadziTrans_zdf1_closure' > cgi.o: In function `r1hm_info': > (.text+0xb6): undefined reference to `cgizm2006zi9zi6_NetworkziCGIziMonad_zdf4_closure' > cgi.o: In function `r1ho_info': > (.text+0x11a): undefined reference to `cgizm2006zi9zi6_NetworkziCGI_runCGI_closure' > cgi.o: In function `r1ho_info': > (.text+0x121): undefined reference to `mtlzm1zi0_ControlziMonadziTrans_zdf1_closure' > cgi.o: In function `r1hq_info': > (.text+0x17e): undefined reference to `cgizm2006zi9zi6_NetworkziCGI_output_closure' > cgi.o: In function `s1hE_info': > (.text+0x2aa): undefined reference to `cgizm2006zi9zi6_NetworkziCGI_handleErrors_closure' > cgi.o: In function `r1hk_srt': > (.rodata+0x0): undefined reference to `mtlzm1zi0_ControlziMonadziTrans_zdf1_closure' > cgi.o: In function `r1hm_srt': > (.rodata+0x4): undefined reference to `cgizm2006zi9zi6_NetworkziCGIziMonad_zdf4_closure' > cgi.o: In function `r1ho_srt': > (.rodata+0xc): undefined reference to `cgizm2006zi9zi6_NetworkziCGI_runCGI_closure' > cgi.o: In function `r1ho_srt': > (.rodata+0x10): undefined reference to `mtlzm1zi0_ControlziMonadziTrans_zdf1_closure' > cgi.o: In function `r1hq_srt': > (.rodata+0x14): undefined reference to `cgizm2006zi9zi6_NetworkziCGI_output_closure' > cgi.o: In function `s1hE_srt': > (.rodata+0x38): undefined reference to `cgizm2006zi9zi6_NetworkziCGI_handleErrors_closure' > collect2: ld gab 1 als Ende-Status zurück However runghc cgi.hs produces a Hello World > Content-type: text/html; charset=ISO-8859-1 > > Hello World! Why is that so? What can I do to compile CGI-programs? (GHC 6.6, libghc6-cgi-dev 2006.9.6-3) -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.7 (MingW32) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org iD8DBQFGV+k211V8mqIQMRsRA62mAKCW0lp+zeniR+3Mi12+9dzRFMZm6QCaArpd R67KMtScgzFFDV0+GDbH75A= =Wuax -----END PGP SIGNATURE-----

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instance Monad AppF - Faster than the list monad?
by Robin Green 26 May '07

26 May '07

The following Haskell 98 module implements a generalisation of Prelude.ShowS for any type. Should be pretty easy to incorporate this into code which currently uses the list monad non-trivially, and get better performance - but can this be right? Surely someone would have published this before if that was true? I haven't actually done any performance tests. Anyway, with this module you end up using function composition instead of list concatenation - except when converting from a list. module Data.List.AppF where import Control.Monad (MonadPlus (mplus, mzero), msum) -- Generalisation of ShowS newtype AppF a = AppF { unAppF :: [a] -> [a] } instance Monad AppF where (>>=) = (msum .) . flip map . appFToList return = AppF . (:) instance MonadPlus AppF where mzero = AppF id mplus x y = AppF $ unAppF x . unAppF y -- Use this to convert Maybe a into AppF a, or indeed any other -- MonadPlus instance. maybeToMonadPlus :: MonadPlus m => Maybe a -> m a maybeToMonadPlus = maybe mzero return listToAppF :: [a] -> AppF a listToAppF = AppF . (++) appFToList :: AppF a -> [a] appFToList = ($ []) . unAppF

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Why this exception occurs ?
by L.Guo 26 May '07

26 May '07

Hi. In GHCi ver 6.6, why this happens ? Prelude Data.ByteString> Data.ByteString.pack $! Prelude.map (`rem` 256) $ [0..511] "*** Exception: divide by zero -------------- L.Guo 2007-05-25

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The danger of Monad ((->) r)
by Tomasz Zielonka 26 May '07

26 May '07

Hello! In a CGI application I was gathering SQL statements I wanted to run in the final transaction. Because I use haskelldb, it was most convenient to use (Database -> IO ()) as the type of the statement or a group of statements. In this representation concatenating two statement groups so they are executed in sequence can be done with: concatStmts s1 s2 = \db -> s1 db >> s2 db My mistake was that I forgot about db and wrote: concatStmts s1 s2 = s1 >> s2 And it was accepted because I had the Monad instance for ((->) r) in scope (from Control.Monad.Trans I guess)! Of course it doesn't do the right thing - s1 is simply ignored. Fortunately I noticed it quite quickly, but I think a similar bug can cause someone a big headache. Perhaps we could move this instance to some separate module, so people won't import it if they don't want it or even don't know about its existence? Best regards Tomek

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Resend: seeking code review
by Justin Bailey 26 May '07

26 May '07

I'm resending my message of yesterday with the code as a zipped attachment. Any and all feedback is welcome. My apologies in advance for the double-post! >From the documentation: "This module provides a set of functions for building simple command-line interfaces. It allows interfaces which collect values (such as Integers, Dates, and more structured values), build lists of values, and use simple menus. It is not intended to build complex interfaces with full cursor control. It is oriented towards line-based interfaces." Justin

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