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GADT and typeclasses [was: Language extensions]
by oleg＠pobox.com 28 May '07

28 May '07

Philippa Cowderoy wrote: > For example, GADTs let you implement monads as interpreters by defining a > datatype representing the abstract syntax tree that describes a > computation - you can't get this to type without at a minimum existential > types and for many monad operations you need the full power of GADTs to > declare a corresponding constructor. I'm yet to see the example of that need. I have seen the examples that the need for GADT was _claimed_ -- but then it turns out the example is implementable without GADT after all. Here are a few such examples: implementing State monad in a free term algebra Initial (term) algebra for a state monad http://www.haskell.org/pipermail/haskell-cafe/2005-January/008241.html Implementing an interpreter in HOAS with fix Even higher-order abstract syntax: typeclasses vs GADT http://www.haskell.org/pipermail/haskell/2007-January/019012.html

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Parsec question
by Andrew Coppin 27 May '07

27 May '07

Greetings. I'd like to write a parser that takes some Haskell source code and seperates it into two piles - comments, and everything else. I have a parser that recognises single-line comments, and another that recognises multi-line comments. What I'd like to do is make a big parser that returns [Either String String], which all the comments in one side and all the rest in the other side. But I can't figure out how to construct such a parser... Any hints?

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Broadcast signals between threads
by Joachim Breitner 27 May '07

27 May '07

[Arg. Just lost the mail and have to re-type it. Sorry if I’m not as verbose as I should be]. Hi, I’m writing a TCP server app ATM. It has one thread per client. Some of the clients want to be notified if the internal state changes, while others are happily chatting with the server, possible modifying the internal state. What I need now is a way for the chatting thread to signal “anyone interested” that the state has changed. The semantics should be roughly these: * All listening threads run some wait-for-signal command that blocks until the signal is sent. * The state changing thread notifies the others by some signal-sending-command, which should not block. * It does not matter if no one is listening, or if threads miss signals because they are doing something else for the moment, or if several sent signals are handled only once. Here is my implementation so far: newtype MSignal a = MS (MVar a) newMSignal = MS `liftM` newEmptyMVar sendMSignal (MS mv) v = do forkIO $ takeMVar mv >> return () -- Cleanup afterwards putMVar mv v receiveMSignal (MS mv) = readMVar mv It builds on MVar’s promise that takeMVar and readMVar commands are handled FIFO, so by spawning a takeMVar thread, the sender ensures that exactly after all currently waiting threads are served the value is taken out of the MVar and everything is done. readMVar will both take the value and put it back, so that the next waiting thread (or the sending thread’s cleanup thread) can take the value. Do you think this is a sensible way to do it? Any pitfalls that I have overlooked? Might this be useful enough to be included in some mainstream library? Greetings, Joachim -- Joachim Breitner e-Mail: mail(a)joachim-breitner.de Homepage: http://www.joachim-breitner.de ICQ#: 74513189

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FFI and multi-threading
by Maurício 27 May '07

27 May '07

Do you guys know of a good example of Haskell calling functions to a thread-safe library (like an existing interface to a real library)? I have a function to which I need to alocate memory for each thread calling it, but I don't know how to do that. Thanks, Maurício

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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 26 May '07

26 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 26 May '07

26 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 26 May '07

26 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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