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Listing package dependencies from GHC
by Dougal Stanton 08 May '07

08 May '07

If I'm doing development between ghci and vim, all the different dependencies I need get linked in when required without me asking. Similarly if I call "ghc --make" from the command line. But I have to write them in manually to my *.cabal file otherwise the compilation process will fail. Until now I've just loaded the program in to ghci and noted down all the dependencies it links in. Am I missing a trick? There must be an easier way, especially for multiple source files. cheers, D.

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Using dump in Netwotk.Pcap
by Harald ROTTER 08 May '07

08 May '07

Dear Haskellers, as a Haskell newbie I really enjoyed Austin Seipp's article on writing a packet sniffer in Haskell using the libpcap. I played around with Network.Pcap and tried to dump packets into a pcap file using the dump function: dump :: Ptr PcapDumpTag -> Ptr PktHdr -> Ptr Word8 -> IO () As compared to the loop function (used by Austin's packet sniffer) loop :: Ptr PcapTag -> Int -> Callback -> IO Int type Callback = PktHdr -> Ptr Word8 -> IO () the dump function takes a "Ptr PktHdr". If I want to "dump" inside a Callback, I have the problem of getting a "PktHdr" argument, but I need a "Ptr PktHdr" to dump. I tried to use "with": with :: Storable a => a -> (Ptr a -> IO b) -> IO b but PktHdr is unfortunately not an instance of the "Storable" class. How is it possible to use the "dump" function (that needs a Ptr PktHdr) inside a Callback that only passes PktHdr ? Any help is appreciated. Thank you Harald. " Ce courriel et les documents qui y sont attaches peuvent contenir des informations confidentielles. Si vous n'etes pas le destinataire escompte, merci d'en informer l'expediteur immediatement et de detruire ce courriel ainsi que tous les documents attaches de votre systeme informatique. Toute divulgation, distribution ou copie du present courriel et des documents attaches sans autorisation prealable de son emetteur est interdite." " This e-mail and any attached documents may contain confidential or proprietary information. If you are not the intended recipient, please advise the sender immediately and delete this e-mail and all attached documents from your computer system. Any unauthorised disclosure, distribution or copying hereof is prohibited."

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How to parse a date and time?
by Justin Bailey 08 May '07

08 May '07

Looking at the libraries documentation, it seems like parsing a string into a time and date should be as simple as: import Data.Time.Format (parseTime) myDate = parseTime "..." where "..." is some string. But these functions don't seem to exist! I can't find many references to them in the wild, and the module "Data.Time.Format" is not found under GHC 6.6 or Hugs. I am running Windows, if it makes a difference. Are the docs just out of date[1] or is it my install? How can I parse a string into a Data.Time.Calendar.Day value (or some other time value)? Thanks for any help! Justin [1] http://haskell.org/ghc/docs/latest/html/libraries/time/Data-Time-Format.html

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The Trivial Monad
by Adrian Neumann 08 May '07

08 May '07

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA512 I've read this blogpost about the "trivial monad" http://sigfpe.blogspot.com/2007/04/trivial-monad.html, because I still don't understand what this monad thingy is all about. The author defines three functions: data W a = W a deriving Show return :: a -> W a return x = W x fmap :: (a -> b) -> (W a -> W b) fmap f (W x) = W (f x) bind :: (a -> W b) -> (W a -> W b) bind f (W x) = f x and asks the reader to prove the tree monad laws for them. However I don't understand the type signatures for bind and fmap. I'd say (and ghci's type inference agrees) that bind and fmap have the type bind:: (a->W b) -> W a -> W b fmap:: (a->b) -> W a -> W b They take a function f and something and return what f does to that. I don't see why they should return a function. This of course makes it hard for me to prove the monad laws. The first however works nonetheless: 1) bind f (return a)= f a => bind f (return a)= bind f (W a) = f a Can someone explain bind and fmap (and possible law 2 and 3)? Thanks, Adrian -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.6 (Darwin) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org iD8DBQFGOuQS11V8mqIQMRsRCmngAJ9NwQMwXeS/PSM1NUsVA8gxPuA0KACfSLiA ItqRZW5a4XyQ099bhMtSWmU= =/8i/ -----END PGP SIGNATURE-----

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Re: a question concerning type constructors
by oleg＠pobox.com 07 May '07

07 May '07

> In Haskell, is it possible to declare a type constructor with a variable > number of type variables e.g. > > data Tuple * .... > > allowing the following declarations: > > t: Tuple > u: Tuple Bool > v: Tuple Bool Int > w: Tuple Bool Int Char Although the data constructor such as the `Tuple' is not possible literally, the declarations like the above can be achieved. Please see the examples in http://www.haskell.org/pipermail/haskell/2006-September/018487.html

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Re: [Haskell-cafe] GHC Hangs
by Josiah Manson 07 May '07

07 May '07

> http://www.haskell.org/ghc/dist/6.6.1/ghc-6.6.1-i386-windows-test1.exe > > Might be worth trying that to see if there's still a problem. > There is still a problem when I use the version you linked to. At first I was hopeful, because the first time I tried compiling bisect.hs it worked. All subsequent attempts, however, have failed.

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GHC Hangs
by Josiah Manson 07 May '07

07 May '07

Hello Haskellers, I have a problem where GHC will hang after compiling a file. The problem seems to occur with my computer, but not with everyone else's. I hope that someone my be able to diagnose my problem. My computer is an Acer Aspire 5670, which runs on an Intel Core Duo T2300 and runs Windows XP Home Edition. My version of GHC is The Glorious Glasgow Haskell Compilation System, version 6.6. When supplied the file bisect.hs with the following contents: sign a | a < 0 = -1 sign a | a >= 0 = 1 bisect f a b = if b - a < eps then c else if sign (f a) == sign (f c) then bisect f c b else bisect f a c where c = (a + b) / 2 eps = 0.000000000000001 main = print $ bisect cos 0 2 Running produces this output at which point nothing further happens. The same thing happens without -dcore-lint -debug, but I have included it because someone suggested it would provide useful diagnostics. C:\data\code\other\haskell\simple>ghc --make bisect.hs -dcore-lint -debug [1 of 1] Compiling Main ( bisect.hs, bisect.o ) The file bisect.o is generated, but GHC is stopped and not using any CPU time. As well, there are two other processes that were spawned and must also be killed separately named gcc.exe and as.exe. Interestingly, if I do multiple iterations of running ghc then killing it. I eventually get a working executable. As far as I can tell, GHC is deadlocked, which makes me wonder if dual cores are causing the problem. BTW, problems like this have occurred in other free software like Open Office, but leaves most other programs unaffected. Josiah

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An interesting toy
by Andrew Coppin 07 May '07

07 May '07

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Arbitrary precision?
by Andrew Coppin 07 May '07

07 May '07

Greetings. Haskell has arbitrary precision integers, in the form of the Integer type. Is there a type somewhere that implements arbitrary precision fractional values?

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symbol $
by Maverick 07 May '07

07 May '07

Hi Do you mind tell what is mean by the symbol $ in this piece of code? Thanks a lot. > mcompile :: Prog -> State Int Code > mcompile (Assign n e) = return $ compileExpr e ++ [POP n] > mcompile (If e p1 p2) = do cur <- fresh > pp1 <- mcompile p1 > pp2 <- mcompile p2 > return $ (compileExpr e ++ (JUMPZ cur):pp1 ++ > JUMP (cur+1):(LABEL cur):pp2 ++ [LABEL (cur+1)]) > mcompile (While e p) = do cur <- fresh > mp <- mcompile p > return $ [LABEL cur] ++ compileExpr e ++ [JUMPZ (cur+1)] ++ mp ++ [JUMP cur,LABEL (cur+1)] -- View this message in context: http://www.nabble.com/symbol-%24-tf3701618.html#a10351407 Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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