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RE: How do I create dynamic exceptions
by Bayley, Alistair 22 Jul '03

22 Jul '03

> The documentation for Data.Dynamic is pretty clear for this and I > believe with GHC6 you can derive Typeable. > > {-# NOINLINE myExceptionTyCon #-} > -- the documentation suggests a fully qualified name > myExceptionTyCon = mkTyCon "MyException" > > instance Typeable MyException where > typeOf _ = mkAppTy myExceptionTyCon [] > I disagree that the docs are clear. I could see that I needed a value of type TypeRep (a datatype with no constructors - so I can't create one directly), and also the mkApplyTy, mkFunTy, and applyTy functions, but it wasn't clear how to use them to get a TypeRep (which one do I use? and how?). An example like this is more helpful. Is there somewhere else I could have read about this? (I took a quick look at the wiki first.) I should have realised I could derive Typeable with ghc6; there was an itch in the back of my brain (now I remember reading about deriving Typeable in ghc6) that was trying to tell me there was an easy way... This is what I have at present, but it still doesn't catch the exception: > module Main where > import Prelude hiding (catch) > import Control.Exception > import Data.Dynamic > data MyException = MkExc Int String > deriving Typeable > mkErr :: Int -> String -> MyException > mkErr n s = MkExc n s > temp :: IO () > temp = do > putStrLn "line 1" > throwIO (DynException (toDyn (mkErr 2 "my temp"))) > handler :: Dynamic -> IO () > handler e = do > case (fromDynamic e) of > Nothing -> putStrLn ("dynamic cast failed") > Just (MkExc n s) -> putStrLn ("exception: " ++ (show n) ++ " " ++ s) > main :: IO () > main = catchDyn temp handler ***************************************************************** The information in this email and in any attachments is confidential and intended solely for the attention and use of the named addressee(s). This information may be subject to legal professional or other privilege or may otherwise be protected by work product immunity or other legal rules. It must not be disclosed to any person without our authority. If you are not the intended recipient, or a person responsible for delivering it to the intended recipient, you are not authorised to and must not disclose, copy, distribute, or retain this message or any part of it. *****************************************************************

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How do I create dynamic exceptions
by Bayley, Alistair 22 Jul '03

22 Jul '03

Exceptions, again... The docs for Control.Exception say (for Dynamic exceptions): "When using dynamic exceptions it is advisable to define a new datatype to use for your exception type, to avoid possible clashes with dynamic exceptions used in other libraries." So I went with that and created a datatype for my exceptions: > data MyException = MkExc Int String ... but now it seems I have to install this as an instance of Typeable, and I don't see how to do that. > instance Typeable MyException where > typeOf ? = ? ***************************************************************** The information in this email and in any attachments is confidential and intended solely for the attention and use of the named addressee(s). This information may be subject to legal professional or other privilege or may otherwise be protected by work product immunity or other legal rules. It must not be disclosed to any person without our authority. If you are not the intended recipient, or a person responsible for delivering it to the intended recipient, you are not authorised to and must not disclose, copy, distribute, or retain this message or any part of it. *****************************************************************

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Ignoring ExitExceptions
by Tim Jolly 21 Jul '03

21 Jul '03

I'm looking for a way to return an arbitrary exit code to the OS. The standard `exitWith' function does the job, but unfortunately it also has the side-effect of reporting an exception (I'm using GHC 6.0, with Control.Exception imported for other purposes). When a fatal error occurs, my program will already have issued a meaningful error message, and the added "*** Exception: exit: ExitFailure 2" will serve only to confuse users. Unfortunately it appears that the only way to return an exit code is to let an ExitException pass unhandled. All I'm really asking for is a way to use the exitWith function as defined in Haskell 98, along with the exception extensions provided in Control.Exception. Is there perhaps some way to prevent ExitExceptions from being reported, or a way to exclude certain types of exception from the handling in Control.Exception? Or can anyone suggest an alternative approach? -- TJ

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How to catch an exception
by Bayley, Alistair 21 Jul '03

21 Jul '03

Trying to get the hang of exceptions... I would expect this program: > module Main where > import Control.Exception hiding (GHC.Prelude.catch) > temp :: IO () > temp = do > putStrLn "line 1" > ioError (AssertionFailed "my temp") > handler :: Exception -> IO () > handler e = putStrLn ("exception: " ++ (show e)) > main :: IO () > main = catch temp handler .. to output: line 1 exception: AssertionFailed: my temp (or whatever "show" produces for the AssertionFailed exception) ... but all I get is: line 1 Fail: my temp This implies that the handler is not called. So what am I doing wrong? ***************************************************************** The information in this email and in any attachments is confidential and intended solely for the attention and use of the named addressee(s). This information may be subject to legal professional or other privilege or may otherwise be protected by work product immunity or other legal rules. It must not be disclosed to any person without our authority. If you are not the intended recipient, or a person responsible for delivering it to the intended recipient, you are not authorised to and must not disclose, copy, distribute, or retain this message or any part of it. *****************************************************************

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segfault in haskell!
by David Roundy 21 Jul '03

21 Jul '03

I've managed to get a segfault in haskell! And without even using the FFI... actually my code uses the FFI, but the changes that triggered the segfault don't involve that, they just use Text.Regex. The code that triggers the segfault is the function produced by: filetype_function :: IO (FilePath -> FileType) filetype_function = do binsfile <- def_prefval "binariesfile" "_darcs/prefs/binaries" bins <- (liftM lines $ readFile binsfile) `catch` (\e-> if isDoesNotExistError e then return [] else ioError e) let rs = map (mkRegex.fixregex) bins isbin f = or $ map (\r-> matchRegex r f /= Nothing) rs ftf f = if isbin f then BinaryFile else TextFile in return ftf and the valgrind report on the segfault is: ==8705== Invalid free() / delete / delete[] ==8705== at 0x4015D6A4: free (vg_clientfuncs.c:185) ==8705== by 0x40399A34: (within /lib/libc-2.3.1.so) ==8705== by 0x40399C13: regfree (in /lib/libc-2.3.1.so) ==8705== by 0x8117683: s67U_entry (in /home/droundy/darcs/darcs) ==8705== Address 0x1 is not stack'd, malloc'd or free'd ==8705== ==8705== Invalid read of size 4 ==8705== at 0x40399A58: (within /lib/libc-2.3.1.so) ==8705== by 0x40399C13: regfree (in /lib/libc-2.3.1.so) ==8705== by 0x8117683: s67U_entry (in /home/droundy/darcs/darcs) ==8705== Address 0x22 is not stack'd, malloc'd or free'd Unfortunately inserting a "trace f $" before the isbin f shows that it is segfaulting on the string "Patch.lhs". This looks to me like a bug in either Text.Regex or glibc (with my bet being on Text.Regex. Any ideas how to track it down and/or work around it? Oh yeah, and I'm running ghc 6.0. Maybe this is fixed in CVS? -- David Roundy http://www.abridgegame.org

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Help with propositional formulas
by Douglas O. Matoso 20 Jul '03

20 Jul '03

Hi, I have an school assignment that asks to implement an truth table of propositional formulas. I'm having difficulties in the following part: data Prop = Var Char | Not Prop | And Prop Prop | Or Prop Prop type Subst = [(Char,Bool)] - Define a function bools :: Int -> [[Bool]] that calculates all possible lists of logical values of a specific length. For example, bools 2 should give the following list: [[False,False], [False,True ], [True ,False], [True ,True ]] - Define a function substs :: Prop -> [Subst] that calculates all possible substitutions for the variables of a proposition. For example, substs p2 should give the following list: [[('A',False),('B',False)], [('A',False),('B',True) ], [('A',True) ,('B',False)], [('A',True) ,('B',True) ]] I would be thankful for any help. Douglas Matoso ************************************************************************************************** ** Este email foi scaneado contra virus e conteudo malicioso ** **************************************************************************************************

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how implement a UArray with a ForeignPtr?
by David Roundy 19 Jul '03

19 Jul '03

Hello. I was wondering if there might be any relatively simple way to implement a type equivalent to UArray, but with the memory stored in a ForeignPtr? It seems like this would be a commonly useful situation, where you have a C function that returns a pointer to an array of data, and it would be nice to be able to access that data without making a copy of it. In case you're wondering, the reason for not allocating the memory within haskell itself and passing it to the C side is that I want to use mmap, which really needs its own finalizer. >From browsing the ghc base libraries, it looks like I'll pretty much just need peekElemOff and unsafePerformIO (which should be safe as long as I don't give a copy of the ForeignPtr to anyone else who might try to write to it--and provided the file isn't opened for write while I'm trying to read it). Does this sound like the right idea, or is there some problem? Might this actually turn out to be slow, which would defeat the whole purpose? -- David Roundy http://www.abridgegame.org

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PARCELAS PUNTA DE CHOROS O $2.000.000 SOLO JULIO Consulte Facilidades
by Tu Tierra 18 Jul '03

18 Jul '03

OFERTA DE INVIERNO PARCELAS PUNTA DE CHOROS CHILE VALOR PARCELA $2.000.000 CHILE CHILE A solo 120 km al norte de La Serena Parcelas de 5075 m2 Fono 09-7979918 -51-311479 Un Entorno Privilegiado de Flora y Fauna A Minutos de la Caleta de Punta de Choros Escritura Inmediata www.tutierra.cl A 2500 Metros de la Playa Grande www.tutierra.cl PROMOCIÓN VENTA DE INVIERNO VALOR PARCELA $2.000.000 USTED DEFINE EL PIE DIFERENCIA 15 CHEQUES TOME SU CUPO OFERTA HASTA EL 31 DE JULIO FORMULARIO DE CONSULTAS AL CONSULTAR RECIBIRÁ TODA LA INFORMACIÓN DEL LOTEO, FOTOGRAFÍAS ANTECEDENTES LEGALES, EMPLAZAMIENTO, UBICACIÓN Y ANTECEDENTES GENERALES PARCELAS DE AGRADO PUNTA DE CHOROS WWW.TUTIERRA.CL VENTA(a)TUTIERRA.CL 09-7979918 SI NO DESEA RECIBIR MAS INFORMACIÓN COMUNIQUESE A ESTE MAIL no deseo recibir mas información (Nota: la legislación chilena no contempla el envío de publicidad por correo electrónico. Respetamos la normativa internacional que establece que ningún e-mail puede considerarse SPAM si se le estipula a las personas instrucciones precisas de como removerse de esta lista

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Case expressions, matching, and "constants"
by Bayley, Alistair 18 Jul '03

18 Jul '03

I've just debugged a program that used a case expression, but where I was trying to match on constants rather than literals. Here's a contrived example: > module Main where > one = 1 > two = 2 > > test n = > case n of > one -> "one" > two -> "two" > _ -> "three" > > main = putStrLn (test 2) This initial version seems to me to be the "natural" way to write the case expression, but it doesn't work because the first alternative always succeeds. This is what I've turned it into to get it to work. It seems a bit clumsy; is there a better way to write this? > test n = > case True of > _ | n == one -> "one" > | n == two -> "two" > | otherwise -> "three" ***************************************************************** The information in this email and in any attachments is confidential and intended solely for the attention and use of the named addressee(s). This information may be subject to legal professional or other privilege or may otherwise be protected by work product immunity or other legal rules. It must not be disclosed to any person without our authority. If you are not the intended recipient, or a person responsible for delivering it to the intended recipient, you are not authorised to and must not disclose, copy, distribute, or retain this message or any part of it. *****************************************************************

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new debian packages and archive: haskell experimental
by Isaac Jones 18 Jul '03

18 Jul '03

Greetings, I've set up a Debian archive (apt source) for experimental Haskell packages and backports. For instance, in the "unstable" section, you might find bleeding edge packages which will eventually be uploaded to Debian if they work (like the arrow preprocessor). In the "stable" section, you'll find ghc6 and newer versions of ghc5 than what you'll find in the official Debian archives (thank Ian Lynagh for the ghc work). See the last section of the Debian Haskell User's Wiki Page for more info including information about how you can contribute packages: http://www.haskell.org/hawiki/DebianUsers People using Debian Stable (woody) can install ghc6 by adding this line to your /etc/apt/sources.list file: deb http://www.syntaxpolice.org/haskell-experimental/ stable/ then run "apt-get update && apt-get install ghc6" (perhaps these instructions can be included on ghc6's web page?) If you're running anything besides stable, you know what to do :) peace, isaac

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