- Haskell-Cafe - Haskell.org

ENQUIRY
by MR. DAVID LARSON 25 Jul '03

25 Jul '03

FROM: MR. DAVID LARSON ANTHONY EZE & CO. SIR, SUBJECT: TRUST NEED YOUR KIND RESPONSE TO THE FOLLOWING REQUEST: I HAVE A MANDATE TO SOURCE FOR A FINANCIAL MANAGEMENT PERSONNEL ASIDE AFRICA, BE IT YOUR PROFFESSION OR NOT, ITS NATURE DOES NOT COUNT, BUT ONLY HONESTY IS NEEDED AT ITS PEAK AND WANTED AS MOST PRIORTY TO ACTUALIZE THIS EXPECTED GOAL. HAS CLOSE LINK WITH THE PERSONNELS IN THE CORRIDORS OF POWER IN MY COUNTRY HAS BEEN DELEGATED TO SOURCE FOR A COMPETENT ASSOCIATE OUTSIDE AFRICA IF YOU ARE WORTHY OF BEEN TRUSTED WITH HUGE CAPITAL FOR ESTABLISHMENT IN YOUR COUNTRY CONTACT ME FOR FULL PERSONAL SECRETIVE INFORMATION OR DETIALS FOR COMMENCEMENT AND TERMS OF REWARD FOR YOUR EXPERTED INTEREST AND CONTRIBUTION. THE VOLUME OF CAPITAL WILL BE DISCLOSED TO YOU ON MY RECEIPT OF YOUR RETURN CORRESPONDENCE INDICATING YOUR WILLINGNESS TO PER-TAKE. KINDLY RESPOND TO THIS MAIL INCLUDING YOUR PERSONAL CONTACT DATA'S FOR FULL DISCLOSURE OF PROCEEDURES. JUST WAITING FOR YOUR REVERT THANKS MR. DAVID LARSON

1 0

RE: How do I create dynamic exceptions
by Bayley, Alistair 23 Jul '03

23 Jul '03

> From: Ross Paterson [mailto:ross@soi.city.ac.uk] > > On Tue, Jul 22, 2003 at 02:59:12PM +0100, Bayley, Alistair wrote: > > > 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 > > You're catching a Dynamic wrapped as a Dynamic (catchDyn already does > fromDynamic, as indicated by its type). Change the handler to > > > handler :: MyException -> IO () > > handler (MkExc n s) = putStrLn ("exception: " ++ (show n) > ++ " " ++ s) > > any other dynamic exception fails through to the top level. Thanks. I was applying some advice from Hal Daume: > so, what you want is probably something like: > > > if rc < 0 > > then throwIO (DynException (toDyn ("Err", rc))) > > else ... > > and then when you catch, use fromDyn(amic) to get the value out if you can > predict its type... ... but in this case what I wanted was (just one more layer of indirection): > handler :: Exception -> IO () > handler e = do > case e of > DynException err -> case (fromDynamic err) of > Nothing -> putStrLn "dynamic cast failure" > Just (MkExc n s) -> putStrLn ("exception: " ++ (show n) ++ " " ++ s) > _ -> putStrLn "unhandled exception" > main :: IO () > main = catch temp handler -- use catch rather than catchDyn ***************************************************************** 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. *****************************************************************

1 0

RE: instance Typeable (Re: How do I create dynamic exceptions)
by Simon Peyton-Jones 22 Jul '03

22 Jul '03

| The above should work, but I wonder when it is possible to simply derive | Typeable? GHC 6.0 supports deriving( Typeable ), but for some strange reason it's not in the manual. It will be. Simon

2 1

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. *****************************************************************

2 1

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. *****************************************************************

3 2

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

1 0

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. *****************************************************************

2 1

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

1 0

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 ** **************************************************************************************************

2 1

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

2 1