June 2003 - Haskell-Cafe

RE: How do I create an IOError exception?
by Bayley, Alistair 05 Jun '03

05 Jun '03

Ahh... I see now (thanks). This advice (in the documentation for System.IO.Error.ioError) threw me: "The ioError variant should be used in preference to throw to raise an exception within the IO monad because it guarantees ordering with respect to other IO operations, whereas throw does not." After reading this I assumed that I should also use the exceptions in System.IO.Error i.e. the IOError type. But of course this is not the case; ioError has type Exception -> IO a. -----Original Message----- From: Hal Daume III [mailto:hdaume@ISI.EDU] Sent: 04 June 2003 16:32 To: Bayley, Alistair Cc: haskell-cafe(a)haskell.org Subject: Re: How do I create an IOError exception? In Control.Exception, there's: > data Exception = ... > | DynException Dynamic > | ... and > throwIO :: Exception -> IO a 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... HTH. - Hal -- Hal Daume III | hdaume(a)isi.edu "Arrest this man, he talks in maths." | www.isi.edu/~hdaume On Wed, 4 Jun 2003, Bayley, Alistair wrote: > (I know I'm asking some noddy questions, but hey, that's what this list is > for...) > > How do I create IOError exceptions? System.IO.Error has two functions that > create IOErrors: userError and mkIOError. However, both of them take a > String (I assume containing a description of the problem), and mkIOError > takes optional Handle and FilePath args. > > http://www.haskell.org/ghc/docs/latest/html/base/System.IO.Error.html > > What I'd like to do is stuff some more information into the exception (a bit > like Java exceptions), along the lines of: > > > mkIOError :: a -> IOError > > where I could choose to use it like this (ociHandleAlloc is a foreign C > function): > > > handleAlloc2 handleType env ptr = do > > rc <- ociHandleAlloc env ptr handleType 0 0 > > if rc < 0 > > then ioError (mkIOError ("Couldn't allocate handle", rc)) > > else peek ptr > > > When I catch the exception (currently in main) I would like to interrogate > it to get the values I stuffed in it (in this case a (String, Int) tuple), > and then do some error reporting by calling another function which decodes > the error number. > > Is this the wrong way to go about error handling? Or is it relatively simple > to create your own IOErrors? Have I missed something in the docs? > ***************************************************************** 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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FFI Help
by Matthew Donadio 04 Jun '03

04 Jun '03

Hi all, I am just starting to experiment with FFI, and am running into a problem. I was to create an FFI to the lgamma(3) found in many of the newer libm implementations. My code follows the sig. The lgamma function works. The gamma function core dumps (I am using ghc 5.04.3) on me. gdb reports a SIGSEGV in signgam(), but I'm not sure why. I believe that I need to use the monad because signgam is only valid after lgamma returns. Does anyone have an idea what I am doing wrong? Thanks. -- Matthew Donadio <m.p.donadio(a)ieee.org> > module Gamma (gamma, lgamma) where > import System.IO.Unsafe > foreign import ccall "math.h lgamma" lgammaC :: Double -> IO Double > foreign import ccall "math.h signgam" signgamC :: IO Int > lgamma :: Double -> Double > lgamma x = unsafePerformIO $ lgammaC x > gamma :: Double -> Double > gamma x = unsafePerformIO $ gammaIO x > gammaIO :: Double -> IO Double > gammaIO x = do lg <- lgammaC x > s <- signgamC > return $ fromIntegral s * exp lg

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How do I create an IOError exception?
by Bayley, Alistair 04 Jun '03

04 Jun '03

(I know I'm asking some noddy questions, but hey, that's what this list is for...) How do I create IOError exceptions? System.IO.Error has two functions that create IOErrors: userError and mkIOError. However, both of them take a String (I assume containing a description of the problem), and mkIOError takes optional Handle and FilePath args. http://www.haskell.org/ghc/docs/latest/html/base/System.IO.Error.html What I'd like to do is stuff some more information into the exception (a bit like Java exceptions), along the lines of: > mkIOError :: a -> IOError where I could choose to use it like this (ociHandleAlloc is a foreign C function): > handleAlloc2 handleType env ptr = do > rc <- ociHandleAlloc env ptr handleType 0 0 > if rc < 0 > then ioError (mkIOError ("Couldn't allocate handle", rc)) > else peek ptr When I catch the exception (currently in main) I would like to interrogate it to get the values I stuffed in it (in this case a (String, Int) tuple), and then do some error reporting by calling another function which decodes the error number. Is this the wrong way to go about error handling? Or is it relatively simple to create your own IOErrors? Have I missed something in the docs? ***************************************************************** 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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RE: Help: writing ffi bindings to a C library
by Bayley, Alistair 04 Jun '03

04 Jun '03

Thanks. This is very helpful. I'm making much better progress now. Is alloca the idiomatic technique when you want to create a pointer to a pointer? Or are there other ways? Initially I tried to pass a Ptr (Ptr SomeType), but I couldn't because Ptr has no constructors To create values of type Ptr I must use the functions in Foreign.Marshal.Alloc (and Foreign.Marshal.Utils). Correct? -----Original Message----- From: Marcin 'Qrczak' Kowalczyk [mailto:qrczak@knm.org.pl] Sent: 02 June 2003 10:46 To: haskell-cafe(a)haskell.org Subject: Re: Help: writing ffi bindings to a C library Dnia pon 2. czerwca 2003 09:58, Bayley, Alistair napisał: > Yes, I had considered this (writing an abstraction layer in C). I have > decided initially to try doing it in Haskell, because I thought that > Haskell would be a better choice for writing abstraction layers (not as > verbose as C). You can write it in Haskell too and I would recommend that - unless the C library uses some passing convention Haskell cannot handle (e.g. passing structs by value, or having variable argument count, or relying heavily on macros). > Getting back to my question, can I use functions that take twice-deref'ed > pointers as args? This is a function that effectively returns some of its > work in "out" arguments Sure. You can use libraries documented in the Haskell FFI proposal. > e.g. a C function like this: > > int someCFunction(SomeType **ptrptr, int i) > > (It would create an opaque structure and point *ptrptr at it. If it fails > then it would return a negative value.) > > Is it simply a case of declaring it like this (in ghc): > > data SomeType > > type SomeTypeRef = Ptr SomeType > > foreign import ccall "blah.h someCFunction" someCFunction :: > > Ptr SomeTypeRef -> Int -> Int foreign import ccall "blah.h someCFunction" someCFunction :: Ptr SomeTypeRef -> CInt -> IO CInt haskellWrapper :: Int -> IO (Maybe SomeTypeRef) haskellWrapper arg = alloca $ \ptr -> do res <- someCFunction ptr (fromIntegral arg) if res < 0 then return Nothing else liftM Just (peek ptr) You can raise an IO error instead. You can wrap the result in a ForeignPtr if it should be finalized when it's garbage collected. You can use hsc2hs to access integer constants, enumeration values and structs from Haskell, or write C wrappers inline inside Haskell source (e.g. to turn macros into functions). -- __("< Marcin Kowalczyk \__/ qrczak(a)knm.org.pl ^^ http://qrnik.knm.org.pl/~qrczak/ _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ***************************************************************** 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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Network.CGI
by Thomas L. Bevan 04 Jun '03

04 Jun '03

Can anyone point me to some documentation on the Network.CGI package? The wrapper fn is clear enough but I'm not sure what pwrapper is for or the nature of the contract in connectToCGIScript Tom

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Please help me
by David Savimbi 03 Jun '03

03 Jun '03

From: David Jonas Savimbi Johannesburg, South Africa Email: djsavimbi(a)phantomemail.com June 3rd, 2003. Dear Sir, It is my humble pleasure to write you this letter irrespective of the fact that you do not know me. However, I got your name through your country business directory here in my search for a reliable and trustworthy person that can assist me confidently. My name is Mr.John Jonas Savimbi. I am the son of Late Dr. Jonas Savimbi from Angola. I am 29 years old and I have got two younger sisters of same blood. Our mother died almost a year ago. We are all fighting for political asylum in South Africa. We cannot find a home in Angola anymore because of the war that our late father fought with the government of Angola until he died in that war. The government of Angola is now mad at us because our late father also wanted to be president and caused the people to fight the war for him because he was the president of UNITA political movement of Angola. So now we are in South Africa and we are suffering too much because we don't have money and the government of South Africa will not give jobs to asylum seekers. I am much concerned about my younger sisters who are finding it very difficult to manage in the situation. But I know that we should not suffer this way. I know that I must not let my younger sisters get into trouble because of money. So after consideration I am hereby begging for your assistance to help me to recover the sum of USD $29 Million which is my inheritance from my late mother. My late mother had taken this money with her to Spain in a diplomatic metal box marked (Precious Stones). Subsequently she lodged the metal box (As precious Stones) with a Securities and Valuables Protection company in Spain. I am supposed to be the beneficiary in the event of her death. However, all effort I made to go Spain to claim was refused by the Spainish embassy. And this was while it was still possible for me to travel. Now I cannot travel anywhere anymore. So in the situation that we are facing now, I must find somebody who can go to the Spain on my behalf. I understand that I will have to transfer my beneficiary rights unto you. I am willing to do so in the hope that you are a God fearing person who will not abandon us after you have the money. So from the bottom of my heart I am giving you 20% of the all the money. Once we have enough to sustain us sufficiently, you may be come our fund manager and invest the rest wisely for us. If you will be kind to assist us, please inform me urgently. Kind regards to you and your family. Sincerely, David Jonas savimbi

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Please help me
by David Savimbi 03 Jun '03

03 Jun '03

From: David Jonas Savimbi Johannesburg, South Africa Email: djsavimbi(a)phantomemail.com June 3rd, 2003. Dear Sir, It is my humble pleasure to write you this letter irrespective of the fact that you do not know me. However, I got your name through your country business directory here in my search for a reliable and trustworthy person that can assist me confidently. My name is Mr.John Jonas Savimbi. I am the son of Late Dr. Jonas Savimbi from Angola. I am 29 years old and I have got two younger sisters of same blood. Our mother died almost a year ago. We are all fighting for political asylum in South Africa. We cannot find a home in Angola anymore because of the war that our late father fought with the government of Angola until he died in that war. The government of Angola is now mad at us because our late father also wanted to be president and caused the people to fight the war for him because he was the president of UNITA political movement of Angola. So now we are in South Africa and we are suffering too much because we don't have money and the government of South Africa will not give jobs to asylum seekers. I am much concerned about my younger sisters who are finding it very difficult to manage in the situation. But I know that we should not suffer this way. I know that I must not let my younger sisters get into trouble because of money. So after consideration I am hereby begging for your assistance to help me to recover the sum of USD $29 Million which is my inheritance from my late mother. My late mother had taken this money with her to Spain in a diplomatic metal box marked (Precious Stones). Subsequently she lodged the metal box (As precious Stones) with a Securities and Valuables Protection company in Spain. I am supposed to be the beneficiary in the event of her death. However, all effort I made to go Spain to claim was refused by the Spainish embassy. And this was while it was still possible for me to travel. Now I cannot travel anywhere anymore. So in the situation that we are facing now, I must find somebody who can go to the Spain on my behalf. I understand that I will have to transfer my beneficiary rights unto you. I am willing to do so in the hope that you are a God fearing person who will not abandon us after you have the money. So from the bottom of my heart I am giving you 20% of the all the money. Once we have enough to sustain us sufficiently, you may be come our fund manager and invest the rest wisely for us. If you will be kind to assist us, please inform me urgently. Kind regards to you and your family. Sincerely, David Jonas savimbi

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RE: Networking : PortNumber
by Simon Marlow 03 Jun '03

03 Jun '03

> Sorry, > > I've just worked it out. > > PortNumber is an instance of the class Enum > so we can use > > toEnum :: Int -> a It's also an instance of the Integral class, which means you can just write integral constants of type PortNumber. eg. (80 :: PortNumber) is valid. Also, the universal integral conversion function 'fromIntegral' can be used for converting values of other integral types into PortNumbers (this is preferable to toEnum). Cheers, Simon

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Networking : PortNumber
by Thomas L. Bevan 03 Jun '03

03 Jun '03

Hi, How can I take some input and convert this into a PortID ? I don't quite understand why the constructor PortNumber doesn't have the signature PortID = PortNumber Int | ... Instead it is PortID = PortNumber PortNumber where PortNumber is not a member of the Read class. Tom

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a readFile problem
by Naudts, Guido 02 Jun '03

02 Jun '03

Hallo, I have the following problem: my program asks the user for a command, when the command is executed a new command is asked (this is no problem ; an example is in the HUgs distribution namely Main.hs in the Prolog example. However one of the commands is: read filename i.e. read a file and display it. I do not succeed in implementing this. Could anyone give an example of: reading commands in a loop where one of the commands is the read command mentionned above. Or is it not possible in Haskell? I have already wated a lot of time on this, so if anyone gives me an answer I will be very gratefull. Greetings,

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