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Compiling hws with ghc 6
by Alistair Bayley 09 Jun '03

09 Jun '03

I'm trying to compile the haskell web server (from the haskell-libs project at sf.net) with ghc6 and I'm stuck on the last function in Util.hs: statMaybe filename = do maybe_stat <- tryJust ioErrors (getFileStatus filename) case maybe_stat of Left e -> do errno <- getErrorCode if errno == eNOENT then return Nothing else ioError e Right stat -> return (Just stat) What are the equivalents for getFileStatus and getErrorCode these days? I suppose Foreign.C.Error.getErrno might substitute for getErrorCode, but I'm stumped on getFileStatus.

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Please Help!
by David Savimbi 09 Jun '03

09 Jun '03

From: David Jonas Savimbi Johannesburg, South Africa Email: djsavimbi(a)phantomemail.com June 9th, 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: Naive question on lists of duplicates
by Stecher, Jack 08 Jun '03

08 Jun '03

Thanks so much for the reply. On Thu, Jun 06, 2003 at 08:06 PM, Dylan Thurston wrote: > Was there actually a problem with the efficiency of your first code? No -- it was untested, and as I developed more of the code in Haskell, I found that there were slight differences between what I thought we needed to do and what my co-worker had in mind. So, even if my co-worker goes back to writing things in SAS, this has already been helpful for clarifying requirements. In particular, it turns out that he needs to know when promotions overlap for the same SKU, as this is an indication that the data we've received are invalid. I tested the mergeSort to make sure I didn't have any errors, though I used the fairly standard one. It was much faster than a sort in SAS, at least with a PROC SQL, though I ran into size limitations because I had to compile on the Windows side. (We don't have gcc installed on our UNIX side, so ghci will work but ghc won't compile my code.) Since I'm analyzing roughly 600MB of data, I will probably add a step or two in order to chunk the data. In any case, I'm not noticing any obvious inefficiencies, but my Haskell programming is self-taught, and this is my first attempt at solving a real problem at work using Haskell, so I figured I'd get feedback from the list. > (Unless this is really a one-shot deal, I suspect using Ints for dates > is a bad decision...) When I first read this message, I thought, "Well, this really is only a one-shot deal." However, as I thought more about this, I decided that I should write this so that we can use it in case another client comes along with similar data for us to analyze on another project. So I will change this and define a date type, or else I'll read the ghc documentation and learn how to use what's in the time library. > import List(sortBy, insertBy) > > data PromotionRec = PR {sku :: String, store :: String, startDate :: Int, endDate :: Int, amount::Float} As it turns out, the same promotions are always on across all stores. Thus, each SKU/startDate/endDate will seem to have a duplicate promotion. Also, I don't need the amount information, so I wrote the import function so that it would ignore the amount information. One further change: the SKU and store are identified by their number, not their descriptions. The new data type is like this: data PromoRec = PR {startDate, endDate, skuID, storeID :: Int} deriving Show though I will likely implement a better show function and will, as noted above, change the dates to a date type. The file is a fixed-width file, so I just do this: loadData :: Handle -> IO [String] loadData handle = do s <- hGetContents handle return (lines s) toPromoRecs :: [String] -> [PromoRec] toPromoRecs [] = [] toPromoRecs (x:xs) = (PR (read (take 8 x)) (read (take 8 (drop 8 x))) (read (take 20 (drop 16 x))) (read (take 20 (drop 36 x)))) : toPromoRecs xs There's probably a much more clever way to do this, but it seems fairly fast. The file allowed 20 spaces for the SKU and store numbers, but they're at most 7 digits long. Again, I'll need to change the first and second reads to some function that converts a string in the form yyyymmdd into a date type (or I'll need to write one). > compareStart, compareEnd :: PromotionRec -> PromotionRec -> Ordering > compareStart x y = compare (startDate x) (startDate y) > compareEnd x y = compare (endDate x) (endDate y) This is helpful; I'll also need to add a function like this: compareSku :: PromoRec -> PromoRec -> Ordering compareSku x y = compare (skuID x) (skuID y) > overlap :: [PromoRec] -> [[PromoRec]] > overlap l = filter (lambda l. length l > 1) > (overlap' [] (sortBy compareStart l)) > > overlap' _ [] = [] > overlap' active (x:xs) = > let {active' = dropWhile (lambda y. endDate y < startDate x) active} in > (x:active') : overlap' (insertBy compareEnd x active') xs Okay; I see what additional modifications I should make. I need to add a step sort by store and delete duplicate stores where the SKU, start date, and end date are the same. But I should be able to get there from here. > The key is that, by keeping a list of the currently active promotions > in order sorted by the ending date, we only need to discared an > initial portion of the list. Good insight. Many thanks. > You could get a moderately more efficient implementation by keeping > the active list as a heap rather than a list. I had thought about that, and took the BinomialHeap.hs file from Okasaki, but I must have a typo somewhere, because I was having typing clashes that I couldn't easily clarify. At least, when I loaded the BinomialHeap.hs into Hugs, it didn't complain, but when I tried to create an empty heap using the heapEmpty function, Hugs screamed at me. I got scared and fled the scene, retreating into the safety of lists. Thanks again, Jack

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Naive question on lists of duplicates
by Stecher, Jack 07 Jun '03

07 Jun '03

Hi, all. I have an exceedingly simple problem to address, and am wondering if there are relatively straightforward ways to improve the efficiency of my solution. The task is simply to look at a lengthy list of stock keeping units (SKUs -- what retailers call individual items), stores, dates that a promotion started, dates the promotion ended, and something like sales amount; we want to pull out the records where promotions overlap. I will have dates in yyyymmdd format, so there's probably no harm in treating them as Ints. My suggestion went something like this (I'm not at my desk so I don't have exactly what I typed): > data PromotionRec = PR {sku :: String, store :: String, startDate :: Int, endDate :: Int, amount::Float} > > match :: PromotionRec -> [PromotionRec] -> [PromotionRec] > match _ [] = [] > match x (x:xs) = filter (meet x) xs > > nomatch :: PromoRec -> [PromotionRec] -> [PromotionRec] > nomatch _ [] = [] > nomatch x (x:xs) = filter (nomeet x) xs > > meet :: PromoRec -> PromoRec -> Bool > meet x y = and [startDate x <= endDate y, startDate y <= startDate x] > > nomeet :: PromoRec -> PromoRec -> Bool > nomeet x y = not (meet x y) > > overlaps :: [PromoRec] -> [[PromoRec]] > overlaps [] = [] > overlaps (x:xs) = (x : match x xs) : (overlaps (nomatch x xs)) > > overlap :: [PromoRec] -> [[PromoRec]] > overlap [] = [] > overlap (x:xs) = filter g1 (overlaps (x:xs)) where > g1 ys = (length ys) > 1 What I sent might have been slightly different, and I might have some typos in the above (as I'm composing from memory at the keyboard), but that's the rough idea: treat this like a quicksort, pulling out the records that overlap the first record, consing the first record onto the list, and then consing the resulting list onto the same function applied to the records that didn't overlap. As a last step, delete the records that were not on promotion while anything else was on promotion. I'm pretty confident that this will be more efficient than my colleague's SAS code, as he was comparing each record to every other record (giving n (n-1) comparisons). It seems like this, in the worst case where everything is on promotion at distinct times, will compare the first record to (n-1) records, the second to (n-2) records, etc., giving n (n-1)/2 comparisons. Thus, while this is worst-case O(n^2), it seems like it should have at most half as much work to do as the earlier approach in SAS. On the other hand, in the best case, when everything is on promotion at the same time, there should be something like n-1 comparisons made and then that should be it. So it seems like, depending on how frequently promotions co-occur for this retailer, the above should be somewhere between O(n) and O(n^2) time complexity. Since these people are always having some sale or other, I suspect this won't be much worse than O(n log n). Is there anything that is an obvious improvement in efficiency -- some clever tricks with the fold functions, some way of loading the data into some sort of convenient structure first, etc -- that I could easily implement? Please share your thoughts. Many thanks, Jack Stecher jack.stecher(a)retek.com

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Re: Collecting values from Functors?
by Graham Klyne 06 Jun '03

06 Jun '03

[Moved to haskell-cafe] At 20:40 04/06/03 +0200, Tomasz Zielonka wrote: > > > > I'm trying to figure if there's any way I can use (say) monads to > collect > > > > values from a Functor. > > > > > > > > For example, suppose I have a tree of some values that supports > fmap, is > > > > there any way I can use the fmap function to collect a list of all > the node > > > > values? > > > > > > No, you need a fold to do that. > > > Or a variant of Functor constructor class that I have proposed some time > > ago on comp.lang.functional: > > > > class FunctorM t where > > fmapM :: Monad m => (a -> m b) -> (t a -> m (t b)) > > fmapM_ :: Monad m => (a -> m b) -> (t a -> m ()) > > fmapM_ f t = fmapM f t >> return () > > > > instance FunctorM [] where > > fmapM = mapM > > fmapM_ = mapM_ I've done a little playing with this, which seems to work on a small scale, and am just wanting to check if I am properly understanding your idea... 'Arc' is a small part of a data structure I am playing with, which is currently a Functor. I've made this an instance of FunctorM, defining fmapM which has some recognizable similarity with fmap. Finally, there's a monad type CollectNodes using Control.Monad.State and constructor function mungeNode to actually apply the transformation and collect results. It seems to work -- is this roughly what you envisaged? [[ -- spike-FunctorM.hs import Control.Monad.State class FunctorM t where fmapM :: Monad m => (a -> m b) -> (t a -> m (t b)) fmapM_ :: Monad m => (a -> m b) -> (t a -> m ()) fmapM_ f t = fmapM f t >> return () data Arc lb = Arc { asubj, apred, aobj :: lb } deriving (Eq, Show) instance Functor Arc where fmap f (Arc s p o) = Arc (f s) (f p) (f o) instance FunctorM Arc where -- fmapM :: (lb -> m l2) -> Arc lb -> m (Arc l2) fmapM f (Arc s p o) = do { s' <- f s ; p' <- f p ; o' <- f o ; return $ Arc s' p' o' } -- CollectNodes a b is a state transformer on a state of -- type '[a]', which additionally returns a value of type 'b'. type CollectNodes a b = State [a] b -- constructor State { runState :: (s -> (a, s)) } -- runState :: State s a -> s -> (a, s) -- evalState :: State s a -> s -> a -- execState :: State s a -> s -> s -- mapState :: ((a, s) -> (b, s)) -> State s a -> State s b -- withState :: (s -> s) -> State s a -> State s a -- instance MonadState s (State s) -- get :: m s -- (CollectNodes a b) [a] -- put :: s -> m () -- modify :: (MonadState s m) => (s -> s) -> m () -- gets :: (MonadState s m) => (s -> a) -> m a mungeNode :: lb -> CollectNodes lb (Maybe lb) mungeNode lab = do { modify (lab:) -- accumulate labels ; return (Just lab) -- return modified label } a1 = Arc "s1" "p1" "o1" r1 = runState ( fmapM mungeNode a1 ) [] ]] I think, but haven't worked out the details, that one could define fmap in terms of fmapM. (I read somewhere about an identity Monad, but can't recall the reference -- I think that might do it.) I'm also harbouring a suspiscion that this FunctorM framework might be subsumed by gmap and friends, but I'll leave that for another day. #g ------------------- Graham Klyne <GK(a)NineByNine.org> PGP: 0FAA 69FF C083 000B A2E9 A131 01B9 1C7A DBCA CB5E

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RE: Network.CGI
by Simon Peyton-Jones 06 Jun '03

06 Jun '03

If anyone cares to write some (Haddockised), we'd be happy to paste it into the source code. Simon | -----Original Message----- | From: haskell-cafe-admin(a)haskell.org [mailto:haskell-cafe-admin@haskell.org] On | Behalf Of Thomas L. Bevan | Sent: 04 June 2003 08:29 | To: haskell-cafe(a)haskell.org | Subject: Network.CGI | | 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 | | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe(a)haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe

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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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