- Haskell-Cafe - Haskell.org

RE: functional graph
by Hal Daume 30 Jul '03

30 Jul '03

> But it looks the Graph class works on types a and b > where a is the Node type where b is the Edge type, or > just the other way around :), I don't have the code > right now. Yes, 'Graph n e' is a graph whose nodes are labelled with elements of type n and whose edges are labelled with elements of type e. > The above rules out my way of representing the data, > or doesn't it (i.e. is there some fancy, magical way > of doing this) ? Yes. (I.e., it rules it out.) > The second intuitive solution is to take a datatype > like > data GraphNode|Teacher etc > |Subject etc > |Student etc > > Should this be the way to do it? Yes. > And then yet another question: I need some kind of way > to remember, which possibilities I already had. > > In example in my C++ program I used a couple of > array's with in each array pointers to the > StudentObjects, TeacherObjects. You could use the same here, or use a FiniteMap. If i'm understanding you correctly you miss the fact that there's no way to look up a node given it's label. This is because the graph module doesn't assume that node labels are unique (otherwise it wouldn't need to generate the Node labels itself). I generally bundle a 'Graph n e' together with a 'FiniteMap n Node' so that I can look up Node identifiers based on node labels. > But then I yet have another problem. How can I > guarantee, to do a complete path traversal, each time > in exactly the same way? I don't understand what you mean. In your case, what is a "complete path traversal." Regardless, if you have the same graph and the same functions for traversing it, since Haskell is referentially transparent you should also "traverse it in the same way..." - Hal

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Parsing CSV files
by Shawn P. Garbett 30 Jul '03

30 Jul '03

I did a small search for parsing a comma seperated file in Haskell and didn't find anything-- so I put together some code to do this. It doesn't handle whitespace very well, this would be a nice addition if someone has an idea out there. Also the rows method, I had trouble just using two do loops without brackets, I couldn't prevent it from being ambiguous. Anyway here's the code: ------------------------- import Parsec import System.IO -- Code to parse a comma seperated file content :: Parser String content = many1 (noneOf "\"") cell :: Parser String cell = do char '\"' w <- content; char '\"' <?> "end of cell" return w; <|> return "" -- Empty cell separator :: Parser () separator = skipMany1 (char ',') cells :: Parser [String] cells = sepBy1 cell separator contents :: Parser [String] contents = sepBy1 content separator; rows :: Parser [[String]] rows = do c <- cells newline do { cs <- rows; return (c:cs); } <|> return [c] csv :: Parser [[String]] csv = do r <- rows eof return r -- Main routine main :: IO () main = do result <- parseFromFile csv "simple.txt" case (result) of Left err -> print err Right sb -> print sb ------------------- Shawn Garbett

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Using exceptions in imperative code
by Bayley, Alistair 29 Jul '03

29 Jul '03

I've got resource acquisition (and release) code which looks like this at present: > connect = do > envHandle <- envCreate > errHandle <- handleAlloc oci_HTYPE_ERROR envHandle > serverHandle <- handleAlloc oci_HTYPE_SERVER envHandle > connection <- dbLogon "user" "password" "database" envHandle errHandle > session <- getSession errHandle connection > return (envHandle, errHandle, serverHandle, connection, session) > disconnect envHandle errHandle serverHandle connection session = do > sessionEnd errHandle connection session > serverDetach errHandle serverHandle > handleFree oci_HTYPE_SERVER serverHandle > handleFree oci_HTYPE_ERROR errHandle > handleFree oci_HTYPE_ENV envHandle > terminate How should I structure it to handle exceptions raised by the various functions (envCreate, handleAlloc, dbLogon, etc)? I've started to restructure it like this: getEnv = do catchDyn ( do envHandle <- envCreate getErr envHandle ) (putStrLn "envCreate failed") getErr envHandle = do catchDyn ( do errHandle <- handleAlloc oci_HTYPE_ERROR envHandle getServer envHandle errHandle ) (do reportOCIException envHandle handleFree oci_HTYPE_ENV envHandle ) getServer envHandle errHandle = do catchDyn ( do serverHandle <- handleAlloc oci_HTYPE_SERVER envHandle getServer envHandle ) (do reportOCIException envHandle handleFree oci_HTYPE_ERROR errHandle handleFree oci_HTYPE_ENV envHandle ) ... but this is so awkward. How should I structure it? ***************************************************************** 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

XMLParser
by Ron de Bruijn 29 Jul '03

29 Jul '03

Hi there, I used the PARSEC parser combinator library to create an following XML-Parser. It's not exactly XML, but it's a usefull format for saving various data. The big difference there is, is that I don't have attributes, but those could easily be added, although I don't need them. Below is the complete code (20! lines of code for the core). I was excited when I saw it also worked like I had imagined, but then I tested it sometimes with increasingly big data. Using my test function below I noticed that there's some exponentional behaviour. I don't see a reason for this. Although I use the try combinator, it should at max double the time, because the parser uses the other parser when there occurs a parse error in the one. My definition of xmlParser looks also a bit odd, but without at least one of the functions of(subEnTop,topEnSub) it will fail parsing. I also don't understand why. Then I question myself: why does my program work with the downloaded version, and not with the PARSEC library that ships with GHC 5.0.4.2, I atleast thought that Parsec hasn't changed for a long time? Does anyone have an explanation for the above problems? Greets Ron module XMLCreator where --this module can parse XML import Parsec import GHC.Show import System.Time --datatype om XML te representeren: XML bestaat altijd uit een topelement en een aantal nestingen data XML a = TopElement String [XML a] | SubElement String a deriving Show type Level = Int --not finished Pretty Printer ppXML::XML a->Level->String ppXML (TopElement name (xml:xmls)) level = concat(take level (repeat " "))++openTag name ppXML _ _ = "hallo" --ppXML (NestedElement name info) level = openTag::String->String openTag s = "<"++s++">" --run :: Show a => Parser a -> String -> IO () run p input= do printTijd case (parse p "" input) of Left err -> do{ putStr "parse error at "; print err } Right x -> do print x printTijd --prints time(used above for printing the start time and the endtime of the algoritm printTijd = do time<-getClockTime (putStrLn.show) time test n = run xmlParser (concat(replicate n str)) --test n = parseTest xmlParser (concat(replicate n str)) str::String str = "<TopicList><TopicItem><Readed>1</Readed><ForumTitle>forumTitle</ForumTitle><TopicTitle>topicTitle</TopicTitle><TopicID>100100</TopicID><TopicStarter>someOne</TopicStarter><LastReplier>someOtherOne</LastReplier><ReplierID>1234</ReplierID><MessageID>messageID</MessageID><SubForumTitle>PW</SubForumTitle><SubForumID>14</SubForumID><Replies>2</Replies><ReplyTime>1050767736</ReplyTime><StartTime>1050762676</StartTime></TopicItem><FileInfo><Version>5</Version><TimeCreated>1059155053</TimeCreated><Application>SomeProgram</Application><About>aboutText</About></FileInfo></TopicList> "; --determine which sign can not be in the elementname or in the informationpiece signs = noneOf ['<','>'] --parses a piece of xml xmlParser::Parser ([XML String]) xmlParser = choice [try (many1 sub),try topEnSub,try subEnTop, many1 top] --xmlParser = choice[try(many1 sub),many1 top] getName::Parser String getName = do teken '<' name<-many1 signs teken '>' return name where teken t = do skipMany space char t skipMany space end::String->Parser String end name = do skipMany space string ("</"++name++">") sub::Parser (XML String) sub = do name<-getName info<-many1 signs end name return (SubElement name info) top::Parser (XML String) top = do name<-getName moreXml<-xmlParser end name return (TopElement name moreXml) subEnTop::Parser [(XML String)] subEnTop = do x<-many1 sub y<-many1 top return (x++y) topEnSub::Parser [(XML String)] topEnSub = do x<-many1 top y<-many1 sub return (x++y) __________________________________ Do you Yahoo!? Yahoo! SiteBuilder - Free, easy-to-use web site design software http://sitebuilder.yahoo.com

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Re: Can't compile hmake
by Thomas Davie 29 Jul '03

29 Jul '03

On Tuesday, Jul 29, 2003, at 11:16 Europe/London, Keith Wansbrough wrote: >> I was wondering if idiocy was my problem there, I don't see the >> directory though: >> >> [nimbus2:project/hmake-3.08/targets] tatd100% ls -la >> total 0 >> drwxr-xr-x 3 tatd100 staff 102 Jul 29 10:42 . >> drwxr-xr-x 14 tatd100 staff 476 Jul 29 10:43 .. >> drwxr-xr-x 7 tatd100 staff 238 Jul 29 10:44 powerpc-Darwin6 >> >> Am I being incredibly stupid here? > > 'Fraid so ;-/ `harch` means "powerpc-Darwin6" in your case. The > current host architecture name. I suspected as much. In which case I have the problem that the file doesn't exist :/ [nimbus2:hmake-3.08/targets/powerpc-Darwin6] tatd100% ls -l total 32 -rw-r--r-- 1 tatd100 staff 4 Jul 29 10:43 buildwith -rw-r--r-- 1 tatd100 staff 428 Jul 29 10:54 config.cache -rw-r--r-- 1 tatd100 staff 3 Jul 29 10:43 ghcsym -rw-r--r-- 1 tatd100 staff 721 Jul 29 10:43 hmake3.config drwxr-xr-x 3 tatd100 staff 102 Jul 29 10:44 obj Thanks for putting up with my idiocy, I'm not used to the unix build tools and where it puts things. Tom Davie

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Re: Can't compile hmake
by Thomas Davie 29 Jul '03

29 Jul '03

On Tuesday, Jul 29, 2003, at 11:03 Europe/London, Keith Wansbrough wrote: >>> Yes, after configuring, try editing the generated config file as >>> follows: >>> In targets/`harch`/config, change the line >>> READLINE="" >>> to >>> READLINE="-DUSE_READLINE=1 -L/sw/lib/ -lreadline" >>> then proceed to 'make' as usual. > [..] > >> to configure, then editing the config.cache file - I have tried the >> exact text you specified and also this: >> READLINE="-DUSE_READLINE=1 -L/sw/lib/ -lreadline -I/sw/include/ >> -include readline/readline.h" >> But still with no joy. I'm currently getting this error: > > He said to edit targets/`harch`/config, not config.cache. > config.cache is just information for the next run of ./configure; it > doesn't affect the build process. I was wondering if idiocy was my problem there, I don't see the directory though: [nimbus2:project/hmake-3.08/targets] tatd100% ls -la total 0 drwxr-xr-x 3 tatd100 staff 102 Jul 29 10:42 . drwxr-xr-x 14 tatd100 staff 476 Jul 29 10:43 .. drwxr-xr-x 7 tatd100 staff 238 Jul 29 10:44 powerpc-Darwin6 Am I being incredibly stupid here? Thanks Tom Davie

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Can't compile hmake
by Thomas Davie 29 Jul '03

29 Jul '03

Hi, I'm just trying to get started with haskell and have been having some problems getting hmake to compile. For some reason it doesn't detect libreadline. Lib readline is installed as evidenced by this: [computer:~/Desktop/hmake-3.08] username% fink list | grep readline i readline 4.3-5 Comfortable terminal input library i readline-shlibs 4.3-5 Comfortable terminal input library I've been attempting to force it to link against the library by using ./configure --buildopts="-L/sw/lib/ -lreadline -I/sw/include/ -include readline/readline.h" but with no success, I'm getting the error "gcc: readline/readline.h: No such file or directory", despite the file /sw/includes/readline/readline.h very definitely existing. Has anyone got any ideas for where I could go from here? FYI, I'm running Mac OS X.2.6 Thanks Thomas Davie

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lazy Printing question
by AJ 26 Jul '03

26 Jul '03

Hi everyone, I have written the following program to find "magic numbers" i.e. integers 'n' such that both (n+1) and (n/2+1) are perfect squares. -- Program to find magic numbers Import IO main :: IO () main = do print (filter magicP sqs) sqs :: [Int] sqs = [x*x | x <- [1,3..mAX]] magicP :: Int -> Bool magicP x | ((x-1) `mod` /= 0 = False | otherwise = (((x-1) `div` 2) + 1) `elem` sqs) mAX :: Int mAX = 20000 -- End of listing If I try to run the program (compiled using GHC 6), it calculates all members of the list and then prints the whole list in the end. Since Haskell is 'lazy' I was expecting behaviour similar to HUGS where it prints the numbers as it finds them. Does this behaviour have something to do with the monadic IO in Haskell? I am a Haskell newbie and can't even claim having understood the "gentle introduction" properly. Now my question is - How do I reproduce such "lazy printing" behaviour in GHC? Wait... I thought of another one :-), how do I speed up this program? - AJ

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Type design question
by Konrad Hinsen 25 Jul '03

25 Jul '03

I am a Haskell newbie working on my first serious test case, and I would like some feedback from the experts to make sure I am not doing anything stupid ;-) My applications are numerical (one goal of my current tests being to check how much of a performance penalty I will pay for using Haskell as opposed to C/C++) and involve a lot of geometry. Therefore I started by defining some appropriate types, in particular vectors. So here we go: -- Most general: element of a vector space. -- Operations: addition, subtraction, and multiplication by a scalar class Vect v where (<+>) :: Floating a => v a -> v a -> v a (<->) :: Floating a => v a -> v a -> v a (<*>) :: Floating a => a -> v a -> v a -- Vector space with a scalar product -- Adds dot product and norm class Vect v => VectSProd v where dot :: Floating a => v a -> v a -> a norm :: Floating a => v a -> a norm x = sqrt (dot x x) data Floating a => Vector a = Vector !a !a !a deriving (Eq, Show) -- Standard vector in 3D space instance Vect Vector where (<+>) (Vector x1 y1 z1) (Vector x2 y2 z2) = Vector (x1+x2) (y1+y2) (z1+z2) (<->) (Vector x1 y1 z1) (Vector x2 y2 z2) = Vector (x1-x2) (y1-y2) (z1-z2) (<*>) s (Vector x y z) = Vector (s*x) (s*y) (s*z) instance VectSProd Vector where dot (Vector x1 y1 z1) (Vector x2 y2 z2) = x1*x2+y1*y2+z1*z2 cross :: Floating a => Vector a -> Vector a -> Vector a cross (Vector x1 y1 z1) (Vector x2 y2 z2) = Vector (y1*z2-z1*y2) (z1*x2-x1*z2) (x1*y2-y1*x2) I would like to keep the numerical representation as general as possible, in particular to postpone decisions about precision (Float, Double, eventually arbitrary precision arithmetic) as much as possible. Therefore the "Floating a" constraint. Which leads to my first question: doing it the way I did, I find myself having to add a "Floating a" constraint to almost every function specification with a vector argument. I would prefer to specify once and for all that vector elements need to be "Floating", everywhere. Is there a way of doing that? Next, I would like to define some stuff for simulations of atomic systems. There are two kinds of environments for such simulations, infinite (no boundaries) and periodic (topologically a torus surface). The essential difference is the way that distances are calculated. -- General simulation universe properties class Universe u where distanceVector :: Floating a => u -> (Vector a) -> (Vector a) -> (Vector a) distance :: Floating a => u -> (Vector a) -> (Vector a) -> a distance u v1 v2 = norm (distanceVector u v1 v2) -- Infinite universe data InfiniteUniverse = InfiniteUniverse instance Universe InfiniteUniverse where distanceVector u v1 v2 = v2 <-> v1 -- Periodic universe data Floating a => OrthorhombicUniverse a = OrthorhombicUniverse a a a instance Floating a => Universe (OrthorhombicUniverse a) where distanceVector (OrthorhombicUniverse a b c) (Vector x1 y1 z1) (Vector x2 y2 z2) = Vector (fmod (x2-x1) a) (fmod (y2-y1) b) (fmod (z2-z1) c) where fmod x y = x - y*truncate (x/y) And here I run into a problem. The "Floating a" constraint on OrthorhombicUniverse has the same purpose as for vectors, the three arguments are the dimensions of the box that is periodically replicated. The compiler complains about the definition of distanceVector: ERROR "/home/hinsen/haskell/md/Universe.hs":24 - Inferred type is not general enough *** Expression : distanceVector *** Expected type : (Universe (OrthorhombicUniverse a), Floating b) => OrthorhombicUniverse a -> Vector b -> Vector b -> Vector b *** Inferred type : (Universe (OrthorhombicUniverse a), Floating a) => OrthorhombicUniverse a -> Vector a -> Vector a -> Vector a I do, in fact, want "Floating a" and "Floating b" to enforce the same type, but how can I do that? BTW, in case anyone wonders why I used a class for "Universe" instead of simply defining data Universe = InfiniteUniverse | OrthorhombicUniverse a a a the reason is that I want to leave the option of adding differently shaped universes later on. Konrad.

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RE: segfault in haskell!
by Simon Marlow 25 Jul '03

25 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 I've found one possible cause of this: we were calling regfree on the regular expression structure even if regcomp failed. This will be fixed in GHC 6.0.1. Workaround: don't pass any bogus regular expressions to regcomp. Cheers, Simon

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ENQUIRY
by MR. DAVID LARSON 25 Jul '03

25 Jul '03

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

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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

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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 19 Jul '03

19 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 19 Jul '03

19 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. *****************************************************************

5 4

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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Re: infinite types
by blaat blaat 18 Jul '03

18 Jul '03

Oh yeah, and the person who _guesses_ what ex6 does in the supplied code wins a chocolate bar supplied by me ;-) Cheers, l4t3r _________________________________________________________________ Add photos to your messages with MSN 8. Get 2 months FREE*. http://join.msn.com/?page=features/featuredemail

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Re: infinite types
by blaat blaat 18 Jul '03

18 Jul '03

Uhm, of to my holliday in France. But, ah, oh, the joy of programming ;-). I thought I would humor this list by showing where the infinite types (is it?) discussion ended. I found it quite nice and the code is short so here goes ;-) To test: save the mail after the marker and run it in Hugs. There are some examples at the end of the file. ------- cut from here, save as "Reactive.hs" --------------------------------- -- A reactive system is modelled by the type -- -- R m = m -> (Nottaken | Taken (R m) [m]) -- -- It is _not_ a coalgebraic definition, it is _not_ an arrow, it is -- a recursive type in this case used to model asynchronous message -- passing systems. -- -- A system may be thought of as collection of (UML) -- transitions: s0 - event[guard]/action* -> s1 where states are -- hidden in the transitions, and a next-state is a transition, -- or something, I guess. Uhm, huh? ;-) -- -- Note that no explicit distinction between in- and output messages -- is made. This allows liberal, although slightly odd ;-), structural -- composition operators a la Broy, Bird-Meertens, Arrows, Fudgets, -- whatever. In a nutshell: stream processing functions. -- -- After reading the following oddball operators, you, uhm, might -- really want to define your own. Really ;-) -- -- This formalization has weak algebraic rules on the composition -- operators. -- -- Load this code in hugs and test the examples by typing ex0 to -- ex6. module Reactive where type R m = m -> Response m data Response m = Nottaken | Taken (R m) [m] ---------------------------------------------------------------------- -- -- onTaken: helper function onTaken::Response m -> (R m -> [m] -> a) -> a -> a onTaken Nottaken f g = g onTaken (Taken s oo) f g = f s oo ---------------------------------------------------------------------- -- -- The behavior of a system is, of course, a mapping from an input -- stream of messages to an output stream of messages. behavior::R m->[m]->[m] behavior s [] = [] behavior s (i:ii) = onTaken (s i) (\s0 oo-> oo ++ behavior s0 ii) (i:behavior s ii) ---------------------------------------------------------------------- -- Copy is a simple system which copies all input to its output. -- -- +++++ -- + + -- +++++ ---------------------------------------------------------------------- copy::R m copy m = Taken copy [m] ---------------------------------------------------------------------- -- Lift is a simple system which is the lifting of a function from -- messages to messages to a reactive system. -- -- +++++++++ -- + ^ + -- + f + -- + + -- +++++++++ ---------------------------------------------------------------------- lift::(m->m) -> R m lift f m = Taken (lift f) [f m] ---------------------------------------------------------------------- -- -- If we have a system on messages of type b, and translator functions -- between another type a, we can change the interface to a system -- on messages of type a. (This is a solution for not -- making an explicit distinction between input and output messages; -- now we can partition the messages of subsystems) -- -- +++++++++ -- + + -- + s:R a +:R b -- + + -- +++++++++ interface::(a->b)->(b->a)->R b->R a interface t f s i = onTaken (s (t i)) (\s1 oo->Taken (interface t f s1) (map f oo)) (Nottaken) ---------------------------------------------------------------------- -- -- The parallel composition of two systems s and r is the system which -- if s takes an input then makes s process it, otherwise if r takes an -- input then makes r process it, otherwise does not process it. -- -- Supposed to model the parallel composition of a collection of -- event handlers (like: sound subsystem, graphics subsystem, etc). -- -- +++++ -- + s + -- + + + -- + r + -- +++++ -- -- ( (a <+> b) <+> c = a <+> (b <+> c) ) parallel::R m->R m->R m parallel s0 s1 i = onTaken (s0 i) (\s2 oo->Taken (parallel s2 s1) oo) (onTaken (s1 i) (\s2 oo->Taken (parallel s0 s2) oo) (Nottaken)) infixr 5 <+> (<+>) = parallel ---------------------------------------------------------------------- -- -- The sequential composition of systems s and r takes an input if -- s accepts the input and then makes r process all output of s. -- All output by s which is not taken by r _passes through_ r. -- -- Supposed to model pipes (where exceptions or messages to the -- outside world leak through). -- -- -- +++++++ -- + s.r + -- +++++++ -- -- ( (a <.> b) <.> c = a <.> (b <.> c) ) -- sequential::R m->R m->R m sequential s0 s1 i = onTaken (s0 i) (\s2 oo-> onTaken (seqloop s1 oo) (\s3 oo1-> (Taken (sequential s2 s3) oo1)) (Nottaken)) (Nottaken) seqloop::R m->[m]->Response m seqloop s [] = Taken s [] seqloop s (i:ii) = onTaken (s i) (\s1 oo0-> onTaken (seqloop s1 ii) (\s2 oo1 -> Taken s2 (oo0++oo1)) (Nottaken)) (onTaken (seqloop s ii) (\s1 oo -> Taken s1 (i:oo)) (Nottaken)) infix 6 <.> (<.>) = sequential ---------------------------------------------------------------------- -- -- Feedback on a system s feeds _all_ output of s back to s. All -- output which is not accepted by s _passes through_ s (and the -- loop). -- -- Feedback folds all the transitions taken over fed back output -- into _one_ big transition... -- -- +++++ -- -+-++ s +-+-- -- | +++++ ^ -- +--------+ ---------------------------------------------------------------------- feedback::R m->R m feedback s i = onTaken (s i) (\s0 oo0-> onTaken (feedloop s0 oo0) (\s1 oo1-> Taken (feedback s1) (oo1)) (Nottaken)) (Nottaken) feedloop::R m->[m]->Response m feedloop s [] = Taken s [] feedloop s (i:ii) = onTaken (s i) (\s0 oo-> feedloop s0 (ii++oo)) (onTaken (feedloop s ii) (\s0 oo -> Taken s0 (i:oo)) (Nottaken)) feed = feedback ---------------------------------------------------------------------- -- small examples: ---------------------------------------------------------------------- test s = take 40 (behavior s (repeat 1)) ex0 = test copy ex1 = test (lift (*2) <.> lift ((-)1)) ex2 = test (lift (+1) <+> lift (*2)) ex3 = test (lift (+2) <+> copy <.> lift (+3)) ex4 = test (feed copy) -- infinite computation ex5 = test (interface (\i->'a') (\a->5) copy) ---------------------------------------------------------------------- -- bigger example: ---------------------------------------------------------------------- data Msg = Tick | Out Int | Add Int | AddResult Int | Write Int Int | Fetch Int | Read Int Int deriving (Show) add::Maybe Int -> R Msg add Nothing (Add j) = Taken (add (Just j)) [] add (Just i) (Add j) = Taken (add Nothing ) [AddResult (i+j)] add _ _ = Nottaken register::Int->Int->R Msg register i _ (Write j r) | i == j = Taken (register i r) [] register i r (Fetch j) | i == j = Taken (register i r) [Read i r] register _ _ _ = Nottaken controller::R Msg controller Tick = Taken controller [Fetch 0, Fetch 1] controller (Read i j) = Taken controller [Add j, Write (1-i) j] controller (AddResult i) = Taken controller [Out i, Write 1 i] controller _ = Nottaken ex6 = test (interface (\_->Tick) (\(Out i)-> i) (feed (register 0 0<+>register 1 1<+>add Nothing<+>controller))) ---------------------------------------------------------------------- -- This code is licensed under the Big-Kazooba-Ritual license: -- -- You may copy, modify or distribute this software provided that -- you first perform the ritual described in section a. -- -- If you ever meet one of the authors of this work you can be asked, -- and must comply, to perform the ritual as described in section a. -- -- You must add this license to any derivatives of this work. -- -- a. Lift your left leg, close your eyes and jump in the air. -- Now tap the point of your nose exactly two times with your -- right index finger. To complete: say the words -- "thank you Big Kazooba" -- -- You may open your eyes now. -- -- The disabled may opt not to perform this ritual but have a -- volunteer perform the ritual on their behalf. -- -- Anyone, of suited position, may opt to command any number of -- subordinates to perform the ritual instead. -- -- Thank you, -- l4t3r ---------------------------------------------------------------------- _________________________________________________________________ MSN 8 helps eliminate e-mail viruses. Get 2 months FREE*. http://join.msn.com/?page=features/virus

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RE: Hugs Humor
by Simon Marlow 17 Jul '03

17 Jul '03

Jerzy Karczmarczuk writes: > I am abhorred by the fact that adding ... :: Rational > changes the lexical meaning of a literal. It doesn't. A literal with a decimal point always means (fromRational (X%Y)) for some appropriate X and Y. Adding a type signature changes the dynamic meaning of the literal, not the lexical meaning (something we're used to in Haskell, no?). Cheers, Simon

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Re: Case expressions, matching, and "constants"
by Graham Klyne 17 Jul '03

17 Jul '03

At 12:03 17/07/03 +0100, Bayley, Alistair wrote: >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 'case' is somewhat redundant. How about: > test n | n == one = "one" > | n == two = "two" > | otherwise = "three" #g ------------------- Graham Klyne <GK(a)NineByNine.org> PGP: 0FAA 69FF C083 000B A2E9 A131 01B9 1C7A DBCA CB5E

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Worldwide Partners program
by The TechNews 17 Jul '03

17 Jul '03

The TechNews, July 2003 Production Mini-plants in mobile containers. Worldwide Partners program Science Network will supply to countries and developing regions the technology and necessary support for production in series of Mini-plants in mobile containers (40-foot). The Mini-plant system is designed in such a way that all the production machinery is fixed on the platform of the container, with all wiring, piping, and installation parts; that is, they are fully equipped... and the mini-plant is ready for production." More than 700 portable production systems: Bakeries, Water purification, Dehydrated food, Steel Nails, Fruit juice preparation, Tire Retreading, Reinforcement Bar Bending for Construction Framework, Sheeting for Roofing, Ceilings and Façades, Plated Drums, Aluminum Buckets, Injected Polypropylene Housewares, Pressed Melamine Items (Glasses, Cups, Plates, Mugs, etc.), Mufflers, Construction Electrically Welded Mesh, Plastic Bags and Packaging, Mobile units of medical assistance, Sanitary Material, Hypodermic Syringes, Hemostatic Clamps, etc. Science Network has started a Co-investment program for the installation of small Assembly plants to manufacture in series the Mini-plants of portable production on site, region or country where required. One of the most relevant features is the fact that these plants will be connected to the World Trade System (WTS) with access to more than 50 million raw materials, products and services and automatic transactions for world trade. Due to financial reasons, involving cost and social impact, the best solution is setting up assembly plants on the same countries and regions, using local resources (labor, some equipment, etc.) Science Network participates at 50% (fifty percent) for investment of each Assembly plant. If you are interested in being a Science Network partner in your country or region, you can send your CV to: Mini-plants <A HREF="mailto:letters@the-tech-news.com">Worldwide Partners program</A> By Robert B. Lethe, The TechNews, Editor ------------------------------------------------------------------------- If you received this in error or would like to be removed from our list, please return us indicating: remove or un-subscribe in subject field, Thanks. <A HREF="mailto:editor@the-tech-news.com"> Editor</A> © 2003 The TechNews. All rights reserved.

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Old alternative syntax for list comprehensions?
by Peter Pudney 17 Jul '03

17 Jul '03

I recently wanted to generate an infinite list of pairs [(i,j) | i <- [1..], j <- [1..]] Of course, this list comprehension never gets around to generating pairs (2,_). The solution I was after is easily done: [(i,s-i) | s <- [2..], i <- [1..s-1]] But I have a vague recollection of an alternative syntax, something like [(i,j) \ i <- [..], j <- [1..]] that generated a list something like [(1,1),(1,2),(2,1),(1,3),(2,2),(3,1),...]. Did I dream this, or was it a feature of Miranda*, Gopher or Hugs many years ago? Peter

4 3

Re: infinite types
by blaat blaat 16 Jul '03

16 Jul '03

> > > type R m = m -> Maybe (R m, [m]) > > > > I don't think there's an extension of Haskell with regular type > > unification. It's certainly possible, but there's an equivalent in > > standard Haskell: > > > > newtype R m = MkR (m -> Maybe (R m, [m])) > >It will possibly be more convenient to break the cycle in another place, >where >we will wrap and pattern match anyway: > >type R m = m -> Response m >data Response m > | NotTaken > | Taken (R m) [m] Thanks everyone for responding so swiftly. The trick by Marcin really cleaned up my code, thanks! Cheers, l4t3r _________________________________________________________________ Tired of spam? Get advanced junk mail protection with MSN 8. http://join.msn.com/?page=features/junkmail

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Haskell book recommendation?
by bjkwak 16 Jul '03

16 Jul '03

Hi! Searching for a new programming language for my work, I got interested in functional languages. After several weeks of googling and searching, I decided to learn Haskell. I have downloaded and read some tutorials from the Haskell home page, but most of them are incomplete and I decided to buy a book or two. I have found a list of books on Haskell in the Haskell home page, but I cannot decide from the information given in the page. Could any body give me some advice? I have some experience in C, but never programmed in functional language before. I have read about 1/4 of the book "ML for the working programmer" by L.C. Paulson. Thanks! Byung-Jae

3 4

re: infinite types
by blaat blaat 16 Jul '03

16 Jul '03

Sorry if this mail starts a new thread. I am not subscribed to haskell-cafe and am new to hotmail. Uhm, as far as the example goes. I was trying to define a small (shallow encoding of) a reactive systems language. Because I wanted to try something else than monads I defined the following recursive type for a reactive system. type R m = m -> Maybe (R m, [m]) Supposedly a reactive system is a system which may not take a message (the Maybe), or do take it and respond with an updated version of itself, and a sequence of outgoing messages. For example, a copying machine might be defined as: id:R m id m = Just (id, [m]) A version for a ?Meally? machine embedding (always take an incoming message, and respond with one outgoing message) could be written as type Meally i o = i -> (Meally i o, o) where id is defined as id::Meally a a id m = (id, m) I like the formalization because (a) it is concise, (b) structural composition operators can be defined trivially, (c) hides the state of a machine, (d) might possibly, in the long run, somewhere, even work out equally well as a monadic approach. Does it make sense? Are there any known references to this approach? Cheers, l4t3r PS: Actually, I just realized this might be the co-algebraic approach, so I guess I am asking for a haskell which allows co-algebraic types. _________________________________________________________________ The new MSN 8: smart spam protection and 2 months FREE* http://join.msn.com/?page=features/junkmail

3 2

infinite types
by blaat blaat 16 Jul '03

16 Jul '03

Hi al! In my holliday I like to hobby a bit with haskell. For a small formalization I wanted to try out a specification which uses infinite types. Does a (extended) haskel compiler exist which allows infinite types? Chees, l4t3r _________________________________________________________________ The new MSN 8: advanced junk mail protection and 2 months FREE* http://join.msn.com/?page=features/junkmail

2 1

RE: functional "updatable" graph
by Hal Daume 16 Jul '03

16 Jul '03

>> I think that the >> difficulties you are >> facing are from the fact that you are trying to >> express a purely >> functional "updatable" graph. > > Should I understand from this, that this is a > difficult problem and that there exist no easy way to > do this at the moment? Well, I think he was being a bit ironic. You can either have a functional graph or an updatable graph, but not both. It sounds like either would do and given that later in this message you discuss that you don't like imperativeness, then a functional graph (ala the Functional Graph Library -- FGL) would suit you perfectly. > About the graphs:Well, I never had graphs at school > yet(first year student)... Although it is somewhere in > my textbooks... They're very simple. You have a collection of nodes, N, and a collection of edges between elements of N. Nodes and edges can have labels. So for instance you could have Teachers and Classes as the nodes and the edges could be between the teacher and the class(es) which go together. > To solve my problem... I need an mutable array, but in > Haskell my program will then be unbelievable > imperative. No, you just need a graph :). > I have been following the discussion about the graph > and so, but the arrow classes and so on, are like > something too sophisticated for me at this moment I > think, although I have some idea of how they work, I > really don't know when to apply them. I just found > out, how Monads work, so... THe discussion about arrows is a complete divergence. You don't need anything about them. > But I certainly believe that my problem must be fixed > before it's easy to program in Haskell. Theorem: this problem is already fixed. Proof: it is easy to program in Haskell. QED. :) - Hal

2 1

Simple monads
by Mark Carroll 15 Jul '03

15 Jul '03

Not really seeing why Unique is in the IO monad, not deeply understanding the use of Haskell extensions in the State source, and wanting to try to learn a bit more about monads, I thought I'd try to write my own monad for the first time: something for producing a series of unique labels. This is how it turned out: ========================================================================== module Label (Label, Labeller, newLabel) where import Monad newtype Label = Label Int deriving (Eq, Ord) newtype Labeller a = Labeller (Int -> (Int, a)) instance Monad Labeller where return r = Labeller (\n -> (n, r)) (Labeller g) >>= y = let f m = let (r, n) = g m Labeller h = y n in h r in Labeller f newLabel :: Labeller Label newLabel = Labeller (\n -> (n + 1, Label n)) runLabeller :: Labeller a -> a runLabeller (Labeller l) = snd (l minBound) labelTest :: Labeller [Int] labelTest = do Label a <- newLabel Label b <- newLabel Label c <- newLabel Label d <- newLabel return [a,b,c,d] main = print (runLabeller labelTest) ========================================================================== I was thinking that maybe, (a) People could point out to me where I'm still confused, as revealed by my code. Is it needlessly complicated? (b) My code may be instructive to someone else. -- Mark

12 24

Handling large ini files.
by Thaddeus L. Olczyk 15 Jul '03

15 Jul '03

A while ago I looked at Haskell. I got stuck on a problem which basically caused me to stop using Haskell. Realising that I never asked anyone else how they would approach the deal, I decided to ask before I put the final chapter on Haskell. I have a program that requires a lot of customisation. As a result there is an initialization file with many entries ( typically 30-80 entries ). Some of these entries need to be accessed quite often, so that opening the file and reading in on demand is not really an option. So the basic question is how do I handle access to these parameters? PS: I've thought of Hugh's approach to global variables, but even with compound data structures it really isn't practical for such a large number. Not even when creating compound structures to hold the data. Worse you wind up with data structures which are not coherent having only the fact that each is a customisation in common. Thaddeus L. Olczyk ----------------------- Think twice, code once.

4 3

RE: Designing a Haskell database interface
by Tim Docker 14 Jul '03

14 Jul '03

I wrote an experimental wrapper around the sybase db client libraries. This was my first attempt at using the ffi, and I was impressed with the fact that I got enough working purely using haskell. My simplistic approach was to embed everything in the IO monad, wrapping each sybase call trivially, and then building slightly higher level functions on top of each. Of course, the fun with handling queries is that you want to get values of types defined by the query string (ie unknown at compile time) from the database and into the typed haskell world. In my approach, the top level interface looked like: open :: String -> String -> String -> IO Process -- open a connection to the database returning an abstract -- process object doquery :: Process -> String -> IO v -> IO [v] -- Takes: a db process; the SQL query string; and -- a (no parameter) IO action that, when executed -- during a query returns a value of type v derived -- from the "current" row -- -- Returns the list of values of type v derived from -- each row. stringv :: Process -> CInt -> IO String doublev :: Process -> CInt -> IO Double intv :: Process -> CInt -> IO Int datetimev :: Process -> CInt -> IO DateTime -- Returns the string/double/int/datetime value of the specified -- column in the "current" row You could then write queries something like this: dbp <- DB.open "SERVER" "user" "password" rows <- DB.doquery dbp queryText $ do name <- DB.stringv dbp 1 v1 <- DB.doublev dbp 2 v2 <- DB.doublev dbp 3 return (label,v1,v2) At which point rows would have type [(String,Double,Double)]. This did the job, but I'd be interested in better ways of doing it! Tim > -----Original Message----- > From: Bayley, Alistair [mailto:Alistair_Bayley@ldn.invesco.com] > Sent: Monday, July 14, 2003 4:25 PM > To: haskell-cafe(a)haskell.org > Subject: Designing a Haskell database interface > > > I'm making slow progress on an Oracle OCI binding. I've got the noddy > session setup and database connection stuff working, so now > I'm looking at > how results should be returned from SQL queries. In Haskell, > there doesn't > seem to be any consistent way of returning results from SQL > queries, unlike > (say) Java's JDBC interface. It may be a bit early to propose > a standard SQL > dbms interface design, but... does anyone have any idea(s) > about how one > ought to look? > > Here's a simple survey of the sql dbms interfaces I've come > across so far (I > found HaskellDB the most complex, and difficult to understand). My > assumptions about how these various libraries work might be > quite wrong: > > > HaskellDB: > dbQuery returns IO [row r], where > - dbQuery is implemented by adoQuery (this library uses Odbc): > adoQuery :: IConnection () -> PrimQuery -> Rel r -> IO > [AdoRow r] > - "AdoRow a" is an instance of class Row > - "class Row row a" declares one function: rowSelect > rowSelect :: Attr r a -> row r -> a > - type AdoRow implements rowSelect with adoRowSelect: > adoRowSelect :: Variant a => Attr r a -> AdoRow r -> a > - Attr has one constructor: Attr Attribute, and Attribute is just a > synonym for String. > So (I think) the return types are determined by the types > of the phantom > types in "Attr r a" and "AdoRow r". And this is where my head > explodes... > > > LibPQ: > fetchAllRows returns tuple pair of the connection and row : (DBI a, > [[String]]) > - so, a row is a list of list of Strings. > > > HaSql: > haSQLObtainQueryResults returns SQL [a], where > - SQL is some custom IO+State Monad > - the type of a is the return type of a function you pass to > haSQLObtainQueryResults to (I think) convert an Odbc pointer > into a Haskell > type. > > > MySql-hs: > mysqlQuery returns a tuple of (Integral, [[Maybe String]], > [MysqlField]), > where > - the first element (Integral type) is the number of rows > - the second element is a list of list of Strings - the result set. > - the third element is metadata. MysqlField is a record > describing a > database column. > > > The most sophisticated implementation wrt type information seems to be > HaskellDB. My initial goal was to use this library and > provide an Oracle > database driver for it, but the HaskellDB seems to be quite > dependent on > Trex, which AFAICT is a Hugs library. > > I was thinking of splitting it into two parts: a library that > submits SQL > queries and returns the results, and a library that > constructs queries with > the relational calculus and generates SQL from them. The > relational calculus > bit was what I was interested in, but for now I want to work > on getting data > out of my Oracle database. > > > Also... > > I was wondering how you might go about mapping arbitrary dbms > types to a > Haskell result set. > > In an ideal world, you can store any values you like in a "relational" > database. However, most SQL dbms products give you just > numbers, text, and > dates. Some dbms's (PostgreSql and Oracle) allow > users/programmers to create > new types and let the dbms treat them in the same manner as > the built-in > ones i.e. with equality and ordering predicates, and > indexing. The built-in > support for the three basic types (numbers, text, dates) is > reflected in the > JDBC API; it has methods like: getByte, getShort, getFloat, getDouble, > getBigDecimal, getInt, getString, getDate, getTime, > getBoolean, while other > types are handled by methods like getBinaryStream and getObject. > > So how would you convert a (say) PostgreSql Point or Box > value to a Haskell > type? Would the approach taken by HaSql be the best (the user > provides a > function that converts binary data into a Haskell value)? > > > ***************************************************************** > 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. > ***************************************************************** > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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Designing a Haskell database interface
by Bayley, Alistair 14 Jul '03

14 Jul '03

I'm making slow progress on an Oracle OCI binding. I've got the noddy session setup and database connection stuff working, so now I'm looking at how results should be returned from SQL queries. In Haskell, there doesn't seem to be any consistent way of returning results from SQL queries, unlike (say) Java's JDBC interface. It may be a bit early to propose a standard SQL dbms interface design, but... does anyone have any idea(s) about how one ought to look? Here's a simple survey of the sql dbms interfaces I've come across so far (I found HaskellDB the most complex, and difficult to understand). My assumptions about how these various libraries work might be quite wrong: HaskellDB: dbQuery returns IO [row r], where - dbQuery is implemented by adoQuery (this library uses Odbc): adoQuery :: IConnection () -> PrimQuery -> Rel r -> IO [AdoRow r] - "AdoRow a" is an instance of class Row - "class Row row a" declares one function: rowSelect rowSelect :: Attr r a -> row r -> a - type AdoRow implements rowSelect with adoRowSelect: adoRowSelect :: Variant a => Attr r a -> AdoRow r -> a - Attr has one constructor: Attr Attribute, and Attribute is just a synonym for String. So (I think) the return types are determined by the types of the phantom types in "Attr r a" and "AdoRow r". And this is where my head explodes... LibPQ: fetchAllRows returns tuple pair of the connection and row : (DBI a, [[String]]) - so, a row is a list of list of Strings. HaSql: haSQLObtainQueryResults returns SQL [a], where - SQL is some custom IO+State Monad - the type of a is the return type of a function you pass to haSQLObtainQueryResults to (I think) convert an Odbc pointer into a Haskell type. MySql-hs: mysqlQuery returns a tuple of (Integral, [[Maybe String]], [MysqlField]), where - the first element (Integral type) is the number of rows - the second element is a list of list of Strings - the result set. - the third element is metadata. MysqlField is a record describing a database column. The most sophisticated implementation wrt type information seems to be HaskellDB. My initial goal was to use this library and provide an Oracle database driver for it, but the HaskellDB seems to be quite dependent on Trex, which AFAICT is a Hugs library. I was thinking of splitting it into two parts: a library that submits SQL queries and returns the results, and a library that constructs queries with the relational calculus and generates SQL from them. The relational calculus bit was what I was interested in, but for now I want to work on getting data out of my Oracle database. Also... I was wondering how you might go about mapping arbitrary dbms types to a Haskell result set. In an ideal world, you can store any values you like in a "relational" database. However, most SQL dbms products give you just numbers, text, and dates. Some dbms's (PostgreSql and Oracle) allow users/programmers to create new types and let the dbms treat them in the same manner as the built-in ones i.e. with equality and ordering predicates, and indexing. The built-in support for the three basic types (numbers, text, dates) is reflected in the JDBC API; it has methods like: getByte, getShort, getFloat, getDouble, getBigDecimal, getInt, getString, getDate, getTime, getBoolean, while other types are handled by methods like getBinaryStream and getObject. So how would you convert a (say) PostgreSql Point or Box value to a Haskell type? Would the approach taken by HaSql be the best (the user provides a function that converts binary data into a Haskell value)? ***************************************************************** 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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Multiple "pointers" to "objects"
by Ron de Bruijn 14 Jul '03

14 Jul '03

Hi there, I was almost certain that Haskell was a great language until I wanted to make a real usefull program and got the following problem. I have data Lesson = Lesson Teacher SomeOtherProperties deriving Show data Subject = Subject Name [Teacher] SomeOtherProperties deriving Show data Teacher = Teacher TimeTable SomeOtherProperties What I want is that when I put some lesson in my timetable, the resources needed for that lesson are used up, so for example the timetable of a teacher will fill with each lesson that it gives. The problem is that when I "model" it this way, the state of the teachers that can give a certain subject will not change (suppose I have some function that fills one timeslot of the timetable of a teacher). It makes it even harder, because of the fact that one teacher can teach multiple subjects. In an OO-language I would simply let each element of the list of TeacherObjects of Subjects point to some TeacherObject, so it remembers it state, but that's anti-Haskell. It's ofcourse possible to put a list of Subjects that a Teacher teaches in the data declaration of the teacher. But then there is no way of saying efficiently (O(1) Just a pointer or index):"Give me a list of all teachers that give Physics", and that's just what I need. I could use a hashtable which includes the teachersobjects as values and the subjects as keys, but that isn't a very beautiful solution. This would give me(building of Hashtable O(n) and getting all teachers of some subject O(1)), so it would do. I am almost sure there exist some nice way of doing this, because otherwise Haskell would be completely useless IMHO, but I don't know it. Do you have any idea? Greets Ron __________________________________ Do you Yahoo!? SBC Yahoo! DSL - Now only $29.95 per month! http://sbc.yahoo.com

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Re: functional "updatable" graph
by Ron de Bruijn 13 Jul '03

13 Jul '03

Are you sure that your data structure shouldn't be expressed by a graph? In that case, there is a very recent thread (last post today) on functional graph algorithms. I think that the difficulties you are facing are from the fact that you are trying to express a purely functional "updatable" graph. Should I understand from this, that this is a difficult problem and that there exist no easy way to do this at the moment? About the graphs:Well, I never had graphs at school yet(first year student)... Although it is somewhere in my textbooks... To solve my problem... I need an mutable array, but in Haskell my program will then be unbelievable imperative. I have been following the discussion about the graph and so, but the arrow classes and so on, are like something too sophisticated for me at this moment I think, although I have some idea of how they work, I really don't know when to apply them. I just found out, how Monads work, so... But I certainly believe that my problem must be fixed before it's easy to program in Haskell. Greets Ron __________________________________ Do you Yahoo!? SBC Yahoo! DSL - Now only $29.95 per month! http://sbc.yahoo.com

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Re: Representing cyclic data structures efficiently in Haskell
by Sarah Thompson 12 Jul '03

12 Jul '03

>>From reading your message, it seems you want a graph library of sorts. > The important bit is 'of sorts', unfortunately! Most graph libraries I've seen to date (although I must admit that they have been written in C and C++, including one I perpetrated personally) don't really work that well for circuits because, for anything other than trivially simple components, the connections between nodes need to be labelled. A component representing something simple like (say) an 'and' gate would be easy enough to represent as a node of an ordinary directed graph. In such a case, an incoming arrow would always be an input of the gate, and an outgoing arrow would always be the output. I need to be able to represent more complex components, up to and including the complexity of things like CPUs, where you might have lots and lots of inputs and outputs with substantially different meanings. In the case of a CPU, it would never be acceptable to (for example) swap a chip select output for an address or data line. This makes me think that an off-the-shelf graph library won't quite be what I'm after, so I will almost certainly need to write the code. What I'm more interested in at the moment is a view on the best kinds of approach that might be appropriate. I'd considered something like embedding the type in the IO monad, with links between components implemented as IORefs, but I'd really prefer something cleaner (pure functional). If the code ends up horribly complicated in order to get decent performance, I might as well fold and use C++ instead. I'm currently wondering about going for an adjacency list approach (as used by most graph libraries), with the tuples in the adjacency list extended to include input/output labels, resulting in a 4-ary tuple rather than the usual 2-ary. But, I don't want to be stuck with representing this as a list -- I really need log N lookups or performance will stink horribly as circuits get big. Maybe a pair of finite maps, allowing the edges to be traversed in either direction? >If FGL seems like overkill to you, I have my own little mini graph >library which basically looks like: > > - An ADT, Node: newtype Node = Node Int deriving ... > - The 'Graph n e' data structure, containing: > - a FiniteMap from n -> Node > - an (growable?) array of type IOArray (Node,Node) e > >the array represents the edges in the top half (i do some index fiddling >to get this efficient) and the finitemap represents the node labellings. >It's a pretty stupid interface and works a lot better when the graphs >are fixed size (that way you don't need to grow the array). But it >works. > > The finite map from n to Node seems like a good idea. Does representing the adjacency list as a flat array cause any problems? Sarah -- ---------------------------------------------- / __ + / Sarah Thompson **** / / (_ _ _ _ |_ / * / / __)(_|| (_|| ) / sarah(a)telergy.com * / / + / http://findatlantis.com/ / ---------------------------------------------- -- ---------------------------------------------- / __ + / Sarah Thompson **** / / (_ _ _ _ |_ / * / / __)(_|| (_|| ) / sarah(a)telergy.com * / / + / http://findatlantis.com/ / ----------------------------------------------

4 3

accessing Haskell-Cafe from oe
by Jeff Aimt 12 Jul '03

12 Jul '03

Hello, I discovered recently (thanks google) that Haskell Cafe was also named "fa.haskell", but can't find a news server which would let me access it like any other newsgroup. If someone accesses haskell cafe using a free news server, could he say me which one ? Thanks by advance.

3 2

RE: Template Haskell question
by Simon Peyton-Jones 12 Jul '03

12 Jul '03

Yes, sorry, as Ian says, type splices just aren't implemented at the moment. The error message is uninformative. This is useful info though -- someone wants type splices! Simon | -----Original Message----- | From: haskell-cafe-admin(a)haskell.org [mailto:haskell-cafe-admin@haskell.org] On Behalf Of Ashley | Yakeley | Sent: 27 June 2003 23:57 | To: haskell-cafe(a)haskell.org | Subject: Template Haskell question | | Can anyone tell me what's wrong with this? | | -- ghc -fglasgow-exts -c TH.hs | module TH where | { | import Language.Haskell.THSyntax; | | class HasZero a where | { | zero :: a; | }; | | aninstance :: TypQ -> Q [Dec]; | aninstance t = [d| | | instance HasZero $t where -- error here | { | zero = 0; | }; | | |]; | | $(aninstance [t|Int|]) | $(aninstance [t|Integer|]) | } | | | $ ghc -fglasgow-exts -c TH.hs | TH.hs:14: Malformed context in instance header | | All I want to do is spin off a number instances for a number of types... | | -- | Ashley Yakeley, Seattle WA | | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe(a)haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe

2 1

Hugs Humor
by Steffen Mazanek 12 Jul '03

12 Jul '03

Hello, a quit funny hugs session: Prelude> 0.5::Rational 1 % 2 Prelude> 0.1::Rational 13421773 % 134217728 Prelude> 13421773/134217728 0.1 I do not know how this fraction is calculated, but it does not fit my expectations :-) Ok, ok, it is no bug... Ciao, Steffen -- Steffen Mazanek - www.steffen-mazanek.de - GPG: 791F DCB3 Haskell, that's where I just curry until fail, unwords any error, drop all undefined, maybe break, otherwise in sequence span isControl and take max $, id: (d:[])

9 19

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by webmaster 12 Jul '03

12 Jul '03

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Re:Decimal Literals
by jmaessen＠alum.mit.edu 12 Jul '03

12 Jul '03

Ketil Z. Malde <ketil(a)ii.uib.no> writes: > I.e. for 3.14, allow any rational number in [3.14,3.15), or > perhaps (3.135,3.145]? Or restrict it to just the rational number in > range with the least total digits? We can get this effect using a version of (%) adapted specifically for converting decimal floating-point constants. Thus: Data.Ratio> 31415926536%1000000000 3926990817 % 125000000 Data.Ratio> let n %% d = approxRational (n%d) (1%(2*d)) in 31415926536 %% 10000000000 312689 % 99532 We can imagine the compiler replacing floating constants by fromRational (n%%d) rather than fromRational (n%d). Of course, then trailing zeros suddenly become significant: Data.Ratio> let n %% d = approxRational (n%d) (1%(2*d)) in 314 %% 100 22 % 7 Data.Ratio> let n %% d = approxRational (n%d) (1%(2*d)) in 3140 %% 1000 135 % 43 This might be good or bad, depending on your perspective. -Jan-Willem Maessen jmaessen(a)alum.mit.edu

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constructors in O'Haskell
by Peter Padawitz 11 Jul '03

11 Jul '03

How may I hide constructors in O'Haskell 'cause export lists don't work in this language? Peter

1 0

Monad constructors
by Peter Padawitz 11 Jul '03

11 Jul '03

Why must every Monad instance be a datatype or newtype, even if there will be just a single constructor? Peter

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Re: Representing cyclic data structures efficiently in Haskell
by C.Reinke 08 Jul '03

08 Jul '03

> What is the best way to represent cyclic data structures in Haskell? There used to be some work on direct cyclic representations at UCL: Dynamic Cyclic Data Structures in Lazy Functional Languages Chris Clack, Stuart Clayman, David Parrott Department of Computer Science, University College London, October 1995 The link to the project, from http://www.cs.ucl.ac.uk/staff/C.Clack/research/report94.html is broken, but the paper seems to be online at http://www.cs.ucl.ac.uk/teaching/3C11/graph.ps If you want to go for this direct cyclic representation, the interplay with lazy memo functions is also interesting: J. Hughes, Lazy memo-functions, Functional Programming Languages and Computer Architecture, J-P. Jouannaud ed., Springer Verlag, LNCS 201, 129-146, 1985. (lazy memo-functions remember previous input-output pairs based only on pointer-info, which is sufficient to write functions over cyclic structures that, instead of infinitely unrolling the cycles, produce cyclic results) (not online?-( Claus

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I NEED YOUR HELP
by johnmboma＠sify.com 08 Jul '03

08 Jul '03

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