February 2004 - Haskell-Cafe

[Haskell-cafe] ITCC 2004, April 5 -7 2004 - Call For Participation, Las Vegas, USA
by jolly＠egr.unlv.edu 29 Feb '04

29 Feb '04

My apologies if this is a duplicate email. CALL FOR PARTICIPATION - ITCC 2004 http://www.itcc.info International Conference on Information Technology- ITCC 2004 is going to be held in Las vegas, NV USA.ITCC is an international forum which brings together researchers and practitioners working on different aspects of Information Technology. It is a technical congregation where the latest theoretical and technological advances on Information Technology are presented and discussed. This year's conference will bring together the world's most respected authorities on Information Retrieval, Operating Systems, Networks,Image/Video Processing, Digital Library, Coding & Data Compression,Watermarking, Simulation, Computer Graphics, Information Technology: Education, Curriculum & Accreditation, Information/Website Security, Data Mining and plethora of IT related technologies.(Pls check the advanced Program at our website). Whether you are a speaker, tutorial presenter, and/or sponsor, you will find ITCC 2004 as the best forum to share and demonstrate your knowledge with the depth and breadth of Information Technology.For registration details,Visit our Website http://www.itcc.info/ Any questions/Comments may be forwarded to it(a)egr.unlv.edu We look forward to your participation and hope to see you in Las Vegas! If you do not wish to be included in this list, and/or do not wish to receive further announcements regarding ITCC, please reply at jolly(a)egr.unlv.edu Your email address will be removed immediately from the distribution list. With Best Regards Vasu Jolly The ITCC 2004 Conference Secretary

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Outstanding context : (Num b, Random b)
by "horsh 26 Feb '04

26 Feb '04

Hi, I am very new to haskell. Could anyone please explain why these two things are not equivalent: m2 = do a <- (drawInt 1 10) print a drawInt :: Int -> Int -> IO Int drawInt x y = getStdRandom (randomR (x,y)) m1 = do b <- getStdRandom (randomR (1,10)) print b the second one produces *** Binding : m1 *** Outstanding context : (Num b, Random b) in Hugs. thanks horsh

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Perl-ish =~ operator
by ozone＠algorithm.com.au 24 Feb '04

24 Feb '04

In my effort to turn Haskell into a language more like Perl (muahaha)[1], I got a bit fed up and implemented something like Perl 5's =~ binding operator (a.k.a. "regex" operator); I thought maybe somebody else here might find it useful. Perl has the concept of 'contexts': a function does something different depending on what type its caller expects back from the function. Sounds like a perfect abuse of type classes, to me :). Code follows: --- {-# OPTIONS -fglasgow-exts #-} {- Need this for "instance Foo [String]" declarations -} module PLRE where -- Perl-Like Regular Expressions import Text.Regex -- Perl-Like =~ operator, which changes behaviour depending on its calling -- context class RegExContext a where (=~) :: String -> String -> a instance RegExContext Bool where s =~ re = case matchRegex (mkRegex re) s of Nothing -> False Just x -> True instance RegExContext [String] where s =~ re = case matchRegex (mkRegex re) s of Nothing -> [] Just x -> x boolContextTest string regEx = case string =~ regEx of True -> print True False -> print False stringListContextTest string regEx = case string =~ regEx of (a:x) -> print ("First match: " ++ a) _ -> error "No subexpression matches" --- Some test output for you: *PLRE> boolContextTest "foo" "^f" True *PLRE> boolContextTest "foo" "^g" False *PLRE> stringListContextTest "foo" "^(.)" "First match: f" *PLRE> stringListContextTest "goo" "^(.)" "First match: g" *PLRE> stringListContextTest "" "^(.)" *** Exception: No subexpression matches Note that you have a fairly severe restriction if you want to use =~ in your code: the Haskell compiler must be able to determine a concrete type for the context that =~ is used in. i.e. if stringListContextTest was defined as: stringListContextTest string regEx = case string =~ regEx of (a:x) -> print a _ -> error "No subexpression matches" The compiler can't concretise a type for 'a', and it'll complain about not having an instance for RegExContext [a] (which is fair enough). Even with this restriction, I'm sure it'll still be useful. It shouldn't be a bit leap to define other Perl-ish operators in this fashion, such as !~, or even s/.../. Have the appropriate amount of fun! 1. Actually, I wanted to turn Haskell into a language more suitable for text processing, but that doesn't sound as evil. -- % Andre Pang : trust.in.love.to.save

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RE: [Haskell-cafe] More help for the newb
by Matthew Morvant 21 Feb '04

21 Feb '04

Thanks for the catching the typo. However, I keep getting type errors. The error that I get seems self-explanaotry, by telling me what to change the type to. But when I change it I get another error that seems to tell me to change it back. -----Original Message----- From: Philippa Cowderoy [mailto:flippa@flippac.org] Sent: Saturday, February 21, 2004 10:02 AM To: Matthew Morvant Subject: Re: [Haskell-cafe] More help for the newb On Sat, 21 Feb 2004, Matthew Morvant wrote: > 36 code3 :: [b] -> [Char] > > 37 code3 = do{char '!'; digit; char 's'; many (do char 's'); return "Suc"} > +++ do{char '%'; char 's'; digit; many (do digit): return "Suc"} > You appear to have a colon instead of a semicolon, which isn't good. Consider "many digit" rather than "many (do digit)", too. -- flippa(a)flippac.org

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Thinner secure email?
by Justin Walsh 21 Feb '04

21 Feb '04

Re: my last goof up Hi, I know, being a totally ignorant Haskell newbie, that I should soon get to know my pecking order. But this e-paper shuffle is so damned annoying. i.e. "Hello World" In .txt mode is 11 bytes. In NoteTab Lite .txt (with url; say "Ripose.com.au") is 28 bytes In WordPad .doc 4,603 kbytes In MS Word .doc 19,456 kbytes. I would have though that the first project might have been the email itself. X does not seem to have been considered here. Why is so much rubbish data going down the line? Am I asking too many question? On the squeak-dev list I would be getting; Well $#1*? stop winging and fix it. I would, if I knew that Haskell is the right tool to use, if a tool is really needed at all. Does anybody get my reasoning? At my age 64.5 I'm too old to start another language, then find out that it's another dead end. I'm not trolling (I forgot to mention, I hate trolls too). With respect Justin walsh --- Outgoing mail is certified Virus Free. Checked by AVG anti-virus system (http://www.grisoft.com) Version: 6.0.585 / Virus Database: 370 - Release Date: 2/11/04

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5 Kb limit with notetab lite attachment
by Justin Walsh 21 Feb '04

21 Feb '04

Is this better? Please note that the entire email I first sent is 7,153 bytes and in attachment This one plus attachment is under 5 kb. I am using notetab lite. How does that rank with security i.e. I don't want to scare people with attachments. Note: I am not good at this sort of thing. Subject: Your message to Haskell-Cafe awaits moderator approval > Your mail to 'Haskell-Cafe' with the subject > > Rational Haskell? > > Is being held until the list moderator can review it for approval. > > The reason it is being held: > > Message body is too big: 6180 bytes with a limit of 5 KB >

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More help for the newb
by Matthew Morvant 21 Feb '04

21 Feb '04

I have gotten past the syntax errors and am now into type errors. Hopefully if I get some help with one I can fix the rest on my own. Here is the funct 36 code3 :: [b] -> [Char] 37 code3 = do{char '!'; digit; char 's'; many (do char 's'); return "Suc"} +++ do{char '%'; char 's'; digit; many (do digit): return "Suc"} ERROR "C:\Documents and Settings\mmorvant\Desktop\haskell.hs":37 - Type error in application *** Expression : many do {...} : return "Suc" *** Term : many do {...} *** Type : Parser [Char] *** Does not match : [Char]

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Rational Haskell?
by Justin Walsh 21 Feb '04

21 Feb '04

Please bear with me. Point of departure: My goal is OWL (but replacing the "O" for Ontological with Noological) http://www.bright.net/~jclarke/kant/history.html 2. Noology (as in noumenon or "state of mind") i.e. rational http://en.wikipedia.org/wiki/Ontology Zoology (as in phenomenon or state of body) i.e. Empirical Declaration: None of these OO languages in their current context seem to work for me. Observations: I love AWK, I don't like C++, I Like plain vanilla Prolog and Smalltalk (as procedural and declarative) IDE's, but in there current form useless for my purposes. I'm unsure of Haskell but, feel it has untapped potential at a meta level. So far I have found, that Haskell is a great "what-if" language (the comparison with the spread sheet correct). I am very much a remnant of the "how-to" generation (one cannot get much lower than assembler). I found it difficult to take responsibility and let go of my "goto". But I do need to reach the "when-to" level: my project is Virtual Reality and SAI:(asIn Silicon Assisted Intelligence is gnot AI). Somehow though all of us early (infant) programmers moved pretty seamlessly form goto's and flowcharting to pseudo-code and structure programming techniques: because it works. However in 1989 I did BR and Semantic Modeling (Debenham and Montgomery UTS), realising immediately that it did not work. Since then I still see the Systems Designers equivalent of the flowchart and the goto i.e Entity Relationship Modeling http://www-106.ibm.com/developerworks/rational/library/319.html Why is it so hard for SysDesigners to make the same mind shift we did? Sorry if I sound rude but I'm stuck in an OO rut and I need to reach Process Orientation rather quickly. Any serious suggestions (apart from "just go away")? Justin PS: I don't like dialectical bickering over detail. Please spare me the flaming --- Outgoing mail is certified Virus Free. Checked by AVG anti-virus system (http://www.grisoft.com) Version: 6.0.585 / Virus Database: 370 - Release Date: 2/11/04

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Help a Newbie
by Matthew Morvant 20 Feb '04

20 Feb '04

I am trying (and have been for quite some time) to repair a broken Haskell script. I keep getting stuck on Last generator in do {...} must be an expression. I am betting that this is not uncommon. Can someone please help me understand? code5 :: Parser code5 = do dd <- code3 do ddd <- code3 do dddd <- code3 return "ce" code4 :: Parser code4 = do dd <- code3 do dde <- code3 digit ddd <- many (do digit) q <- code5 return (q : "ss") Thanks in advance for helping Matthew

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RE: [Haskell-cafe] setPermissions bug?
by Simon Marlow 20 Feb '04

20 Feb '04

> On Wed, Feb 18, 2004 at 12:31:34PM -0000, Simon Marlow wrote: > > > I presume that this is a bug in setPermissions? > > > > Yes, it's a bug. However, since > getPermissions/setPermissions doesn't > > really give you enough control in Unix, I suggest you use > the functions > > available from System.Posix instead (fileStatus, fileMode, > setFileMode > > etc.). > > Well, in my case all I want to do is to copy permissions from > one file to > another, so get/setPermissions would have been fine. But yeah, I've > already switched to using setFileMode etc. Actually if you want to copy permissions from one file to another, then get/setPermissions is *not* what you want. It will only copy across the owner's permissions. The problem is that the Permissions type is fully defined and non-abstract; there's no place to store extra platform-specific permission bits. Cheers, Simon

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hOp
by sebc＠macs.hw.ac.uk 19 Feb '04

19 Feb '04

The RTS of GHC is truly an amazing piece of software. By removing the system-specific bits, and just adding a few lines of C and assembly code, we were able to turn it into a standalone micro-kernel that can be extended in our favorite language. A screenshot is available here: http://etudiants.insia.org/~jbobbio/hOp-20040219.png Everything from the base hierarchical library not under the System subdirectory is available. This includes threads and communication primitives and the foreign interface, which means experimenting with writing drivers in Haskell should be reasonably easy. We hope to make a source release soon. -- Sébastien Carlier Jérémy Bobbio

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[Q] What calls what
by Andre Betz 19 Feb '04

19 Feb '04

Hi, I have a question about the following function. I don't know how the definition after "where" are called. For example, in ( D x2 (-y1) , b ++ b2 ) there is x2 defined in the where-block. So does that mean that it calls the iteratin at line 7? But what is with b2, does it also calls the line 7? Who calls the definition in line 3 and who calls the def in line 7? Many thanks Andre 1 test :: ( D , [Int] ) -> ( D , [S] ) -> ( D , [Int] ) 2 test ( D x y , b ) ( D dx dy , st ) = ( D x2 (-y1) , b ++ b2 ) 3 where ( x1 , y1 ) = head $ dropWhile ( ( > 0 ) . snd ) $ iterate inc ( x , y ) 4 5 inc ( n , m ) = ( n+1 , m+1 ) 6 7 ( b2 , x2 ) = head $ do 8 S ( D x3 sty ) pl _ sb <- st 9 guard $ sty == -y1 10 return ( [1,2,3] ++ sb , x3 + sb) -- ********************************************************** ** * André Betz * * * ---------- * * * EMail: mail(a)andrebetz.de * WW * * Web: http://www.andrebetz.de * /__\ * * ICQ: 117759780 * | oo | _WWWWW_ * * Mobil: +49 (0)162 8847 391 * (|_()_|) / o o \ * * Office: +49 (0)9131 82611 27 * \__/ (| __O__ |) * * Home: +49 (0)911 6695 558 * /|\/|\ \ \___/ / * * Tel./Fax: +49 (0)89 1488 224189 * |||||||| /-------\ * * Snail: Hirschelgasse 34 * |||||||||=========| * * D-90475 Nürnberg * |||||||||=========| * ********************************************************** **

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Re: [Haskell] Parsec question: attempted 'notMatching' combinator
by Graham Klyne 18 Feb '04

18 Feb '04

[To Haskell-cafe...] At 16:57 17/02/04 -0500, Andrew Pimlott wrote: >On Tue, Feb 17, 2004 at 07:48:52PM +0000, Graham Klyne wrote: > > Thanks! That got me going, though not with quite what you suggested. > > > > I ended up with this: > > [[ > > notMatching :: Show a => GenParser tok st a -> GenParser tok st () > > notMatching p = do { a <- try p ; unexpected (show a) } <|> return () > > ]] > > which does the required job for me. > >Oops, that does look better. I knew the try had to go somewhere. :-) > >The only remaining problem is when p succeeds but does not consume any >input, eg eof. In this case, the <|> return () cannot distinguish it >from p failing. > >I didn't realize at first what a dirty trick this function uses: It >distinguishes success or failure of p by whether any input from the >first part (before <|> return ()) was consumed. I don't think it is >possible (or desirable!) to get this approach 100% right: By "erasing" >the success of p, you lose the information you need. > >What about a more prosaic implementation: > > notFollowedBy' :: Show a => GenParser tok st a -> GenParser tok st () > notFollowedBy' p = do res <- do a <- try p; return $ Just a > <|> > return Nothing > case res of Just a -> unexpected (show a) > Nothing -> return () I don't see why that would work where the above case does not; i.e. when p consumes no input. Or indeed, your later, preferred version: [[ notFollowedBy' :: Show a => GenParser tok st a -> GenParser tok st () notFollowedBy' p = join $ do a <- try p; return (unexpected (show a)) <|> return (return ()) ]] >This works for the tests I've tried, but there's one little quirk with >error reporting: I never really cared so much about that bit! #g -- > aNoBC = do char 'a' > notFollowedBy' $ do char 'b'; char 'c' > > *Main> parseTest (aNoBC >> char 'e') "abe" > parse error at (line 1, column 2): > unexpected "e" > expecting "c" or "e" > >It seems that parsec both misreports which token is unexpected (should >be "b", and thinks that the failure to match "c" is a problem, even >though notFollowedBy' succeeded. > > > Using your version caused the notMatching parser to be equivalent to: > > return () > > presumably, because the failure was protected by the try combinator? > >Or perhaps more accurately, the success (of p) was protected by the try! >Ie, the unexpected and the try together undid the tell-tale token >consumption of p. > >Andrew ------------ Graham Klyne For email: http://www.ninebynine.org/#Contact

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RE: [Haskell-cafe] setPermissions bug?
by Simon Marlow 18 Feb '04

18 Feb '04

> I've run into a problem using Directory.setPermissions (or > equivalently System.Directory.setPermissions). > > Basically the problem is that > > getPermissions f >>= setPermissions f > > is not a noop. Instead it is equivalent to > > chmod o-r f > chmod g-r f > etc > > I presume that this is a bug in setPermissions? Yes, it's a bug. However, since getPermissions/setPermissions doesn't really give you enough control in Unix, I suggest you use the functions available from System.Posix instead (fileStatus, fileMode, setFileMode etc.). Cheers, Simon

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setPermissions bug?
by David Roundy 18 Feb '04

18 Feb '04

I've run into a problem using Directory.setPermissions (or equivalently System.Directory.setPermissions). Basically the problem is that getPermissions f >>= setPermissions f is not a noop. Instead it is equivalent to chmod o-r f chmod g-r f etc I presume that this is a bug in setPermissions? The documentation claims that Note that to change some, but not all permissions, a construct on the following lines must be used. makeReadable f = do p <- getPermissions f setPermissions f (p {readable = True}) Which certainly implies that it is possible to change only some permissions, contrary to what happens in the actual implementation. It also seems that the Permissions data type could include in it additional (hidden, since it isn't portable perhaps) information in order to make the above code work. Then the only major question would be whether setPermissions f (p {writeable = True}) should make the file world-writeable or just user-writeable. I don't know about that. But at a minimum, it should preserve world-readability. -- David Roundy http://www.abridgegame.org

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Re: [Haskell] updating graphs non-destructively
by Graham Klyne 18 Feb '04

18 Feb '04

[to Haskell-cafe] At 19:29 17/02/04 -0500, S. Alexander Jacobson wrote: >Thank you for the link to FGL. I also looked at >the boilerplate stuff. If *feels* like there >should be a way to embed the graph stuff in the >boilerplate stuff to allow non-destructive update >of arbitrary object graphs without handcoding the >mapping of the datastructure to an object graph? > >Is this possible? Has anyone does this? Any >reason to believe its a bad idea? > >Alternatively, I could decide up front to >represent my data as RDF triples (amenable to a >graph system), but I'd rather take advantage of >Haskell's type system. Hmmm (speaking as an RDF advocate) ... that seems like an odd reason for using RDF. Much of the value of RDF is, I would say, as means to exchange information *between* programs rather than for internal structuring of data. An RDF-graph-based interface to data provides for some uniformity of access and processing that can be useful. In my own work, I have at least two different internal representations for RDF data (neither of which have been designed for efficiency). #g ------------ Graham Klyne For email: http://www.ninebynine.org/#Contact

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Wiki manager change
by Keith Wansbrough 16 Feb '04

16 Feb '04

Hi all... the Haskell Wiki http://haskell.org/hawiki/ is under new management. From now on, please contact Shae Erisson <shae(a)ScannedInAvian.com> with your requests, comments, or feedback on the Wiki site - or, of course, improve and extend the site yourself! Thanks all for your contributions - please keep them coming, and make this resource even more useful. Cheers, --KW 8-) Former Haskell Wiki manager -- Keith Wansbrough <kw217(a)cl.cam.ac.uk> http://www.cl.cam.ac.uk/users/kw217/ University of Cambridge Computer Laboratory.

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[Q] Using Class
by Andre Betz 13 Feb '04

13 Feb '04

Hi, I'm a really beginner to haskell and I have a question about using classes. I don't now really what this function does. So I will try to use it to see what it will put out position :: ( Enum a , Eq b ) => b -> [b] -> [a] position x = position_snd . zip ( enumFrom $ toEnum 1 ) where position_snd [] = [] position_snd ((p,y):ys) | x == y = p : position_snd ys | otherwise = position_snd ys My roblem is now that I don't know how to use it. I tried "head position 2 [1,2,3,4]" but I always get an error. Thanks for help André

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RE: [Haskell-cafe] Template Haskell question?
by Simon Peyton-Jones 12 Feb '04

12 Feb '04

There is a lot of TH stuff in the HEAD that is not in the released GHC 6.2. TH is still under active development, including design changes, largely in response to feedback from people using it -- so keep us posted on template-haskell(a)haskell.org Simon | -----Original Message----- | From: haskell-cafe-bounces(a)haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of | Jeremy Shaw | Sent: 11 February 2004 16:44 | To: Per Larsson | Cc: haskell-cafe(a)haskell.org | Subject: Re: [Haskell-cafe] Template Haskell question? | | Hello, | | I had very similar problems due to bugs in 6.2 that are fixed in cvs head. If you build ghc out of cvs | head that may fix your problem. If not, you can report the bug on the glasgow-haskell- | bugs(a)haskell.org mailing list. | | Jeremy Shaw. | | | On Feb 11, 2004 01:39 AM, Per Larsson <per(a)L4i.se> wrote: | | > I have written a small TH application (module THRecord) which creates update | > functions for records. It is intended to be used like this: | > | > import THRecord | > | > data Record = R {... | > ... | > $(THRecord.generateRecordModifiers (reifyDecl Record)) | > | > It works great, but only if the splice is placed last in the importing module! | > If I place it in the middle of the module it seems to hide subsequent | > declarations and I get 'Variable not in scope ...' compiler errors. If I | > place it at the top of the module I get: | > | > ghc-6.2: panic! (the `impossible' happened, GHC version 6.2): | > nameModule Record {- tc 01D -} | > | > Has anybody a clue of what is happening here? | > | > -- Per | > | > | > _______________________________________________ | > Haskell-Cafe mailing list | > Haskell-Cafe(a)haskell.org | > http://www.haskell.org/mailman/listinfo/haskell-cafe | | | | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe(a)haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe

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Template Haskell question?
by Per Larsson 11 Feb '04

11 Feb '04

I have written a small TH application (module THRecord) which creates update functions for records. It is intended to be used like this: import THRecord data Record = R {... ... $(THRecord.generateRecordModifiers (reifyDecl Record)) It works great, but only if the splice is placed last in the importing module! If I place it in the middle of the module it seems to hide subsequent declarations and I get 'Variable not in scope ...' compiler errors. If I place it at the top of the module I get: ghc-6.2: panic! (the `impossible' happened, GHC version 6.2): nameModule Record {- tc 01D -} Has anybody a clue of what is happening here? -- Per

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Haskell newbie
by Justin Walsh 08 Feb '04

08 Feb '04

Can anyone recommend a very thin Linux/Haskell setup for DHCP cable? Jus. --- Outgoing mail is certified Virus Free. Checked by AVG anti-virus system (http://www.grisoft.com) Version: 6.0.576 / Virus Database: 365 - Release Date: 1/30/04

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Haskell binaries for AIX?
by Bruno Melo 07 Feb '04

07 Feb '04

Maybe, this link can help you: http://www.cin.ufpe.br/~haskell/ghc-AIX/ ## bruno melo ## linux user #334122 www.rootshell.be/~brunoj

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Path names (again?)
by Vincenzo aka Nick Name 06 Feb '04

06 Feb '04

Hi all, I am so often in need for this (I suppose, non-existing) module, that I am going crazy :) I have found discussions in the archives for haskell mailing lists, but is there any, even posix-only, even unix-only, but better portable, implementation of functions to - concatenate file paths - tell if a path is absolute or relative - chase symlinks - find all the files in a directory (yes, that's what I need :)) I am tired of finding bugs in my implementation, is there someone with a better thing to offer? Is there any hope to see such functions in GHC libraries, or are those already there and it's just me who can't find'em? Thanks in advance Vincenzo

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Sat solver
by Ron de Bruijn 05 Feb '04

05 Feb '04

Hi there, I need a complete 3-CNF-Sat solver that can solve sentences of about length 20 (or shorter). Now I use simple model checking, but that's a bit slow , you understand :) I have seen some algorithms on the web and some code-sniplets in papers. But I presume there is some implementation available, so I thought: Let's ask, and don't reinvent the wheel. Greets Ron P.S. Thanks to those who answered my previous question, although I have found another way of expressing my problem. __________________________________ Do you Yahoo!? Yahoo! Finance: Get your refund fast by filing online. http://taxes.yahoo.com/filing.html

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Syntactic extensions
by Niklas Broberg 05 Feb '04

05 Feb '04

Hello, I am using the haskell-src bundle to implement a preprocessor for some syntactic extensions to haskell. This is all fine and well, but I find myself also wanting the preprocessor to let through some already existing extensions, namely: * implicit parameters * pattern guards * multi-parameter type classes with functional dependencies * possibly template haskell The haskell-src bundle already has support for mptcs (without fundeps), and pattern guards are rather straight-forward, but for the rest; Where can I find formal specifications of these extensions in terms of added/changed syntax? Or even better, is there anyone out there who has already extended the haskell-src bundle with support for any of them? regards, Niklas Broberg (d00nibro[at]dtek.chalmers.se) _________________________________________________________________ Add photos to your e-mail with MSN 8. Get 2 months FREE*. http://join.msn.com/?page=features/featuredemail

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Haskell overview
by Mark Carroll 04 Feb '04

04 Feb '04

A colleague with a mathematics and Lisp background is wanting to learn more about Haskell. The books he's looked at concentrate more on building up from the basics and getting the syntax right, etc., whereas really he's looking more of a top-down view that makes Haskell's features and behavior clear and relates them to category theory, etc. Would anyone be able to suggest some good references? -- Mark

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Type Class Query
by Tim Stitt 03 Feb '04

03 Feb '04

I'm experimenting with type classes in Haskell and tried to implement a new class based on nullifying values of various types e.g. nullify 6 = 0 nullify 7.6 = 0 nullify [1,2,3] = [] nullify Tree = Empty etc. I set up my class as follows: class Null a where nullify :: a -> a instance Null Int where nullify x = 0 but get the following error when I invoke nullify 6 on the Hugs command line: ERROR - Unresolved overloading *** Type : (Num a, Null a) => a *** Expression : nullify 6 I then tried the following: instance Num a => Null a where nullify x = 0 but got the following error in Hugs ERROR "C:\zero.hs":5 - Syntax error in instance head (constructor expected) Can anybody explain what I'm doing wrong ? Thanks in advance, Tim.

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Storing functional values
by nickgrey＠softhome.net 03 Feb '04

03 Feb '04

Hi, I'm writing a game in Haskell. The game state includes a lot of closures. For example, if a game object wants to trigger an event at a particular time, it adds a function (WorldState -> WorldState) to a queue. Similarly there are queues which contain lists of functions which respond to events. (CreatureAttackEvent -> WorldState -> WorldState) I'd like to be able to save the game state to disk so that it can be reloaded. Obviously, these closures are now a problem, as they can't be stored. I could obviously, replace all the functional values with non-functional ones, and have a big case statement specifying the behaviour associated with each constant. However, I don't really like this solution. It means that for each type of function which was in the game state (there are lots) I will need a type which lists all the possible behaviours (again, there are lots) that could occur. First of all, these types are likely to be very large. Secondly, I'd rather the behaviours were defined in different modules. For example, I want to have an "Orc" module which defines functions such as "Response to orc being hit", "Response to orc picking something up", etc. This implementation would force me to have a "Monster reponse to being hit" MODULE, containing an orcs response, a goblins response etc. I.e., the modules would be grouped by type of behaviour, rather than type of monster, meaning that adding a new monster type would require modifications accross many modules. (This example is obviously something of a simplification, but hopefully it shows the point.) Obviously, none of this is fatal. I could make it work. But going through my program and replacing all the functions with values and then doing big pattern matches against the values to find the functions seems like a very non-functional way of doing things. Can anyone suggest a neater design?

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Puzzled by error with forall quantifier.
by Theodore S. Norvell 02 Feb '04

02 Feb '04

Can anyone explain the following error message from Hug98 (with extensions enabled)? I have > class Space t > class Space t => HasY v t > where > assignY :: v -> t -> t > getY :: t -> v > varY :: forall w . Space w => v -> (t->t) -> (w->w) I get an error on the last definition "quantifier does not mention type variable v" (and if I quantify v, then it complains that t is not quantified). The idea I'm trying to express is that for any instance of HasY v t there should be a function varY with type v -> (t->t) -> (w->w) for some type w of class Space. Any help much appreciated. Is there any kind of tutorial introduction to "forall" out there? Cheers, Theo Norvell

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Generalized list-comprehension
by Ron de Bruijn 01 Feb '04

01 Feb '04

Hi there, I have written this little function: f :: (Num a, Enum a) => a -> [[a]] f n = [[a]|a<-fu n] ++ [a:[b]|a<-fu n,b<-fu n] ++ [a:b:[c]|a<-fu n,b<-fu n,c<-fu n] fu n = [1..n] This is an example of the function in action: *Mod> f 4 [[1],[2],[3],[4],[1,1],[1,2],[1,3],[1,4],[2,1],[2,2],[2,3],[2,4],[3,1],[3,2],[3, 3],[3,4],[4,1],[4,2],[4,3],[4,4],[1,1,1],[1,1,2],[1,1,3],[1,1,4],[1,2,1],[1,2,2] ,[1,2,3],[1,2,4],[1,3,1],[1,3,2],[1,3,3],[1,3,4],[1,4,1],[1,4,2],[1,4,3],[1,4,4] ,[2,1,1],[2,1,2],[2,1,3],[2,1,4],[2,2,1],[2,2,2],[2,2,3],[2,2,4],[2,3,1],[2,3,2] ,[2,3,3],[2,3,4],[2,4,1],[2,4,2],[2,4,3],[2,4,4],[3,1,1],[3,1,2],[3,1,3],[3,1,4] ,[3,2,1],[3,2,2],[3,2,3],[3,2,4],[3,3,1],[3,3,2],[3,3,3],[3,3,4],[3,4,1],[3,4,2] ,[3,4,3],[3,4,4],[4,1,1],[4,1,2],[4,1,3],[4,1,4],[4,2,1],[4,2,2],[4,2,3],[4,2,4] ,[4,3,1],[4,3,2],[4,3,3],[4,3,4],[4,4,1],[4,4,2],[4,4,3],[4,4,4]] *Mod> This is ofcourse nice and all, but I need a function that does this same trick, but then doesn't only generate listelements of the maximum hard-coded length of three, but to length m, where m is an extra parameter. It's important that the elements are precisely in this order. I tried rewriting the listcomprehension to map, concat and filter, but that only complicated things. There is a possibility where I can write a function that gives the next of a list. So the next of [1,1,1] would be [1,1,2], but that would be somewhat inefficient when the list becomes large. Then I just call this function with (replicate n (head $ fu n)) and (last $ fu n). Then I just apply this function next to the rest, until I reach a state where all elements of the list equal the last value of the inputlist, so I have the required list of length n. Then to simply get the complete list as in function f. I need to map(\x->otherFunction x) [1..], but this "next"-function is almost identical to what the built-in listcomprehension does. I just don't like my solution. Then I can probably use Template Haskell(which I have never used), but that seems overkill. So does anyone has a better solution? Greets Ron __________________________________ Do you Yahoo!? Yahoo! SiteBuilder - Free web site building tool. Try it! http://webhosting.yahoo.com/ps/sb/

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