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Type Class Query
by Tim Stitt 04 Feb '04

04 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 03 Feb '04

03 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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Producing fortran/C code with haskell?
by Vincenzo aka Nick Name 31 Jan '04

31 Jan '04

I seem to recall a discussion, don't know if it was here or on comp.lang.functional, where somebody said he uses haskell to generate fortran code. That fascinated me a lot, because that would mean being able to generate a program already specialized for a specific input, by first reading input in haskell and then producing code (fortran, but could be C either) - and because I guess it can add static safety exploiting haskell types. Since we already have that nice syntax for monads those programs should be readable, too. Where could I find information on such topics, or existing libraries to generate programs with haskell? Is somebody willing to share what (s)he already did? V.

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Favorite/recommended math books
by Greg Buchholz 29 Jan '04

29 Jan '04

I was wondering if the Haskellers out there have any favorite math texts they recommend for someone hoping to improve their programming skills by learning more math. I'm thinking about something at the undergraduate level that you enjoyed reading. What about Mr. Haskell Curry's _Foundations of Mathematical Logic_? Greg Buchholz

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Re: [Haskell-cafe] Interpreting fields in a buffer
by Tomasz Zielonka 28 Jan '04

28 Jan '04

On Mon, Jan 26, 2004 at 02:39:32PM -0800, John Meacham wrote: > On Mon, Jan 26, 2004 at 01:24:37PM +0100, Tomasz Zielonka wrote: > > 3) Roll your own (de)serialization framework > > > > That's what I did. It's a bit complicated, but I will try to describe it > > within a couple of days. Right now all I can say is that it uses TH and > > has a couple of implementations of low-lever decoders, one of which > > reads directly from UArray Int Word8. I managed to achieve throughput of > > 3 MB / s for quite complicated binary protocols, and I think I can > > improve that even further. > > Hi, I would be very interesting in looking at the design of this. Is it > available on the web somewhere? I've put the code in attachment. I compiled in GHC 6.0.1 (this may matter because of Template Haskell. I also used other GHC extensions). There is an Example.hs, which shows how to declare a record for IP header and how to use both supplied parsers. This is not a standalone program, so to try it, compile it with ghc -O2 --make Example and load it in GHCi (you have to compile it first, because the interpreter can't handle unboxed tuples). The library is really very simple, I created it quickly to solve some immediate problems. There is no support for bitfields - I think there would be if I had to deal with them ;) The original library used little endian encoding. I changed to big endian aka network order, but there really should be a possibility to choose. Right now I am not sure how this choice should be available to the user. The ability to easily switch from Parsec to an efficient UArray Parser and the benefit from specializing 'times' came as nice surprises. Well, not exactly unexpected surprises, because I was striving for it a bit, but it was easier than I thought. Template Haskell is used to automatically derive instances for record types. In my application I had to create some instances by hand, because some regions of binary files where not self-describing - they needed additional information from the outside. There is also an encoding part - if someone's interested, I can extract it from the application. The funny thing is that I was only supposed to produce files, not parse them. But I started by doing a parser and thanks to a declarative approach, when I finished the parser, the unparser was ready almost instantly. One way to introduce bitfields and varying endianness would be to design some description language like that in Erlang (some algebraic datatype should suffice). Then we could generate datatypes and class instances from it using TH. Something like that (just a sketch): ipHeaderLayout = Record "IPHeader" [ BitField "Word8" BE [ (4, Just ("iphHeaderLen", "!Word8")) , (4, Just ("iphVersion", "!Word8")) ] , Field "iphTOS" (Unsigned 1 BE) "!Word8" , Field "iphTotalLen" (Unsigned 2 BE) "!Word16" , Field "iphID" (Unsigned 2 BE) "!Word16" , Field "iphFragOff" (Unsigned 2 BE) "!Word16" , Field "iphTTL" (Unsigned 1 BE) "!Word8" , Field "iphProtocol" (Unsigned 1 BE) "!Word8" , Field "iphCheck" (Unsigned 2 BE) "!Word16" , Field "iphSAddr" (Unsigned 4 BE) "!Word32" , Field "iphDAddr" (Unsigned 4 BE) "!Word32" ] $(createDataType ipHeaderLayout) $(createDecodableClassInstance ipHeaderLayout) $(createEncodableClassInstance ipHeaderLayout) Best regards, Tom -- .signature: Too many levels of symbolic links

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Interpreting fields in a buffer
by Dominic Steinitz 28 Jan '04

28 Jan '04

I looked at the Erlang syntax when I wrote my helper functions and I agree it is very nifty. I didn't have the time to investigate how to do it in Haskell but it would be disappointing if it (or something like it) couldn't be done. Dominic. > > Mikael, > > Thanks, that's very helpful and seems to be just the sort of > thing I'm looking for. > > Greg > > > On Jan 26, 2004, at 6:05 PM, Mikael Brockman wrote: > > > > You'll probably want to take a look at Erlang's so called ``bit > > syntax'' > > at http://www.erlang.se/euc/00/bit_syntax.html. It's very nifty, and > > I'd love to see it (or something equally convenient) as a Haskell > > extension. > > > > Hugs and kisses, > > Mikael Brockman > >

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Simple "Maybe" question
by Jim Lewis 27 Jan '04

27 Jan '04

I'm new to Haskell and can't find an example to solve a trivial problem. I have code like this: findBlank :: [] -> Int findBlank str = findIndex (==' ') str But interpreter complains elsewhere of mismatch of Int with Maybe Int. I want to handle the maybe only here and nowhere else. This just makes matters worse: findBlank str = Just findIndex (==' ') str Thanks. Powered by the E-mail PIM - Info Select - www.miclog.com

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elemt from list
by Pajo Patak 27 Jan '04

27 Jan '04

I want to return a list, from and list of lists (all integers), where is the erroe in the code? nthList :: [[a]] -> Int -> [a] nthList ([x]:xs) 1 = [x] nthList ([x]:xs) (n+1) = nthListh xs n _________________________________________________________________ Schützen Sie Ihren Posteingang vor unerwünschten E-Mails. http://www.msn.de/antispam/prevention/junkmailfilter Jetzt Hotmail-Junk-Filter aktivieren!

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