- Haskell-Cafe - Haskell.org

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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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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