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2-CNF Sat algorithm/haskell code
by Ron de Bruijn 17 Mar '04

17 Mar '04

Dear Haskellers, Today I searched over more than an hour on the web to find an implementation of an algorithm that was first written in the 1970's that solves 2-Conjuntive Normal Form logical sentences in polynomial time. The only thing I could find were some homework assignments for students of universities and I learned that there exist an algoritm for 2-CNF. Although I am a student, what I am doing is no homework. I believe the datastructure used was a strongly connected graph (perhaps this rings any bells). If you don't know any implementation in any imperative language or functional language (not to far off when compared to Haskell), a clear pseudo-code description, either written by yourself or a URL to a webpage/paper with that information would also be very welcome. Thanks in advance, Ron de Bruijn P.S. This is really offtopic, but I would like to hear some opinions about this project: http://www.supercompilers.com/ I don't know whether they are for real, but when this works it would greatly impacts the use of inefficient functional programs(when applied to it ofcourse). Like in Haskell, a lot of programs could be written like: [x|x<-someList, somePred x] Although they computationally seen would be very slow, without "supercompilation", with they would perform just as good as some smart way of doing it. __________________________________ Do you Yahoo!? Yahoo! Mail - More reliable, more storage, less spam http://mail.yahoo.com

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Building Haddock on Windows
by Bayley, Alistair 16 Mar '04

16 Mar '04

Has anyone recently built Haddock on Windows? I'm invoking configure and make from Cygwin bash. Configure takes a couple of hours, and then make produces a load of errors (see below). If anyone has a ready-made .exe I'd be happy to use that. I just want to use Haddock, not modify it. Alistair. ---- output from make: [All appears ok up to this point] make[1]: Leaving directory `/cygdrive/h/haskell/haddock-0.6/glafp-utils' make[1]: Entering directory `/cygdrive/h/haskell/haddock-0.6/haddock' ------------------------------------------------------------------------ ===fptools== Recursively making `boot' in src html doc ... PWD = /cygdrive/h/haskell/haddock-0.6/haddock ------------------------------------------------------------------------ ------------------------------------------------------------------------ ==fptools== make boot - --unix -wr; in /cygdrive/h/haskell/haddock-0.6/haddock/src ------------------------------------------------------------------------ test: 504: unknown operand M -optdep-f -optdep.depend -osuf o -H16m -O -package data -package text -package util -package net -fglasgow-exts -cpp Binary.hs BlockTable.hs Digraph.lhs FastMutInt.hs HaddockDB.hs HaddockHH.hs HaddockHtml.hs HaddockLex.hs HaddockModuleTree.hs HaddockParse.hs HaddockRename.hs HaddockTypes.hs HaddockUtil.hs HaddockVersion.hs HsLexer.lhs HsParseMonad.lhs HsParseUtils.lhs HsParser.hs HsSyn.lhs Html.hs Main.hs M: not found make[2]: [depend] Error 127 (ignored) ------------------------------------------------------------------------ ==fptools== make boot - --unix -wr; in /cygdrive/h/haskell/haddock-0.6/haddock/html ------------------------------------------------------------------------ ------------------------------------------------------------------------ ==fptools== make boot - --unix -wr; in /cygdrive/h/haskell/haddock-0.6/haddock/doc ------------------------------------------------------------------------ ------------------------------------------------------------------------ ===fptools== Finished making `boot' in src html doc ... PWD = /cygdrive/h/haskell/haddock-0.6/haddock ------------------------------------------------------------------------ make[1]: Leaving directory `/cygdrive/h/haskell/haddock-0.6/haddock' make[1]: Entering directory `/cygdrive/h/haskell/haddock-0.6/haddock' ------------------------------------------------------------------------ ===fptools== Recursively making `all' in src html doc ... PWD = /cygdrive/h/haskell/haddock-0.6/haddock ------------------------------------------------------------------------ ------------------------------------------------------------------------ ==fptools== make all - --unix -wr; in /cygdrive/h/haskell/haddock-0.6/haddock/src ------------------------------------------------------------------------ test: 504: unknown operand make INSTALLING=0 BIN_DIST=0 - --unix - --no-print-directory -r all test: 504: unknown operand H16m -O -package data -package text -package util -package net -fglasgow-exts -cpp -c Binary.hs -o Binary.o -ohi Binary.hi H16m: not found make[3]: [Binary.o] Error 127 (ignored) H16m -O -package data -package text -package util -package net -fglasgow-exts -cpp -c BlockTable.hs -o BlockTable.o -ohi BlockTable.hi H16m: not found make[3]: [BlockTable.o] Error 127 (ignored) H16m -O -package data -package text -package util -package net -fglasgow-exts -cpp -c Digraph.lhs -o Digraph.o -ohi Digraph.hi H16m: not found make[3]: [Digraph.o] Error 127 (ignored) [and it continues like this for a while...] ----------------------------------------- ***************************************************************** Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. *****************************************************************

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Haskell on Rotor CLI
by s_＠id.ru 16 Mar '04

16 Mar '04

Hi, All! Is there any project for running Haskell on the Rotor CLI? zelych --- Professional hosting for everyone - http://www.host.ru

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AW: [Haskell-cafe] Haskell overview
by Markus.Schnell＠infineon.com 12 Mar '04

12 Mar '04

and for some days now Markus Schnell > -----Ursprüngliche Nachricht----- > Von: haskell-cafe-bounces(a)haskell.org > [mailto:haskell-cafe-bounces@haskell.org] Im Auftrag von Gour > Gesendet: Freitag, 12. März 2004 14:53 > An: haskell-cafe(a)haskell.org > Betreff: Re: [Haskell-cafe] Haskell overview > > > MR K P SCHUPKE (k.schupke(a)imperial.ac.uk) wrote: > > > Is it just me or is "citeseer.nj.nec.com" down? > > Same here :-( > > Sincerely, > Gour > > -- > Gour > gour(a)mail.inet.hr > Registered Linux User #278493 > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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Re: Polymorphic lists...
by MR K P SCHUPKE 12 Mar '04

12 Mar '04

Just running through the implementation of some of the functions I mentioned, with the newtyped tuple version. Thought the run-time indexed code looked neat so I thought i'd post it: First the definition: newtype RNil = RNil () newtype RCons attribute relation = RCons (attribute,relation) class Relation relation instance Relation (RNil ()) instance Relation relation => Relation (RCons attribute relation) emptyRM :: RNil emptyRM = RNil () now the code: class Relation r => IndexRM' r a where indexRM' :: r -> Int -> Maybe a instance IndexRM' RNil a where indexRM' _ _ = Nothing instance IndexRM' r a => IndexRM' (RCons a r) a where indexRM' (RCons (a,_)) 0 = Just a indexRM' (RCons (_,r)) n = indexRM' r (n-1) instance IndexRM' r a => IndexRM' (RCons b r) a where indexRM' (RCons (_,_)) 0 = Nothing indexRM' (RCons (_,r)) n = indexRM' r (n-1) This finds the Nth value where 'n' is a normal integer. The last block repeats twice because one matches the case where the return type is the same, and the other is for then the return type and indexed value differ. This is why the return type is Maybe. With the compile time natural number lookup, the program will fail to compile if the types differ, but with the runtime lookup the type and the success cannot be determined (hence no functional dependancy) and returning Maybe makes the most sense. Regards, Keean.

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Re: [Haskell-cafe] Haskell overview
by MR K P SCHUPKE 12 Mar '04

12 Mar '04

Is it just me or is "citeseer.nj.nec.com" down? Keean.

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Re: [Haskell-cafe] Passing types as arguments
by MR K P SCHUPKE 12 Mar '04

12 Mar '04

You can pattern match on types - as I have been having much fun with recently ... you use multi-parameter type classes (with optional functional dependancies) to match types. Regards, Keean.

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RE: [Haskell-cafe] Passing types as arguments
by Simon Peyton-Jones 12 Mar '04

12 Mar '04

We pass values as proxies for types extensively in the "Scrap your boilerplate" scheme. The paper is on my home page. The 'typeOf' function is a good example. Part II coming up shortly. Simon | -----Original Message----- | From: haskell-cafe-bounces(a)haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of | ajb(a)spamcop.net | Sent: 12 March 2004 03:50 | To: haskell-cafe(a)haskell.org | Subject: [Haskell-cafe] Passing types as arguments | | G'day everyone. | | Some time ago a suggestion came up about passing types as arguments | to functions: | | http://www.haskell.org/pipermail/haskell/2003-June/012184.html | | As a matter of curiosity, the topic has come up again on the wiki: | | http://haskell.org/hawiki/StudyGroup/GraphExamplesInHaskell/WhySum3 | | For this example, we don't just want to pass types, we also want to | pattern match on them. | | I'm curious if anyone else has put any thought to this idea. Is it just | a syntax issue, or are there bad interactions with other parts of the | type system? | | Cheers, | Andrew Bromage | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe(a)haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe

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Passing types as arguments
by ajb＠spamcop.net 12 Mar '04

12 Mar '04

G'day everyone. Some time ago a suggestion came up about passing types as arguments to functions: http://www.haskell.org/pipermail/haskell/2003-June/012184.html As a matter of curiosity, the topic has come up again on the wiki: http://haskell.org/hawiki/StudyGroup/GraphExamplesInHaskell/WhySum3 For this example, we don't just want to pass types, we also want to pattern match on them. I'm curious if anyone else has put any thought to this idea. Is it just a syntax issue, or are there bad interactions with other parts of the type system? Cheers, Andrew Bromage

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Haskell overview
by ATWOOD,JOHN (HP-Corvallis,ex1) 11 Mar '04

11 Mar '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 For lispers, I like to point out this sentence of section 6.4 of the Haskell 98 Report: Haskell provides several kinds of numbers; the numeric types and the operations upon them have been heavily influenced by Common Lisp and Scheme. For mathematicians, I'd recommend the first 5 pages of: Calculating Functional Programs, Jeremy Gibbons, 2002 http://web.comlab.ox.ac.uk/oucl/work/jeremy.gibbons/publications/index.html# calculating Then I'd mention the Acid Rain Theorem, Fusion Theorem, Wadler's _Theorems for Free!_. When I'd show some exemplary fp programs: Bridging the Algorithm Gap: A Linear-Time Functional Program for Paragraph Formatting. Oege de Moor and Jeremy Gibbons, 1999 http://web.comlab.ox.ac.uk/oucl/work/jeremy.gibbons/publications/index.html# para Financial Contracts: An Adventure in Financial Engineering, 2000 Simon Peyton Jones, Jean-Marc Eber, and Julian Seward http://www.lexifi.com/resources.html#paper Functional Specification of JPEG Decompression, and an Implementation for Free, Jeroen Fokker, 1995 http://citeseer.nj.nec.com/72092.html Modeling Web Interactions, Paul Graunke, Robert Bruce Findler, Shriram Krishnamurthi, Matthias Felleisen, 2003 http://citeseer.nj.nec.com/graunke03modeling.html Haskell Server Pages Functional Programming and the Battle for the Middle Tier, Erik Meijer, Dannyy van Velzen, 2000 http://citeseer.nj.nec.com/318262.html Writing High-Performance Server Applications in Haskell, Case Study: A Haskell Web Server, Simon Marlow, 2000 http://www.haskell.org/~simonmar/bib.html (about the 4th one down) And then, the more introductory material: Why Functional Programming Matters, John Hughes http://www.cs.chalmers.se/~rjmh/Papers/whyfp.html and here's a summary of the paper: http://www.cs.auckland.ac.nz/~j-hamer/360/why-fp-matters.html Section 3.1.3 of SICP: http://mitpress.mit.edu/sicp/ Why no one uses functional languages, Philip Wadler, 1998 http://citeseer.nj.nec.com/wadler98why.html here's an old, but still relevant summary of some issues keeping fp from taking over the world: http://www.cs.colorado.edu/~zorn/cs5535/Fall-1997/haskell-summary.html Some general links: http://haskell.readscheme.org/ http://www.haskell.org Hope this isn't too overwhelming, but it was an opportunity for me to gather my thoughts. Enjoy, John Atwood

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Re: Polymorphic lists...
by oleg＠pobox.com 11 Mar '04

11 Mar '04

[Moving to Haskell-cafe from http://www.haskell.org/pipermail/glasgow-haskell-users/2004-March/006390.ht… http://www.haskell.org/pipermail/glasgow-haskell-users/2004-March/006393.ht… I hope y'all don't mind the move: the discussion of polymorphic lists is of general interest and is not limited to GHC. Besides, they serve coffee in here, albeit virtually. However, they charge for it virtually, too.] Keean Schupke wrote: > The list indexed by integers cannot determine the type of the > return value through induction on the class... in other words > it cannot determine the return type of the lookup function > until runtime: you can see this in the class instance for 'tke' > > > instance (TH a (a,b), TH W b) => TH W (a,b) where > > tke (W 0) th@(h,t) f = f h th > > tke (W n) (h,t) f = tke (W$ n-1) t f > > On the other hand indexing by natural numbers allows the compiler > to know the return type (and avoid the use of existentials) > because it is determined at compile time... you can see this because > the recursion termination is done by the type signatures in the instance > not the pattern guards of the function. > > instance Relation r => RIndex Zero (a `RCons` r) a where > rIndex Zero (x `RCons` _) = x > instance RIndex Idx r b => RIndex Idx (a `RCons` r) b where > rIndex (Suc n) (_ `RCons` xs) = rIndex n xs I concur wholeheartedly. I have expressed the same sentiment: http://www.haskell.org/pipermail/haskell/2003-August/012493.html http://www.haskell.org/pipermail/haskell/2003-June/011939.html The reason the previous message implemented indexing by true Int is because Ralf Laemmel specifically asked for it (plus it is much harder to do). Keean Schupke wrote: > It looks like most of this stuff has been done before... but I don't think > there is any of it in the ghc libraries. I needed this code for a real > application, and could not find anything suitable so I rolled my own. > > What do people think - is there a case for getting this stuff in the libs, > should we write a functional pearl? does anyone have any comments about > the code I posted, or how it could be improved? It is indeed a very good question. Just for the sake of it, here's a little bit simplified version of your example. Labels are done differently. > {-# OPTIONS -fglasgow-exts -fallow-undecidable-instances -fallow-overlapping-instances #-} > module Foo where > data Z = Z > data S a = S a > > class Nat a where n2n:: a-> Int > instance Nat Z where n2n _ = 0 > instance (Nat a) => Nat (S a) where n2n _ = 1 + n2n (undefined::a) > > -- keyed access > class (Nat n) => MLookup n a r where > mLookup :: r -> n -> a > instance MLookup Z a (a,r) where > mLookup r _ = fst r > instance MLookup n a r => MLookup (S n) a (a,r) where > mLookup (_,xs) _ = mLookup xs (undefined::n) > instance MLookup n a r => MLookup n a (b,r) where > mLookup (_,xs) n = mLookup xs n > > -- Positional access > class (Nat n) => MIndex n a r | n r -> a where > midx:: r -> n -> a > instance MIndex Z a (a,r) where midx (x,_) _ = x > instance (MIndex n a r) => MIndex (S n) a (b,r) where > midx (_,xs) _ = midx xs (undefined::n) The example now reads > newtype Name a = Name a deriving Show > newtype Size a = Size a deriving Show > newtype Weight a = Weight a deriving Show > > infixr 5 &+ > (&+) = (,) > test = (Name "Box") &+ (Size (3::Int)) &+ (Weight (1.1::Float)) > &+ (Name "AnotherBox") &+ (Size (42::Int)) &+ (Weight (24.09::Float)) > &+ () Note that at run-time, (Name "Box") is the same as "Box". Name is a compile-time-only label that incurs no run-time overhead. So, the essence is the same -- a polymorphic associative list where keys are ephemeral (have no run-time representation). "mLookup r n" finds the n-th association in this list of a type a. In the present representation, labels and values are specified together. There is no need for a type declaration for test. The compiler will figure it out. The name and the size of the first box *Foo> mLookup test Z::(Name String) Name "Box" *Foo> mLookup test Z::(Size Int) Size 3 and of the second one *Foo> mLookup test (S Z)::(Name String) Name "AnotherBox" *Foo> mLookup test (S Z)::(Size Int) Size 42 We can also access the element of the array by their absolute position: *Foo> midx test Z Name "Box" *Foo> midx test (S Z) Size 3 *Foo> midx test (S (S (S Z))) Name "AnotherBox" Decimal types for indices (rather than unary, as above) would make for a nicer interface. We can easily write a right fold > class MFoldr a r where > mfoldr :: (a -> b -> b) -> b -> r -> b > instance (MFoldr a r) => MFoldr a (a,r) where > mfoldr f z r = f (fst r) $ mfoldr f z (snd r) > instance MFoldr a () where > mfoldr f z r = z > instance MFoldr a r => MFoldr a (b,r) where > mfoldr f z r = mfoldr f z (snd r) and find out how many names are in our list *Foo> mfoldr (\ (Name (a::String)) n -> n + 1) 0 test 2 and the total sizes of our boxes *Foo> mfoldr (\ (Size s) a -> a + s) (0::Int) test 45 Incidentally, this polymorphic array may be a model of an extensible record with row polymorphism and even polymorphic labels. Yet another record proposal.

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matching constructors
by Vadim Zaliva 08 Mar '04

08 Mar '04

Hi! I am new to Haskell, but I have background in various programming languages (including Lisp) I have very basic question, if there is a way to match algebraic types constructors besides use of pattern matching. I wrote today code like this: data Flag = Verbose | Input String | Output String | Filter String deriving Show instance Eq Flag where Verbose == Verbose = True Input _ == Input _ = True Output _ == Output _ = True Filter _ == Filter _ = True _ == _ = False findFlag :: Flag -> [Flag] -> Flag findFlag f [] = f findFlag y (x:xs) = if x == y then x else findFlag y xs If there is a cleaner way to implement findFlag and Eq Flag? Vadim -- "La perfection est atteinte non quand il ne reste rien a ajouter, mais quand il ne reste rien a enlever." (Antoine de Saint-Exupery)

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HaRe import problem
by Georg Martius 08 Mar '04

08 Mar '04

Hi, I have a problem with Haskell Refactoring. I use the last snapshot from 26/02/2004 and emacs. If I have a source file with an import of a libary module e.g. Text.XML.HaXml.OneOfN I get: Source files missing for (add files with 'pfe add' or 'pfe chase'): Text.XML.HaXml.OneOfN, needed by Test Cheers, Georg

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Haskell BDD library?
by Hans-Wolfgang Loidl 07 Mar '04

07 Mar '04

Hi, We are just starting on a project of parallelising a symbolic model checking algorithm, which operates on BDDs (binary-decision diagrams), in GpH. Does anyone have a Haskell implementation of such a library around and is willing to share it? Also, pointers to possibly useful BDD libraries, either in Haskell or lesser languages, would be welcome. Cheers, -- Hans-Wolfgang Loidl Ludwig-Maximilians-University Munich Tel: +49 89 / 2180 9864 Institut f. Informatik Fax: +49 89 / 2180 9338 Oettingenstr 67 Email: hwloidl(a)informatik.uni-muenchen.de D 80538 Munich URL: http://www.tcs.informatik.uni-muenchen.de/~hwloidl/

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Fun with Haskell, runST, MArray, and a few queens.
by David Sankel 05 Mar '04

05 Mar '04

Hello Enthusiasts, My fiancee was assigned the n-queens problem in her Data Structures class. It was a study in backtracking. For those unfamiliar with the problem: one is given a grid of n x n. Return a grid with n queens on it where no queen can be attacked by another. Anyway, I decided to try an implementation in Haskell (as I often do with her assignments). Instead of the imperative approach (adding a queen and then getting rid of it), I opted for a functional one (the grid is passed to recursive calls, etc.). The interesting thing about this assignment is the runtimes: (n=10) ghc 58.749s ghc -O 12.580s javac 1.088s The Haskell version takes significantly longer (and it gets worse for larger inputs). So it seems that imperative algorithms are much better for certain problems. Since Haskell is supposed to have the ability to run imperative algorithms, I was wondering if any of you could explain how runST and MArray could be used to solve this problem (or is there a better way?). I am also interested in the run times you get with these two implementations of the n-queens problem. David module Main where import Array import Maybe boardSizeToTest = 10 main = print $ fromJust $ solution (emptyBoard boardSizeToTest) boardSizeToTest --The Board datatype. A n x n array indexed starting with 1. data Board = Board Int (Array (Int,Int) Int) deriving Eq instance Show Board where show (Board n b) = concat [ (printLine b a) ++ "\n" | a <- [n,n-1..1] ] where printLine board row = concat [ (str (board ! (row,a))) | a <- [1..n] ] str (-1) = "q" --It's a queen str 0 = "." --It's an empty spot str a = show a --It's a spot that can be attacked by a queen --Helper function from Gentle Introduction to Haskell mkArray :: (Ix a) => (a -> b) -> (a,a) -> Array a b mkArray f bnds = array bnds [(i, f i) | i <- range bnds] --Another helper function to update arrays (/-) :: (Ix a) => Array a b -> [(a, (b -> b))] -> Array a b (/-) array s = array // (map (\ (a, f) -> (a,f (array!a))) s) --An empty chessboard emptyBoard :: Int -> Board emptyBoard n = Board n $ mkArray (\_->0) ((1,1),(n,n)) --Adds a queen to the board and adds 1 in all the positions the queen --could feasible move. addQueen :: Board -> (Int,Int) -> Board addQueen b@(Board n board) c = Board n $ newBoard // [(c, -1)] where Board _ newBoard = queenHelper b (+1) c --Removes a queen from the board and subtracts 1 in all the positions the queen --could have feasible moved. removeQueen :: Board -> (Int,Int) -> Board removeQueen b@(Board n board) c = Board n $ newBoard // [(c, 0)] where Board _ newBoard = queenHelper b ((-)1) c queenHelper :: Board -> (Int->Int) -> (Int, Int) -> Board queenHelper (Board n board) f (row,column) = Board n $ board /- horizontal /- vertical /- negPosDiag /- posNegDiag /- negNegDiag /- posPosDiag where vertical = [ ((r,column),f) | r <- [1..n] ] horizontal = [ ((row,c),f) | c <- [1..n] ] negPosDiag = [ ((row-i,column+i),f)| i <- [1..(min (n-column) (row-1))]] posNegDiag = [ ((row+i,column-i),f)| i <- [1..(min (column-1) (n-row))]] negNegDiag = [ ((row-i,column-i),f)| i <- [1..(min (column-1) (row-1))]] posPosDiag = [ ((row+i,column+i),f)| i <- [1..(min (n-column) (n-row))]] --Returns all the positions on the board that do not have a queen and cannot --be attacked by a queen already on the board. possiblePositions (Board n board) = filter (\a -> board ! a == 0) [ (row,column) | row <- [1..n], column <- [1..n] ] --Finds a solution given a board and a number of queens to put on it solution :: Board -> Int -> Maybe Board solution board 0 = Just board solution board i = let solutions = catMaybes $ [ solution (addQueen board c) (i-1) | c <- (possiblePositions board) ] in if solutions == [] then Nothing else Just $ head solutions

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[Haskell-cafe] Re: Fun with Haskell, runST, MArray, and a few queens. (Imperative Haskell Version)
by David Sankel 05 Mar '04

05 Mar '04

> I say this is a case of bad code. Of course language <foo> is faster and > better if you write horribly bad code in language <bar>. > Taking the first solution found by searching with google I get times > around 0.015s (real) for the Haskell version and 1.7s for your Java > solution (which also seems to be overcomplicated to me). That's cool. Although the java algorithm is beyond any doubt overcomplicated, I'd like to reproduce it using Haskell's imperative constructs because not all backtracking algorithms have a simple closed form solution. What I've done using monads is a little better in speed, but not the huge improvement I was expecting. Any comments as to what I'm doing wrong here? Updated Table: ghc�� 58.749s ghc -O�� 12.580s ghc -O (monad version) 8.284s javac�� 1.088s David module Main where import Control.Monad.ST import GHC.Arr import Maybe import Ix main = print $ runST something n = 10 -- Used only for output data Board = Board Int (Array (Int,Int) Int) deriving Eq instance Show Board where show (Board n b) = concat [ (printLine b a) ++ "\n" | a <- [n,n-1..1] ] where printLine board row = concat [ (str (board ! (row,a))) | a <- [1..n] ] str (-1) = "q" --It's a queen str 0 = "." --It's an empty spot str a = show a --It's a spot that can be attacked by a queen --Finds a solution given a board and a number of queens to put on it solution board 0 = return True solution board i = do possibleP <- (possiblePositions board) trySolution possibleP where trySolution [] = return False trySolution (c:cs) = do addQueen board c good <- solution board (i-1) if good then return True else do removeQueen board c trySolution cs --Returns all the positions on the board that do not have a queen and cannot --be attacked by a queen already on the board. -- possiblePositions board = -- do -- p <- sequence [ readSTArray board (row,column) >>= -- (\a -> return (a == 0,(row,column))) -- | row <- [1..n], column <- [1..n] ] -- return $ map (\(_,b)->b) $ filter (\(a,_)-> a) p possiblePositions board = do -- let indices = range $! boundsSTArray board let indices = [ (row,column) | row <- [1..n], column <- [1..n] ] p <- sequence $ map (\i -> readSTArray board i >>= (\a -> if a == 0 then return $ Just i else return Nothing )) indices -- return $ map (\(_,b)->b) $ filter fst p return $ catMaybes p arrayToBoard board = do it <- sequence [ readSTArray board (a,b) >>= (\s -> return ((a,b),s)) | a <- [1..n], b <- [1..n] ] return (Board n (array ((1,1),(n,n)) it)) emptyBoard = newSTArray ((1,1),(n,n)) 0 something = do board <- emptyBoard solution board n arrayToBoard board --Adds a queen to the board and adds 1 in all the positions the queen --could feasible move. addQueen b c = do queenHelper b (+1) c writeSTArray b c (-1) --Removes a queen from the board and subtracts 1 in all the positions the queen --could have feasibly moved. removeQueen b c = do queenHelper b (\a -> a - 1) c writeSTArray b c 0 --Helper function to update arrays (/-) array s = do sequence_ [ readSTArray array b >>= \n -> writeSTArray array b $! (c n) | (b,c) <- s ] return array queenHelper board f (row,column) = do board /- (horizontal ++ vertical ++ negPosDiag ++ posNegDiag ++ negNegDiag ++ posPosDiag) where vertical = [ ((r,column),f) | r <- [1..n] ] horizontal = [ ((row,c),f) | c <- [1..n] ] negPosDiag = [ ((row-i,column+i),f)| i <- [1..(min (n-column) (row-1))]] posNegDiag = [ ((row+i,column-i),f)| i <- [1..(min (column-1) (n-row))]] negNegDiag = [ ((row-i,column-i),f)| i <- [1..(min (column-1) (row-1))]] posPosDiag = [ ((row+i,column+i),f)| i <- [1..(min (n-column) (n-row))]]

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Instance Problem
by 98222＠oa.cmbchina.com 05 Mar '04

05 Mar '04

How can I do this: class M a where tok::a->String instance M String where tok s="String" instance Num a=>M a where tok n="Number" instance (for all other types a)=>M a where tok x="Other"

1 0

State Monad
by Georg Martius 04 Mar '04

04 Mar '04

Hi folks, I have a question to the State Monad or Monads in general - I'am not sure. Lets assume I have functions to transform strings. I can use the State Monad as follows: strTrans :: String -> String strTrans s = s ++ s f :: State String () f = do put "hallo" modify strTrans modify strTrans' ... Up to this point everything is clear. Now I have also functions to map from (a, String) -> (a,String). I could write: modifyT :: ((a, String) -> (a, String)) -> a -> State String a modifyT trans a = do str <- get let (a', str') = trans (a, str) put str' return a' f :: State String () f = do put "hallo" modify strTrans i <- modifyT strIntTrans 4 -- strIntTrans :: (Int, String) -> (Int, String) i' <- modifyT strIntTrans i ... But this is obviously awkward. How can I stick two Monads in each other? I could't figure out how to use StateT. Any help would be appreciated. Thanks! Georg

4 4

a rant from a stoned suicidal person
by Chris Moline 04 Mar '04

04 Mar '04

i want to start off by apologizing. im lonely. i dont know anyone. anyone at all who is liek me. im different. everyone else is content to just watch tv all the time and then on weekends go and get drunk. im not. i am not threatening anything here when i say waht i am about to say. i know that waht i am about to say is not appropriate for haskell-cafe. i know this. but this needs to be said. im going to kill myself not right now. no you dont need to do anything. this is a plea. im begging you. ok see here im on my knees. my not-stoned self is going to hate me for this. but he needs the help and i know hes going to forgive me. because im him:) isnt that funny? i have no friends. none at all. im surrounded by motherfucking idiots *IDIOTS* i dont hate them. ok, im lying i hate them. i hate you. i hate this mothierfucking list. all i want is a goddamned friend who is *like me* i like thinking about shit. i like philosophy and math and all that stuff. dont suggest going to college. im sorry but ive found this out the hard way. im a loser. im a motherfucking loser. im too lazy. i have difficulty speling nad pronoundni big words. i do drugs. i know everyone says drugs are bad, but i like them. deal with it however you wnat. sned me to jail if you feel the need to. because i dont care anymroe do you hera that? I DONT FUCKING GIvE A ShIT ANYMORE im going to kill myself. i must. do you why i must? because im going insane. im surrounded by people but im completey alone. I HAVE NO IONE TO TAlK TO I AM LOSER I MUST DIE because im a loser so i am sendign this mesage to this list even though none of you care. none of you wnat ot deal with my problems. thats fine. i respect that. but i have noone and i cant find anyone and so i am begging PLEASE PUT ME OUT OF MY FUCKI*NG MISERY SHOOT ME * chris is crying * so anyways i am sorry for bothering you with this incoherent message but hers your mesage for the day FUCK YOU!

2 1

Re: [Haskell-cafe] State Monad
by Wolfgang Jeltsch 03 Mar '04

03 Mar '04

Am Mittwoch, 3. März 2004 18:15 schrieb Georg Martius: > Thanks for your answer. I got it now. > It works with lift instead of liftM. Yes, of course. > Georg Wolfgang

1 0

Trouble with [in,string]char* with hdirect
by Vincenzo aka Nick Name 03 Mar '04

03 Mar '04

Hi all, I am experimenting with hdirect for the first time, and I can't figure out how to call an haskell function of type String -> IO Int from C. I already can compile and use a function of type Char -> IO Int, so I assume to be using the right command line arguments. I have, in Math.idl ---- module Math { int munlink([in,string]char *s); }; ---- in Math.hs ---- module Math where munlink :: String -> IO Int munlink s = putStr ("Hey!" ++ s ++ "\n") >> return (-1) ---- This is the generated .hs file: module MathProxy where import Prelude (fromEnum, toEnum) import qualified Prelude import qualified Foreign.Ptr (Ptr) import qualified HDirect (unmarshallString) import qualified Math (munlink) munlink_proxy :: Prelude.String -> Prelude.IO Prelude.Int munlink_proxy = wrap_munlink Math.munlink foreign export ccall "munlink" munlink_proxy :: Foreign.Ptr.Ptr Prelude.String -> Prelude.IO Prelude.Int wrap_munlink :: (Prelude.String -> Prelude.IO Prelude.Int) -> Foreign.Ptr.Ptr Prelude.String -> Prelude.IO Prelude.Int wrap_munlink munlink_meth s = do s <- HDirect.unmarshallString s munlink_meth s I get a type error when compiling MathProxy.hs, and the strange thing is that commenting out the type declaration for "munlink_proxy" makes my program work like a charm. Errors from the compiler follow, any help will be highly appreciated. Vincenzo Ciancia ghc -O2 -fglasgow-exts Math.hs -c ihc -fshow-idl-in-comments -s -fhs-to-c --gen-headers -fuse-ints-everywhere -c Math.idl ghc -O2 -c main.c -o main.o ghc -O2 -fglasgow-exts -package hdirect MathProxy.hs -c MathProxy.hs:15: Couldn't match `GHC.Ptr.Ptr GHC.Base.String' against `GHC.Base.String' Expected type: GHC.Base.String -> GHC.IOBase.IO GHC.Base.Int Inferred type: GHC.Ptr.Ptr GHC.Base.String -> GHC.IOBase.IO GHC.Base.Int In the application `wrap_munlink Math.munlink' In the definition of `munlink_proxy': munlink_proxy = wrap_munlink Math.munlink MathProxy.hs:17: Couldn't match `GHC.Base.String' against `GHC.Ptr.Ptr GHC.Base.String' Expected type: GHC.Ptr.Ptr GHC.Base.String -> GHC.IOBase.IO GHC.Base.Int Inferred type: GHC.Base.String -> GHC.IOBase.IO GHC.Base.Int When checking declaration: foreign export ccall "munlink" munlink_proxy :: GHC.Ptr.Ptr GHC.Base.String -> GHC.IOBase.IO GHC.Base.Int

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[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

1 0

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

3 2

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

3 5

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

2 1

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

1 0

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 >

1 0

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]

1 0

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

1 0

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

3 2

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

1 0

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

1 1

[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 * |||||||||=========| * ********************************************************** **

1 0

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

2 1

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

2 1

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

1 0

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.

1 0

[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

2 1

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

1 0

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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