- Haskell-Cafe - Haskell.org

GHC 6.6.1: Where is Graphics.SOE ?
by Dmitri O.Kondratiev 18 Jul '07

18 Jul '07

I am trying to use Graphics.SOE (that was present at least in GHC 6.4) to go through "Simple Graphics" examples as described in Pail Hudak book "The Haskell School of Expression. Learning functional programming through multimedia". It looks like Graphics.SOE does not anymore exist in GHC 6.6.1. Where one can get it or what to use instead of it? Do I understand right that Graphics library in GHC 6.6.1 is split between OpenGL and GLUT modules? Any tutorials on OpenGL and GLUT modules similar to Paul Hudak "Simple Graphics"? Thanks! -- Dmitri O. Kondratiev dokondr(a)gmail.com http://www.geocities.com/dkondr

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"How to Help Mailing Lists Help Readers"
by Justin Bailey 18 Jul '07

18 Jul '07

Andy Oram over at O'Reilly just posted an article analyzing how mailing list readers are helped or hindered in the Perl and Ruby on Rails communities. His most interesting conclusion is that many posters come without the background needed to understand answers. That is certainly been the case on the cafe in many instances. I believe if he had analyzed the Haskell-Cafe list he would have found a much higher percentage of user questions are answered, and sometimes in incredible depth. His conclusion seems to be that, for newbies at least, books and articles that lead the reader gently into the new domain is the answer. As I've learned Haskell over the last year I've found that to definitely be the case. The article is at http://praxagora.com/andyo/professional/mailing_list_follow_up He'll also be speaking at OSCON next week and I look forward to hearing what he has to say. Justin

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RE: haskell for web
by Bayley, Alistair 18 Jul '07

18 Jul '07

> From: glasgow-haskell-users-bounces(a)haskell.org > [mailto:glasgow-haskell-users-bounces@haskell.org] On Behalf Of ??? > > help > haskell for web code http://www.haskell.org/haskellwiki/Applications_and_libraries/Web_programmi… Try a few of these out (whatever meets your needs). For web apps WASH and HAppS seem popular. Feel free to ask the haskell-café list for further help. Alistair ***************************************************************** 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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Re: [Haskell-cafe] Time consumption nub
by Arie Groeneveld 18 Jul '07

18 Jul '07

Miguel Mitrofanov wrote: > AG> Wondering about time space consuming: 'nub' vs 'map > AG> head.group.sort' > > Prelude> :t Data.List.nub > Data.List.nub :: (Eq a) => [a] -> [a] > Prelude> :t Data.List.sort > Data.List.sort :: (Ord a) => [a] -> [a] > > nub uses less information than sort, so it MUST be slower. Ok, so when do I use nub instead of 'map head.group.sort' ? Using nub gave me a lot of trouble in terms of time consumption while handling long lists. @@i=arie

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GHC 6.6.1: Where is Graphics.SOE ?
by Dmitri O.Kondratiev 18 Jul '07

18 Jul '07

*Andrea Rossato* wrote: <haskell-cafe%40haskell.org?Subject=%5BHaskell-cafe%5D%20GHC%206.6.1%3A%20Wh…> >Hi! > >as far as I know what you are looking for (Graphics.SOE) is part of >HGL. Have a look here: >http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HGL-3.1 >Hope I got it right and that this helps. > >All the best, >Andrea Andrea thanks! I tried to install HGL on Win32 and got this unresolved dependency: HGL-3.1>runghc Setup.hs configure Configuring HGL-3.1... configure: Dependency base-any: using base-2.1.1 Setup.hs: cannot satisfy dependency X11-any Any ideas? Thanks, Dmitri -- Dmitri O. Kondratiev dokondr(a)gmail.com http://www.geocities.com/dkondr

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"no-coding" functional data structures via lazyness
by Dave Bayer 18 Jul '07

18 Jul '07

Learning Haskell, the Prelude.ShowS type stood out as odd, exploiting the implementation of lazy evaluation to avoid explicitly writing an efficient concatenable list data structure. This felt like cheating, or at least like using a screwdriver as a crowbar, to be less judgmental. Recently I was playing with prime sieves and various heap data structures, while rereading Chris Okasaki's "Purely Functional Data Structures", and it dawned on me: > merge xs@(x:xt) ys@(y:yt) = case compare x y of > LT -> x : (merge xt ys) > EQ -> x : (merge xt yt) > GT -> y : (merge xs yt) > > diff xs@(x:xt) ys@(y:yt) = case compare x y of > LT -> x : (diff xt ys) > EQ -> diff xt yt > GT -> diff xs yt > > merge' (x:xt) ys = x : (merge xt ys) > > primes = ps ++ (diff ns $ foldr1 merge' $ map f $ tail primes) > where ps = [2,3,5] > ns = [7,9..] > f p = [ m*p | m <- [p,p+2..]] The code is very fast for its size; I haven't seen Haskell code posted on the web that comes close, and it is faster than any of my other tries (I posted this code to http://www.haskell.org/haskellwiki/ Prime_numbers). Effectively, it steals a heap data structure out of thin air by exploiting the implementation of lazy evaluation. It would seem that GHC's core data structures are coded closer to the machine that anything I can write _in_ Haskell. So much for studying how to explicitly write a good heap! So is there a name for this idiom, "no-coding" a classic data structure through lazy evaluation? Are there other good examples?

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Re: Happy Error
by Hugo Pacheco 18 Jul '07

18 Jul '07

I'm sorry, it turned out to be pretty simples. The error appears when there are references to undefined terminal or non-terminal productions. That's it. Sorry for the messages, at least it might be of some help to someone else. hugo On 7/18/07, Hugo Pacheco <hpacheco(a)gmail.com> wrote: > > Hi Simon, > > I'm having what I supose ti be a oarsec internal error when trying to > compile some grammar. > > > I get: > $ happy -g -a -c Parser.y > happy: parE > > > Is this a known bug? I could find no reference to it. > > > I'm using happy version 1.16 and I can't reduce the problem to a single > production. > The grammar file is attached. > > > Thanks in advance, > hugo > > > > > >

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Fwd: Happy Error
by Hugo Pacheco 18 Jul '07

18 Jul '07

I'm forwarding this mail, in case anyone might know about the bug. ---------- Forwarded message ---------- From: Hugo Pacheco <hpacheco(a)gmail.com> Date: Jul 18, 2007 3:45 AM Subject: Happy Error To: simonmar(a)microsoft.com Hi Simon, I'm having what I supose ti be a oarsec internal error when trying to compile some grammar. I get: $ happy -g -a -c Parser.y happy: parE Is this a known bug? I could find no reference to it. I'm using happy version 1.16 and I can't reduce the problem to a single production. The grammar file is attached. Thanks in advance, hugo

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Getting lambdabot to work with 6.6.1
by Vincenz Syntactically 18 Jul '07

18 Jul '07

Dear, After a suggestion from quicksilver, I decided to write this message. To get lambdabot working on 6.6.1 you need to: 1) ensure you have the regexp-base, regexp-compat and regexp-posix from hackage installed 2) If you install them from user, make sure to add --user in the build-script of lambdabot 3) Get hs-plugins from darcs http://www.cse.unsw.edu.au/~dons/code/hs-plugins/, not from hackage (Lemmih informed it's an issue with the .hi parser) 4) Fix Lib/Parser.hs and replace "> 606" with ">= 606" in the #if GLASSGOW_HASKELL around the definition of "pling_name" and co 5) Remove the Maybe-definition of arbitrary in scripts/ShowQ.hs 6) Make sure to uncomment the runplugs section in lambdabot.cabal 7) ... 8) Profit Cheers

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F#
by Edward Ing 18 Jul '07

18 Jul '07

Has anyone tried out F#? Is this a taboo subject here?

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Haskell shootout game
by Hugh Perkins 18 Jul '07

18 Jul '07

Had an idea: a real shootout game for Haskell. The way it would work is: - you email a haskell program to a specific address - it shows up on a web-page The webpage shows the last submitted solution for each person - anyone can select two solutions and click "Fight" -> the scripts "fight" in an arena for a second or so, and the results are published to the website The arena itself comprises: - a 2d grid, of a certain size (or maybe variable size) - each grid cell can be a wall, or one of the opponents - the boundaries of the grid are walls - random blocks of wall are placed around the grid The opponents only perceive what is in a section of space to their front, in a 45 degree arc from either side of the direction they are facing - each player can face along one of the four grid axes Each player takes it in turns to move - at each move the player can: - move one square - turn 90 degrees, in either direction - fire Firing will score one point if the opponent is in the line of fire at that time, and there are no intervening walls. Opponents can see the direction the other opponent is facing, as long as the other opponent is in their view arc, and there are no intervening walls. Each turn is represented by a function something like: doturn :: String -> [[GridValue]] -> (Action,String) -- [[GridValue]] is a map of what Me sees this turn, or has seen previously -- the Strings are a way for the function to pass state to itself between moves data GridValue = Opponent | Me | Wall | Empty data Action = Fire | MoveNorth | MoveSouth |MoveEast | MoveWest | TurnLeft | TurnRight | Wait -- (players can move backwards and sideways) The turn would be run as a separate thread, which either terminates successfully, or is aborted after a fixed time x milliseconds (maybe 10 milliseconds?) The String that doturn produces at the end of a turn is passed back in at the beginning of the next turn (so one could use gread/gshow to serialize/deserialize arbitrary data types, and there is no limitation on what data can be stored in the state). After say 1000 turns, the results are the points of each script. (or we could give each script a number of "lives" and if its loses them all the other script wins outright) This can run on a hosted webserver probably, because each match is part of a webpage request, and lasts a maximum of about a second, so shouldnt be terminated prematurely by cpu-monitoring scripts.

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Ann: Emping 0.3
by Hans van Thiel 18 Jul '07

18 Jul '07

Hello All, Version 0.3 of Emping is available. Emping is a utility that reads a table in a csv (comma separated) format that can be generated from Open Office Calc (spreadsheet), derives all shortest rules for a selected attribute, and writes them to a .csv file that can be read by OO Calc. The shortest rules may be partially ordered by implication (entailment) and equivalence (equality) and the top level is also shown in .csv format. Optionally all logical entailments and equalities are listed as well. If the data set contains ambiguous rules or more occurrences of the same rule, the user is warned. The main difference with V0.2 is that v0.3 can now handle intermediate size files (100 to a few thousand rules) because the transformation of conjunctive form (ands of ors) to disjunctive (ors of ands), which seemed to be exponential in the number of rules, has been optimized by using an intermediate 'maybe tree'. Probably I also used the wrong fold, without strictness, in the prior version, but it would still have been exponential. I 'think' it's now closer to linear to the number of rules, and quadratic to the number of attributes. But tests show the completion time is highly dependent on how well the data is structured. Obviously, much has still to be done. More info, for those who are interested, on http://j-van-thiel.speedlinq.nl/emp/empug.html Thank you, Hans van Thiel

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Why does Data.Map exist when...
by Tony Morris 18 Jul '07

18 Jul '07

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 ...it seems to be a special case of Set? Does Data.Map add anything more useful than Map' below? import Data.Set as Set newtype MyPair a b = MP (a, b) deriving Show instance (Eq a) => Eq (MyPair a b) where MP (a, _) == MP (a', _) = a == a' instance (Ord a) => Ord (MyPair a b) where MP (a, _) `compare` MP(a', _) = a `compare` a' type Map' k a = Set (MyPair k a) - -- Tony Morris http://tmorris.net/ -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.6 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org iD8DBQFGnDgEmnpgrYe6r60RAu4FAJ93Fwcx7ZX08+qO4ZlzRVV52TXpNQCeNr7u ioq0XrWt/Wymfh52W1spiFk= =FC5h -----END PGP SIGNATURE-----

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-O2 compile option can give speed increase over -O. Fasta shootout program test runs.
by Richard Kelsall 17 Jul '07

17 Jul '07

I have been playing with the Fasta program in the shootout to see if I can make it umm faster. Starting from dons program on this page and adding some timing calculations as suggested on this wiki page http://shootout.alioth.debian.org/gp4/benchmark.php?test=fasta&lang=ghc&id=2 http://www.haskell.org/haskellwiki/Timing_computations I added different OPTIONS into the top line of the program did a ghc --make fasta.hs and ran it each time with fasta 2500000 (This is one tenth of the shootout figure.) These runs all keep the existing OPTIONS of -fbang-patterns -fexcess-precision Seconds OPTIONS Added ------- ------------- 40.5 40.5 -funbox-strict-fields 40.4 {-# INLINE rand #-} 17.2 -O 17.0 -O -fvia-C 14.4 -O -optc-march=pentium4 11.5 -O2 11.2 -O3 11.5 -O3 {-# INLINE rand #-} 11.3 -O2 -optc-march=pentium4 There was a bit of variation, I've averaged over two runs. This is on an Intel Pentium D 2.66GHz running W2K and GHC 6.6.1. It seems the -O2 option can give a significant speed increase relative to just the -O option. This is contrary to the documentation which says http://www.haskell.org/ghc/docs/latest/html/users_guide/options-optimise.ht… http://www.haskell.org/ghc/docs/latest/html/users_guide/faster.html it won't make any difference. I guess it's program, architecture and operating system specific, but according to these figures the -O2 option seems well worth a try for programs that need speed. It may be that we sacrifice bounds checking or something important with -O2, I don't know. Richard.

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Re: hMapping polymorphic functions
by oleg＠pobox.com 17 Jul '07

17 Jul '07

hMapping polymorphic functions is indeed quite challenging, but can be done. That was the topic of the message Type-class overloaded functions: second-order typeclass programming with backtracking http://okmij.org/ftp/Haskell/poly2.txt The challenge is how to avoid specifying the context too early or at all and keep the context to the bare minimum. Your particular case is simple however: applying the function (\x->[x]) that applies literally to any argument type and is defined only by one case: > data FnListify = FnListify > > instance TypeCast [a] r => Apply FnListify a r where > apply _ x = typeCast [x] > > list1 = 'a' :*: "a" :*: True :*: HNil > test1 = hMap FnListify list1 *Poly2> test1 "a" :*: (["a"] :*: ([True] :*: HNil)) Within the Poly2 framework, the example becomes > data FnListify = FnListify > > instance TypeCast Otherwise r => GFN n FnListify a r > instance TypeCast [a] r => Apply (GFnA n FnListify) a r where > apply _ x = typeCast [x] > > instance TypeCast [a] r => Apply FnListify a r where > apply _ x = typeCast [x] > > test2 = hMap (GFn FnListify) list1 *Poly2> test2 "a" :*: (["a"] :*: ([True] :*: HNil)) The advantage of making things more complicated than they are is that we can add more clauses to our generic function. For example, we may choose to handle Booleans in a different way, by negating them. We add (perhaps in a different module, so the original code remains as it was) > data OnlyBool > instance TypeCast OnlyBool r => GFN Z FnListify a r > instance Apply OnlyBool Bool HTrue > instance TypeCast HFalse r => Apply OnlyBool a r > > instance Apply (GFnA Z FnListify) Bool Bool where > apply _ x = not x and now, *Poly2> test2 "a" :*: (["a"] :*: (False :*: HNil)) P.S. It may be worth sending me a carbon copy (CC) of any question relating to HList, OOHaskell, continuations, etc. I can no longer afford reading every message on Haskell-Cafe (or even every tenth message).

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A really bad way to say "all isSpace"
by Neil Mitchell 17 Jul '07

17 Jul '07

Hi, Reading through the code to read: read s = case [x | (x,t) <- reads s, ("","") <- lex t] of [x] -> x [] -> error "Prelude.read: no parse" _ -> error "Prelude.read: ambiguous parse" Reading through the code to lex, it appear that it will return [("","")] if and only if all isSpace t. If this is really the case, does it make sense to state all isSpace t? It has a much clearer meaning to me. Thanks Neil

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Haskell for categorists
by Miguel Mitrofanov 17 Jul '07

17 Jul '07

Just being curious. There are a lot of tutorials ensuring the reader that, although Haskell is based on category theory, you don't have to know CT to use Haskell. So, is there ANY Haskell tutorial for those who do know CT? I don't need it, personally, but still...

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More on improving the mailing list
by dons＠cse.unsw.edu.au 17 Jul '07

17 Jul '07

An interesting study on problem resolution and feedback on some technical mailing lists, "How to Help Mailing Lists Help Readers (Results of Recent Data Analysis) " http://praxagora.com/andyo/professional/mailing_list_follow_up/ including graphs! :) With conclusions at the end on how to improve mailing lists by /integrating material into wikis/, and providing both theoretical and practical advice for a given problem. -- Don

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ANNOUNCE: CC-delcont-0.1; Delimited continuations for Haskell
by Dan Doel 17 Jul '07

17 Jul '07

Hello, After a bit of hacking, and much documenting, I'm pleased to announce a preliminary release of a delimited continuation library for Haskell. The implementation is drawn from "A Monadic Framework for Delimited Continuations"[1] by Dybvig, Petyon Jones and Sabry, although it has some abstraction available over said implementation (there is both a monad and a transformer; control operations will work interchangeably with either. In addition, all four control operators from "Shift to Control"[2] are implemented). In addition, I have done the adaptation necessary to include the Haskell implementation of the dynamic scoping investigated in "Delimited Dynamic Binding"[3] by Kiselyov, Shan and Sabry, and it has been included. One can think of it as embeddings of the reader or state monads into the delimited control monads (only more flexible). In the future, I'd like to also, possibly, include something like Oleg's generic zippers, and possibly other useful abstractions based on delimited continuations, if I find them. But that's work for another day, I suppose. If you wish to try it out, a package is available on hackage: Page: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/CC-delcont-0.1 Tabrall: http://hackage.haskell.org/packages/archive/CC-delcont/0.1/CC-delcont-0.1.t… However, I should warn that it's fairly dependent on GHC HEAD at the moment: 1) It uses the Unsafe.Coerce module 2) It uses GADTs to store type class evidence 3) It uses GHC-features that haddock will choke on, so haddock.ghc will be required for docs (and I haven't gotten that working yet to check that the haddock is bug-free), which in turn requires GHC HEAD. 1 & 2 could be relaxed if there's interest. Please feel free to let me know if you find any bugs, or have any suggestions for improvements. I'll follow up this message with a message containing an example program and a discussion thereof. Cheers, Dan Doel 1: http://www.cs.indiana.edu/~sabry/papers/monadicDC.pdf 2: http://www.cs.rutgers.edu/~ccshan/recur/recur.pdf 3: http://okmij.org/ftp/papers/DDBinding.pdf

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problem with IO, strictness, and "let"
by Michael Vanier 17 Jul '07

17 Jul '07

I stumbled across a problem with IO and strictness that I could fix, but I can't understand why the fix works. I've compressed it down into a program which simply computes the number of lines in a file. Here is a version that doesn't work: module Main where import System.IO import System.Environment process_file :: FilePath -> IO () process_file filename = do h <- openFile filename ReadMode c <- hGetContents h let cs = unlines $ lines c hClose h putStrLn $ show $ length cs main :: IO () main = do args <- getArgs process_file (args !! 0) This will return a length of 0 lines for any input file. Obviously, the "let" is not being evaluated strictly (nor would we expect it to be), so that when the evaluation is requested, the file is already closed and the length of the list of lines is 0 (though I might have expected an error). I then tried this: process_file :: FilePath -> IO () process_file filename = do h <- openFile filename ReadMode c <- hGetContents h let cs = id $! lines c -- try to strictly evaluate the let binding hClose h putStrLn $ show $ length cs which also failed exactly as the previous version did (i.e. always returning 0). Then I gave up on "let" and did this: process_file :: FilePath -> IO () process_file filename = do h <- openFile filename ReadMode c <- hGetContents h cs <- return $! lines c hClose h putStrLn $ show $ length cs This works. However, I don't understand why this version works and the previous version doesn't. Can anyone walk me through the evaluation? Also, is there a way to make "let" strict? TIA, Mike

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maybe OT: open standards: ooxml, are haskellers interested ?
by Marc Weber 17 Jul '07

17 Jul '07

At the moment I only know of questions about interfacing the API of Microsoft office eg using COM technology. This shows that at least some haskellers are interested in office documents. That's why I want to mention www.noooxml.org Summary: Microsoft wants to create a new ISO standard for its OOXML format which you will never be able to implement completely. It's also about 6000 pages compared to 600 pages (ODT, OpenOffice is using this) Most haskell libraries are kind of open source. If tey weren't haskell wouldn't be that appealing. More information about topics like this can be found on www.ffii.org Marc Weber

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In-place modification
by Isaac Gouy 17 Jul '07

17 Jul '07

On Jul 15, 1:25 pm, "Hugh Perkins" <hughperk...@gmail.com> wrote: > > or maybe 'pidigits', a lazy pi generator, > This is I/O bound, which isnt interesting, unless you really want to > benchmark I/O to console? a) output is redirected to /dev/null - read the FAQ b) the I/O is cheap delete PiDigitSpigot class and change Console.Write (digits.Next ()); to Console.Write (1); ~0.15s down from ~12.8s ____________________________________________________________________________________ Got a little couch potato? Check out fun summer activities for kids. http://search.yahoo.com/search?fr=oni_on_mail&p=summer+activities+for+kids&…

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When is extra-libraries config in .cabal
by Edward Ing 16 Jul '07

16 Jul '07

I am building and running some haskelldb/HDBC/HSQL libraries on Windows XP (using the MinGW minimals system (MSYS)) with GHC environment. I ran into runtime link problems, foreign functions would not found. eg. atoi,from C standard; recv from sockets. To solve the problem, I added dll's to the build configuration field "extra-libraries", MSVCRT (c runtime), KERNEL32, WSOCK32 (sockets). This solved the problem. 1. But is there a better way to solve the problem? I am expecting that those libraries are available through the OS and do not need explicit loading. I am confuse by what does the loading of the libraries. 2. Does a UNIX/POSIX system not have this problem? Information for understanding this problem would be great. Edward Ing

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Re: [Haskell-cafe] Haskell & monads for newbies
by Andrew Coppin 16 Jul '07

16 Jul '07

Donald Bruce Stewart wrote: > andrewcoppin: > >> I saw a quote somewhere round here that went like this: >> >> "Haskell isn't really suited to heavily I/O-oriented programs." >> "What, you mean like darcs?" >> "...oh yeah." >> >> > > Great quote! :) > TY. :-) Be even greater if I could remember who the heck said it... (It's probably a #haskell quote from the Humour section on the Wiki, as a guess.) Actually, thinking about it, if you wanted to demonstrate just how badly Haskell sucks at I/O, we have: - Darcs - XMonad - Lambdabot - Frag - Wasn't there a stand-alone HTTP server somewhere? - GHC (provides make-like functionallity, and also invokes half a dozen external programs to do its work) - Any others? Yep, I'd say that's pretty conclusive. ;-)

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SHA1 again
by Malte Milatz 16 Jul '07

16 Jul '07

See the following link for a purely functional and straight-forward implementation of SHA1. Disclaimer: Please be kind to me, I haven't done much Haskell (yet). And I know nothing about SHA1 except its specification. http://hpaste.org/1695#a2 It performs better than the SHA1 algorithm in Crypto: It is faster by a factor of approximately e. ;-) It is also competitive (regarding time) with the »unsafe« SHA1 implementation posted here some days ago, although it uses considerably more memory. Of course, you may safely forget it if you're interested in performance near a C implementation such as GNU sha1sum. I'd heartily welcome any comments or suggestions for improvement, as well as benchmark results, especially if they differ from mine. I'm using the standard ghc -O2, by the way. Malte

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Indentation Creep
by Thomas Conway 16 Jul '07

16 Jul '07

Hi All, In the best spirit of Haskelling, I thought I'd try dropping in a completely different data structure in a spot where I thought the existing one was (1) ugly (2) leaking memory. In particular, I wrote a Trie implementation. Now the point is actually not much to do with the data structure itself, but code layout. I mention this particular data structure only because it is the one I was working on, but it seems to come up quite often. Consider the following function: data Trie t = Empty | Trie (TriePtr t) (MaybePtr t) (TriePtr t) type TriePtr t = TVar (Trie t) type MaybePtr t = TVar (Maybe t) data Bit = Zero | One deriving Show dmin p = do mv <- dmin' p case mv of Nothing -> error "dmin: no values" Just (v,_) -> return v dmin' p = do t <- readTVar p case t of Empty -> return Nothing Trie l m r -> do mv <- dmin' l case mv of Nothing -> do mv <- readTVar m case mv of Nothing -> do mv <- dmin' r case mv of Nothing -> error "emit nasal daemons" Just (v,e) -> do if e then writeTVar p Empty else return () return mv Just v -> do re <- null r case re of False -> writeTVar m Nothing True -> writeTVar p Empty return (Just (v,re)) Just (v,e) -> do case e of True -> do me <- empty m re <- null r case me && re of False -> writeTVar m Nothing True -> writeTVar p Empty return (Just (v,me && re)) False -> return mv where empty m = do v <- readTVar m case v of Nothing -> return True Just _ -> return False All that case analysis causes indentation to creep, and lots of vertical space "feels" wasted. Is that just a fact of life, or is there Haskellmagic that I still need to learn? cheers, T. -- Dr Thomas Conway drtomc(a)gmail.com Silence is the perfectest herald of joy: I were but little happy, if I could say how much.

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Re: [Haskell-cafe] Haskell shootout game
by PR Stanley 16 Jul '07

16 Jul '07

Is this some sort of a war game? At 11:14 16/07/2007, you wrote: >This would be a lot of fun! Make sure to take the lessons from >http://haskell.org/haskellwiki/Safely_running_untrusted_Haskell_code >into account. > >regards, > >Bas van Dijk > >On 7/15/07, Hugh Perkins <hughperkins(a)gmail.com> wrote: >>Had an idea: a real shootout game for Haskell. >> >>The way it would work is: >>- you email a haskell program to a specific address >>- it shows up on a web-page >> >>The webpage shows the last submitted solution for each person >>- anyone can select two solutions and click "Fight" >>-> the scripts "fight" in an arena for a second or so, and the results are >>published to the website >> >>The arena itself comprises: >>- a 2d grid, of a certain size (or maybe variable size) >>- each grid cell can be a wall, or one of the opponents >>- the boundaries of the grid are walls >>- random blocks of wall are placed around the grid >> >>The opponents only perceive what is in a section of space to their front, in >>a 45 degree arc from either side of the direction they are facing >>- each player can face along one of the four grid axes >> >>Each player takes it in turns to move >>- at each move the player can: >> - move one square >> - turn 90 degrees, in either direction >> - fire >> >>Firing will score one point if the opponent is in the line of fire at that >>time, and there are no intervening walls. >> >>Opponents can see the direction the other opponent is facing, as long as the >>other opponent is in their view arc, and there are no intervening walls. >> >>Each turn is represented by a function something like: >> >>doturn :: String -> [[GridValue]] -> (Action,String) >> >>-- [[GridValue]] is a map of what Me sees this turn, or has seen previously >>-- the Strings are a way for the function to pass state to itself between >>moves >> >>data GridValue = Opponent | Me | Wall | Empty >>data Action = Fire | MoveNorth | MoveSouth |MoveEast | MoveWest | TurnLeft | >>TurnRight | Wait -- (players can move backwards and sideways) >> >>The turn would be run as a separate thread, which either terminates >>successfully, or is aborted after a fixed time x milliseconds (maybe 10 >>milliseconds?) >> >>The String that doturn produces at the end of a turn is passed back in at >>the beginning of the next turn (so one could use gread/gshow to >>serialize/deserialize arbitrary data types, and there is no limitation on >>what data can be stored in the state). >> >>After say 1000 turns, the results are the points of each script. (or we >>could give each script a number of "lives" and if its loses them all the >>other script wins outright) >> >> >>This can run on a hosted webserver probably, because each match is part of a >>webpage request, and lasts a maximum of about a second, so shouldnt be >>terminated prematurely by cpu-monitoring scripts. >> >> >>_______________________________________________ >>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

can't build hIDE
by Vadim 16 Jul '07

16 Jul '07

Hello, all, I'm following instructions to build hIDE at the page http://www.haskell.org/haskellwiki/HIDE I get "development" version b ycommand darcs get http://darcs.haskell.org/hIDE I succesfully built all dependant packages (Yi), and stuck on error when building HIDE itself: Building hide-plugin-0.1... /opt/ghc-6.6.1/bin/ghc -package-name hide-plugin-0.1 --make -hide-all-packages -i -idist/build/autogen -i. -isrc -odir dist/build -hidir dist/build -package base-2.1.1 -package ghc-6.6.1 -package parsec-2.0 -package FilePath-0.1.0 -DHAVE_LEADING_UNDERSCORE=False -DGHC_NUMERIC_VERSION="6.6.1" -Dlinux_TARGET_OS -Dlinux_HOST_OS -DGHC_LIBDIR="/opt/ghc-6.6.1/lib/ghc-6.6.1" -fglasgow-exts Hide.Plugin.Types Hide.Plugin.Parser Hide.Plugin.Printer Hide.Plugin.Loader Hide.Plugin.LoaderMidLevel Hide.Plugin.TypeCheck [5 of 6] Compiling Hide.Plugin.LoaderMidLevel ( src/Hide/Plugin/LoaderMidLevel.hs, dist/build/Hide/Plugin/LoaderMidLevel.o ) src/Hide/Plugin/LoaderMidLevel.hs:126:26: Not in scope: `moduleFS' I don't know how to find mentioned `moduleFS' Please advise me how should I deal with this error TIA, Vadim.

2 2

hMapping polymorphic functions
by jeff p 16 Jul '07

16 Jul '07

Hello, I am experimenting with type-level HList based programming. The Apply class: class Apply f a r | f a -> r where apply :: f -> a -> r apply = undefined -- In case we use Apply for -- type-level computations only -- Normal function application instance Apply (x -> y) x y where apply f x = f x makes it seem like it should be possible to hMap: class HMap f l l' | f l -> l' where hMap :: f -> l -> l' instance HMap f HNil HNil where hMap f HNil = HNil instance ( Apply f x y, HMap f xs ys ) => HMap f (HCons x xs) (HCons y ys) where hMap f (HCons x xs) = HCons (apply f x) (hMap f xs) a (term-level) polymorphic function over a HList. As written it doesn't work at all. A (local) functional dependency to say that the function determines its argument type is missing. Towards this end I changed the Apply instance to: instance TypeCast a' a => Apply (a -> b) a' b where apply f x = f (typeCast x) This change allows me to apply a polymorphic function to elements of an HList by hand: (\(f :: forall a. a -> [a]) -> HCons (f 'a') (HCons (f "a") HNil)) (\x -> [x]) However, I need to explicitly write the polymorphic type of the argument f. Thus I cannot use hMap. So I have two questions: 1) Am I missing something? 2) Is there some way to pass an arbitrary (i.e. don't know explicit type at compile time) polymorphic function as an argument to another function? thanks, Jeff

1 0

Perfect shuffle on std. libs
by Felipe Almeida Lessa 16 Jul '07

16 Jul '07

Hello! =) I wonder why Oleg's perfect shuffle[1] isn't on any standard library? Is there a reason or is it just a lack of patches? I'd personally like to have at least the function > shuffle :: RandomGen g => g -> [a] -> [a] > shuffle gen list = > let len = length list > ran = [r `mod` k | (k,r) <- zip [len,len-1..2] randoms gen] > in shuffle1 cartas rs' where shuffle1 is Oleg's perfect shuffle and length is the beSureYouReallyWantIntBeforeUsingThisLength ;). Cheers, [1] http://okmij.org/ftp/Haskell/perfect-shuffle.txt -- Felipe.

3 2

Another monad question...
by David LaPalomento 16 Jul '07

16 Jul '07

Hi everyone, I've been playing with the parsers decribed in "Monadic Parser Combinators" (http://www.cs.nott.ac.uk/~gmh/bib.html#monparsing) and I've gotten stumped. I'm trying to get comfortable working monadically, so please excuse my ignorance. Here's the relevant portions of my code: data Parser a = Parser { parse :: (String -> [(a, String)]) } instance Monad Parser where return v = Parser (\inp -> [(v, inp)]) par@(Parser p) >>= f = Parser (\inp -> concat [parse (f v) out | (v, out) <- p inp]) instance MonadPlus Parser where mzero = Parser (\inp -> []) p `mplus` q = Parser (\inp -> (parse p inp ++ parse q inp)) item :: Parser Char item = Parser (\inp -> case inp of [] -> [] (x:xs) -> [(x, xs)]) sat :: (Char -> Bool) -> Parser Char sat p = Parser (\inp -> [ (v, out) | (v, out) <- parse item inp, p v]) lower :: Parser Char lower = Parser (\inp -> parse (sat (\x -> 'a' <= x && x <= 'z')) inp) upper :: Parser Char upper = Parser (\inp -> parse (sat (\x -> 'A' <= x && x <= 'Z')) inp) letter :: Parser Char letter = lower `mplus` upper -- word parses everything as [] word :: Parser String word = mzero `mplus` do x <- letter; xs <- word; return (x:xs) As I noted in the code, no matter what inputs I give it, > parse word "blah blah" always returns []. Any ideas where where my misunderstanding is? David

4 7

AngloHaskell 2007, dates and venue confirmed
by Neil Mitchell 16 Jul '07

16 Jul '07

Hi, We are pleased to announce AngloHaskell 2007 http://www.haskell.org/haskellwiki/AngloHaskell Dates: 10th-11th of August (Friday-Saturday) Location: Cambridge, with talks at Microsoft Research on Friday All the details are on the wiki page, along with free registration. Everyone is invited, we will have a day of talks at MSR, then a day of other activities. There will be plenty of chance for general discussions on anything. If anyone in Cambridge is able to accommodate a few people for the Friday or Saturday night, please add your name to the wiki, and accept our thanks in advance. All that is needed is floor space. Thanks Neil and Philippa

4 5

Re: [Haskell-cafe] Maintaining the community
by Andrea Rossato 15 Jul '07

15 Jul '07

On Sun, Jul 15, 2007 at 07:05:53PM +1000, Donald Bruce Stewart wrote: > So how would you think we can approve? We have to help in more specific > ways, and listen more carefully to what people are asking? I usually do this when I want a newcomer to join my community (for instance my research community): I study him, I read his stuff, I try to get to know him, so that I can find for him a place that can suit his needs, feed his curiosity, and help him improve while maximizing his contribution to the community. I treat him as she were my fellow researcher, and not a student I'm supposed to teach to. Just an example: I find it so much irritating when you give a paper to someone and, before even reading it, the receiver starts asking you if you have already read the just published papers of authors x and y about a topic that could slightly resemble the one of your paper (even if actually does not)? This a the kind of behaviour I always try to avoid with new comers. I suppose they know more then me, even if in different area of knowledge. We are all experts of almost nothing. But I'm talking about a small research community, with one or two new members a year. I have no experience in managing such a large community as this one. I just thought that the fact that I felt repelled by the behaviour of some of the members of this community, and please believe me when I say that that behaviour was far from being intentionally offensive or even disrespectful in my regards, could be useful for someone that has such a difficult task. All my best wishes. Andrea

1 0

[weird stuff] The Dodgy Diagonal
by Conor McBride 15 Jul '07

15 Jul '07

Scary words warning: Polynomial, Functor, Bifunctor, unsafeCoerce# Folks A peculiar query for folks who know more about the internals of Haskell compilers than I do. I attach the full code with all the bits and pieces, but let me pull out the essentials in order to state the problem. I've been mucking around with polynomial types in one parameter > newtype Id x = Id x -- element > newtype K1 a x = K1 a -- constant > data ((Sum1 p q)) x = L1 (p x) | R1 (q x) -- choice > data ((Prod1 p q)) x = P1 (p x) (q x) -- pairing all of which give unremarkable instances of Functor < class Functor p where < fmap :: (s -> t) -> p s -> p t > instance Functor Id > instance Functor (K1 a) > instance (Functor p, Functor q) => Functor (Sum1 p q) > instance (Functor p, Functor q) => Functor (Prod1 p q) I've also been mucking around with polynomial types in two parameters > newtype Fst x y = Fst x > newtype Snd x y = Snd y > newtype K2 a x y = K2 a > data ((Sum2 p q)) x y = L2 (p x y) | R2 (q x y) > data ((Prod2 p q)) x y = P2 (p x y) (q x y) which give unremarkable bifunctors, doing two maps at once > class Bifunctor p where > bimap :: (s1 -> t1) -> (s2 -> t2) -> p s1 s2 -> p t1 t2 > instance Bifunctor Fst where > bimap f g (Fst x) = Fst (f x) > > instance Bifunctor Snd where > bimap f g (Snd y) = Snd (g y) > > instance Bifunctor (K2 a) > instance (Bifunctor p, Bifunctor q) => > Bifunctor (Sum2 p q) > instance (Bifunctor p, Bifunctor q) => > Bifunctor (Prod2 p q) Now, I'm interested in collapsing the diagonal. What? Er, this: > class (Bifunctor b, Functor f) => Diag b f | b -> f where > diag :: b x x -> f x > gaid :: f x -> b x x If the two parameters to a bifunctor are instantiated with the same thing, we should be able to exchange with the functorial representation. I'll just do one way. > instance Diag Fst Id where > diag (Fst x) = Id x > instance Diag Snd Id where > diag (Snd x) = Id x > instance Diag (K2 a) (K1 a) where > diag (K2 a) = K1 a > instance (Diag pb pf, Diag qb qf) => Diag (Sum2 pb qb) (Sum1 pf qf) where > diag (L2 p) = L1 (diag p) > diag (R2 q) = R1 (diag q) > instance (Diag pb pf, Diag qb qf) => Diag (Prod2 pb qb) (Prod1 pf qf) where > diag (P2 p q) = P1 (diag p) (diag q) That looks like a whole lot of doing very little. So, can I (in practice, in this or that compiler) get away with... > dodgy :: Diag b f => b x x -> f x > dodgy = unsafeCoerce# > ygdod :: Diag b f => f x -> b x x > ygdod = unsafeCoerce# ...dodgy for diag and ygdod for giad? Minimal nontrivial experiments in ghc give grounds for cautious optimism, but I'd be delighted to hear from better informed sources. Cheers Conor ------------------

4 3

Troubles with Exercise 4.9 from YAHT
by michael＠schmong.org 15 Jul '07

15 Jul '07

from YAHT Exercise 4.9 Write a function elements which returns the elements in a BinaryTree in a bottom-up, left-to-right manner (i.e., the ï¿½ï¿½ï¿½rst element returned in the left-most leaf, followed by its parentï¿½ï¿½ï¿½s value, followed by the other childï¿½ï¿½ï¿½s value, and so on). The re- sult type should be a normal Haskell list. here's the data type BinaryTree data BinaryTree a = Leaf a | Branch (BinaryTree a) a (BinaryTree a) Here's my first attempt nodeCount :: BinaryTree a -> [b] nodeCount = map putStrLn (show a) a is out scope. I started looking at what types putStrLn and map are. First thing that occurs to me is that I can't use map unless I pass it a list. I'm not allow to use fold yet, that's the next assignment. Any clues on how to proceed?

2 1

Functional dependencies *not* part of the next Haskell standard?
by peterv 15 Jul '07

15 Jul '07

I tried to do something in CAL that I could not solve without functional dependencies. In their support forum, it got mentioned that func.deps propably won't make into the next Haskell standard... Any comments on that? Now, the thing I tried to solve was: data Vector2 a = Num a => V2 a a class Vector a n | a -> n where dot :: a -> a -> n instance Num a => Vector (Vector2 a) a where dot (V2 x1 y1) (V2 x2 y2) = x1 * x2 + y1 * y2 test1 = dot (V2 1.0 2.0) (V2 3.0 4.0) Please note this is just some testing code (the math people in here might be horrified by it ;), although any hints of how to do this in a clearer way are welcome. As side-note, does Haskell have some easy to use good linear algebra package for doing 2D, 3D and 4D math? Then I would not be writing any of this code, although I did find it useful for learning. Now without the funcdep a -> n in the type class declaration I get No instance for (Vector (Vector2 t) n) arising from use of `dot' at l:/Haskell/test/vector.hs:9:8-36 Possible fix: add an instance declaration for (Vector (Vector2 t) n) In the expression: dot (V2 1.0 2.0) (V2 3.0 4.0) In the definition of `test1': test1 = dot (V2 1.0 2.0) (V2 3.0 4.0) CAL seems to have the same problem, and without funcdeps I seem to be stuck in that language. PS(1): In the above I was using GHCI with extensions enabled. PS(2): IMHO CAL is in some aspects easier to get started than Haskell because of their nice Eclipse plugin and GEM Cutter environment for doing learning experiments. Thanks, Peter

15 29

Looking for final year project - using Haskell, or another functional language
by wp 15 Jul '07

15 Jul '07

Hi all, I will soon be doing my last year in computer science. One part of our last year encompasses quite a big project which will go over 3 terms and will account for 3 modules (45 credits). I was thinking in doing something using functional languages (preferably Haskell, as it's the one I know most). Does anybody know anyone who would have a task suitable for such a project which would encompass the whole development life cycle (maybe a sub-project?). I would do this obviously for free; the client can be anyone (industrial, academic, open source, etc. ... ), as long as the project is something serious and for practical usage. I would be happy for any suggestions ... Thanks walter.

9 17

Re: [Haskell-cafe] Type system madness
by Andrew Coppin 15 Jul '07

15 Jul '07

Bryan O'Sullivan wrote: > Andrew Coppin wrote: > >> Oh well, the problem is easily fixed... *sigh* > > I doubt that anybody minds having you talk about Haskell. You've been > responsible for spawning a lot of interesting threads. [And that one about compression that's still going on somewhere... lol!] > All I would suggest is that you take your cue from the other people > who post to the list, and try a few tactics before you post: > > - If you have a chatty one-line comment, do 2,000 other people need to > see it? Mmm. This is why I prefer NNTP. (If a thread becomes tangent, everybody just marks it "ignore" and they don't have to waste time downloading it or reading it.) But yeah, point taken... > - If you have a question to ask, try to spend 2 minutes with Google or > the Haskell wiki to find the anwer. I continue to be surprised at the things that don't seem to be on the Wiki... Google is typically no help at all with anything Haskell-related, because Haskell is so completely obscure. The various haddoc documentation is also frustratingly sparse in places. (E.g., Control.Concurrent.STM.TVar contains *nothing* but terse type signatures. And concurrent programming is already a tricky thing to get right.) Some things seem to be "well known" yet not actually written down anywhere - e.g., the finer points of using "seq" to make stuff go faster. But sure, I do like to try to puzzle a thing out first before posting here. (If something else, I kind of enjoy a challenge...) > - Join us on #haskell on IRC. It's extremely chatty, and you'll be > welcome. Not in my experience, no. (Maybe I ask the wrong way... but almost everybody seems to simply ignore me. Actually, usually when I go there absolutely nobody is speaking at all. What time zone do these people live in?)

4 4

Displaying fractions of a second from a datediff?
by Hugh Perkins 15 Jul '07

15 Jul '07

Hi, Pretty dumb question I know, but..how to I display fractions of a second from a datediff? ie: main = do starttime <- gettime -- do something here that takes a few seconds endtime <- gettime let timediff = diffClockTimes endtime starttime let timediffstring = show( tdSec timediff) ++ " " ++ show( realToFrac(tdPicosec timediff) / 1000000000 ) putStrLn( show(timediffstring) ) return () tdPicosec is negative??? ... but sometimes it is positive... so it's really unclear what the appropriate way to handle this is! Example outputs: "1 -62.0" "0 921.0" ... it could be that the time in seconds in the second example is "0.921", but for the first, is it "1.062" (adding the absolute milliseconds) or is it "0.938" (adding the negative milliseconds)? Tried to figure this out by generating timeDiffs myself then normalizing them, but this doesnt seem to do anything useful ;-) : Prelude> normalizeTimeDiff $ TimeDiff 0 0 0 0 0 0 1000000000000000000000000000000 0000 TimeDiff {tdYear = 0, tdMonth = 0, tdDay = 0, tdHour = 0, tdMin = 0, tdSec = 0, tdPicosec = 10000000000000000000000000000000000}

1 1

Newbie question about tuples
by peterv 15 Jul '07

15 Jul '07

Hi, I have a couple of questions about tuples. Q1) Is it possible to treat a tuple of N elements in a generic way? So instead of writing functions like lift1 e1, lift2 (e1,e2), lift3 (e1,e2,e3) just one function liftN that works on tuples of any length? Q2) (Maybe related to Q1) Can I convert a tuple of length N to a heterogeneous list (using "forall" aka existentially quantified types) and vice versa? Q3) Suppose I want to declare an instance of Num on all tuple types having (Num instances) as elements; is this possible? I tried instance Num a => Num (a,a) where . but this fails I also tried instance Num a => Num ((,) a a) where . but that also fails. I can of course create a new type like newtype Num a => Vector2 a = Vector2 (a,a) and then create an instance for Vector2, but I was wondering if it would be possible without introducing a new type. Thanks, Peter

12 23

Type system madness
by Andrew Coppin 15 Jul '07

15 Jul '07

OK, can somebody explain to me *really slowly* exactly what the difference between an existential type and a rank-N type is? (I couldn't find much of use on the wiki. I have now in fact written some stuff there myself, but since I don't understand it in the first place, I'm having difficulty trying to explain it to anybody else...)

22 73

Re: [Haskell-cafe] sha1 implementation thats "only" 12 times slower then C
by Anatoly Yakovenko 15 Jul '07

15 Jul '07

---------- Forwarded message ---------- From: Anatoly Yakovenko <aeyakovenko(a)gmail.com> Date: Jul 14, 2007 11:09 AM Subject: Re: Your SHA1 To: Dominic Steinitz <dominic.steinitz(a)blueyonder.co.uk> > > 1. Very good. Thanks, it was a fun experiment. > > 2. It has type hash::BS.ByteString -> IO [Word] but hash is a pure function. > > Can this be changed? you can call it safely with unsafePerformIO, I do a lot of hacks to get the performance, like casting a C ptr from a bytestring to a word array, but each call to hash should have no side effects. > > 3. I haven't tried but I assume it only runs with ghc and not hugs? I guess if > > point 1 could be addressed then we could put it in the crypto library (assuming > > you are happy with this) with some sort of conditional flag to use your code if > > the library is being built for ghc but to use the slow version for other > > compilers / interpreters. i would be surprised if this ran on hugs, i havent tried. Also, i haven't verified that my implementation is correct, as far as the sha1 algorithm is concerned. the complexity is the same, so if there any bugs in the math they can be easily fixed without making it slower. > > On a more discursive note, I still don't think we have found the holy grail > > here: idiomatic functional programming (probably not using StorableArray and > > unsafeRead and unsafeWrite) and lightning fast speed. yea, i agree, i am doing a lot of ugly hacks to get things going faster. Actually i think the "cleaner" approach would be to use Harpy extension and do the math with x86 assembly :). This way its at least straight forward, cause right now, i was just randomly changing strictness and unboxing things so ghc generates the fastest code, but a different haskell compiler would have completely different results. > > Dominic. > > > > PS I noticed you have: > > > > splitByN::Int -> BS.ByteString -> [BS.ByteString] > > splitByN nn ll = st : (if (BS.null en) then [] else (splitByN nn en)) > > where > > (st,en) = BS.splitAt nn ll > > > > It's a function I often use: > > > > splitByN n = > > unfoldr k > > where > > k [] = Nothing > > k p = Just (splitAt n p) > > > > Maybe it should be a standard part of List and ByteString? yea i agree, i've seen a couple other implementations as well. seems like its something that should be in Prelude and ByteString. Anatoly

2 1

Frisby (PEG parser) unbiased-choice thoughts
by Isaac Dupree 15 Jul '07

15 Jul '07

since I don't like unexpected behavior happening when something not intended to happen, happens, and it's better documentation (a free assertion) -- I find myself making Haskell comments to specify whether the left-biasedness of each (//) is important, various places in my own code that uses frisby. We examine how could such an operator could behave, having similar effect to (//) but symmetric (call it (=|=); (p1 =|= p2) should be equivalent to (p2 =|= p1) )... I will use original frisby syntax (see <http://repetae.net/john/computer/frisby/>), not the Applicative/Alternative instances I added, to be clearer. "Thought 1" could make a different but valid Alternative instance, after all. Prototype implementations are given... so it's clearly still O(1). thought 1: (=|=) :: P s a -> P s a -> P s a a =|= b = ((peek a <> peek b) `onlyIf` (error "BUG in (=|=) user")) // (a // b) thought 2: (=|=) :: Eq a => P s a -> P s a -> P s a a =|= b = ((peek a <> peek b) `onlyIf` (check)) // (a // b) where check (ra,rb) = if ra /= rb then error "BUG in (=|=) user" else False --This one "unnecessarily" requires Eq just to check a programmer's --assertion of equivalence, but allows overlaps that give the same --result. On second thought... 'a' and 'b' could consume different --amounts of input and produce the same result, so this is buggy. thought 3: symmetricDifference :: P s a -> P s a -> P s a a `symmetricDifference` b = (doesNotMatch a ->> b) // (doesNotMatch b ->> a) feeding arbitrary inputs using quickcheck should hopefully catch incorrect usages of the (=|=)s that can be used in error, since it is well known that one cannot systematically determine for all PEG parsers whether the assertion will hold true (it must be at least as difficult as determining whether any two are exactly equivalent). HOWEVER it would also be interesting to see a similar system that could be restricted enough that one could show the correctness of such a non-intersection claim (I wonder how frequently such claims are fairly trivial to prove correct, anyway) Isaac

1 0

Strange results when trying to create large Bool arrays.
by Richard Kelsall 14 Jul '07

14 Jul '07

My first ever Haskell program just creates an array in memory. I wanted to try creating really big arrays. import Data.Array.IO import System import Text.Printf main = do n <- getArgs >>= readIO . head :: IO Int a <- newArray (1,n) True :: IO (IOUArray Int Bool) printf "Created array 1 .. %8d \n" (n::Int) :: IO () It appears to work up to quite large numbers, but then gets strange. When I give it an array size of 1,000,000,000,000 it returns this Created array 1 .. -727379968 Presumably the Int has overflowed without warning when read from the argument. I guess there must be a switch to make it produce a nice error message rather than overflowing without warning. If I replace all the 'Int's in the program with 'Integer's it works better, but sometimes it freezes when I give it a silly large number rather than returning an 'unable to allocate that much memory' error. And what really puzzles me is this behaviour in the 'Integer' version. 1000 created array 1000000 created array 1000000000 created array 10000000000 created array (approximately my bits of real memory) 100000000000 created array 1000000000000 freezes 10000000000000 created array 100000000000000 created array 1000000000000000 freezes 10000000000000000 created array It seems to randomly claim to have successfully created huge sizes of array. So now I am not sure which of these arrays it is really creating. I would expect it to consistently freeze above my real memory size. Or preferably return a 'not enough memory' error message. I have 1Gb of real memory in my machine. GHC 6.6.1 for Windows, running on W2K. Assuming these errors are in Haskell rather than my code (beginner's fallacy?) could I vote for having the memory, bounds, overflow etc checking switched on by default. I like my errors to be made as visible as possible as soon as possible. Or did I do something wrong? Richard.

17 29

HDBC-ODBC build/install problem.
by Edward Ing 14 Jul '07

14 Jul '07

Hi, I am trying to make HaskellDB work with HDBC-ODBC. I did builds of HDBC/HDBC-ODBC. But when I am building HaskellDB-HDBC-ODBC, I get the following message. -------------- "[1 of 1] Compiling Database.HaskellDB.HDBC.ODBC ( Database/HaskellDB/HDBC/ODBC.hs, dist\build/Database/HaskellDB/HDBC/ODBC.o ) C:\Program Files\Haskell\HDBC-odbc-1.1.2.0\ghc-6.6.1/Database/HDBC/ODBC/Connection.hi Declaration for connectODBC: Failed to load interface for `Database.HDBC.ODBC.ConnectionImpl': Use -v to see a list of the files searched for. Cannot continue after interface file error" -------------- >From this, I know the problem is the linkage between Database.HDBC.ODBC.Connection and Database.HDBC.ODBC.ConnectionImple. (Also I looked at the code to see the reference.) I did a little further investigation. I looked at the package registry area (C:\Program Files\Haskell\HDBC-odbc-1.1.2.0\ghc-6.6.1\Database\HDBC\ODBC) and notice that ConnectionImpl.hi is not there. I went back to the build directory and did find ConnectoinImpl.hi and ConnectionImpl.o. It seems like runghc Setup.hs install, did not install ConnectionImpl.hi. I looked into the file named ".installed-pkg-config" and I saw this: ------------ exposed-modules: Database.HDBC.ODBC hidden-modules: Database.HDBC.ODBC.Connection Database.HDBC.ODBC.Statement Database.HDBC.ODBC.Types Database.HDBC.ODBC.Utils Database.HDBC.ODBC.TypeConv import-dirs: "C:\\Program Files\\Haskell\\HDBC-odbc-1.1.2.0\\ghc-6.6.1" library-dirs: "C:\\Program Files\\Haskell\\HDBC-odbc-1.1.2.0\\ghc-6.6.1" hs-libraries: HSHDBC-odbc-1.1.2.0 extra-libraries: odbc32 -------------- No mention of ConnectionImple.hi. It looks like the setup up script did not install ConnectionImpl.hi. Did ConnectionImpl.o get bound into "libHSHDBC-odbc-1.1.2.0.a" even though ConnectionImpl.hi did not get successfully installed? Does anyone know why the install target does not install ConnectionImpl.hi and how I can get around this problem? (Where is the odbc32 to be found anyways?) Here are a few things I did try which did NOT work: 1. Copy ConnectionImpl.hi over manually. HaskellDB-HDBC-ODBC builds, but at runtime there is a link error. 2. Manually alter ".installed-pkg-config" to add ConnectionImpl.hi as hidden module. Please comment on why these would not work ( I will learn from this.) Help would be appreciated. Edward Ing

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sha1 implementation thats "only" 12 times slower then C
by Anatoly Yakovenko 14 Jul '07

14 Jul '07

So I tried implementing a more efficient sha1 in haskell, and i got to about 12 times slower as C. The darcs implementation is also around 10 to 12 times slower, and the crypto one is about 450 times slower. I haven't yet unrolled the loop like the darcs implementation does, so I can still get some improvement from that, but I want that to be the last thing i do. I think I've been getting speed improvements when minimizing unnecessary allocations. I went from 40 times slower to 12 times slower by converting a foldM to a mapM that modifies a mutable array. Anyone have any pointers on how to get hashElem and updateElem to run faster, or any insight on what exactly they are allocating. To me it seems that those functions should be able to do everything they need to without a malloc. This is the profiling statistics generated from my implementation COST CENTRE MODULE %time %alloc hashElem SHA1 42.9 66.2 updateElem SHA1 12.7 16.7 unboxW SHA1 10.6 0.0 hashA80 SHA1 5.2 0.3 temp SHA1 4.6 0.0 sRotateL SHA1 4.6 0.0 ffkk SHA1 3.3 2.6 hashA16IntoA80 SHA1 3.1 0.1 sXor SHA1 2.9 0.0 do60 SHA1 2.9 2.6 sAdd SHA1 2.3 0.0 do20 SHA1 1.3 2.6 splitByN SHA1 1.2 2.3 do80 SHA1 0.8 2.6 do40 SHA1 0.4 2.6 Thanks, Anatoly

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impossible to write type signature
by jeff p 14 Jul '07

14 Jul '07

Hello, I managed to write a function, which ghc accepts, for which ghc rejects it's own type signature. I remember seeing a thread about this sometime in the last year or so, but I can't seem to find it. Does anyone remember this thread? thanks, Jeff

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[Math] Category theory research programs?
by Creighton Hogg 14 Jul '07

14 Jul '07

Hi Haskell, Sorry to contribute to the noise but given that we've been talking about categories lately, I was wondering if anyone had any opinions on good universities for studying category theory. I'm trying to figure out where to apply for my phd. I want to either be at a place with a strong category theory program or a strong differential geometry program. Thanks, Creighton

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money type ?
by Simon Michael 14 Jul '07

14 Jul '07

Good day all, my budding ledger program could not balance transactions exactly because of rounding error with Double. I *think* I got it working better with Rational (it was late). Another suggestion from #haskell was to multiply all money by 100. I'm tracking multiple currencies/commodities with varying precision so this gets a bit more complicated. Is there a type or library out there that's good for representing money and other quantities while avoiding rounding errors ? Best - Simon

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Simple newbie question - Int and Integer
by Gregory Propf 14 Jul '07

14 Jul '07

So what the hell is the difference between them? Int and Integer. They aren't synonyms clearly. What's going on? ____________________________________________________________________________________ Take the Internet to Go: Yahoo!Go puts the Internet in your pocket: mail, news, photos & more. http://mobile.yahoo.com/go?refer=1GNXIC

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