July 2007 - Haskell-Cafe

In-place modification
by Isaac Gouy 16 Jul '07

16 Jul '07

On Jul 15, 1:25 pm, "Hugh Perkins" <hughperk...(a)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

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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.

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hMapping polymorphic functions
by jeff p 15 Jul '07

15 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

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Perfect shuffle on std. libs
by Felipe Almeida Lessa 15 Jul '07

15 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.

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Another monad question...
by David LaPalomento 15 Jul '07

15 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

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