August 2007 - Haskell-Cafe

Remember the future
by Andrew Coppin 25 Aug '07

25 Aug '07

I've seen comments in various places that monads allow you to "borrow things from the future". That sounds completely absurd to me... can anybody explain?

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Re: [Haskell-cafe] Style
by Marc A. Ziegert 25 Aug '07

25 Aug '07

whops... i did check it, but that was a copypaste mistake. buggy: > tm_parallelizable_v1 = \n -> sum . takeWhile (>0) $ map (div n) fives > where fives = iterate (*5) 1 should be: > tm_parallelizable_v1 = \n -> sum . takeWhile (>0) $ map (div n) fives > where fives = iterate (*5) 5 - marc Am Freitag, 24. August 2007 schrieben Sie: > Hi Marc > > First off, thanks for your reply. > >> tm_parallelizable_v1 = \n -> sum . takeWhile (>0) $ map (div n) fives > >> where fives = iterate (*5) 1 > Did you check this one? IMHO I think it's producing the 'wrong' answer. > > *Main> tm_parallelizable_v1 100 > 124 > (0.00 secs, 0 bytes) > > *Main> tm 100 > 24 > (0.00 secs, 0 bytes) > > If comparing the result to the other variants is accepted as a sort > of proof. ;-) > > But calculating the number of trailing zero's of n! is a matter of > counting powers of five in the factorized n!: f.e.: > > 10! = 1 2 3 2^2 5 2*3 7 2^3 3^2 2*5 > --> 5^2 --> picking up enough powers of 2 --> (2*5)^2 = 100 > > So you will have to correct your 'fives' to f.e. > > fives = tail $ iterate (*5) 1 > > > > Regards > > > @@i > > >

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Help using CGIT
by Rich Neswold 25 Aug '07

25 Aug '07

Hello! I've been having a tough time trying to use the CGI monad transformer (CGIT). Hopefully someone can show me my misstep(s). I have a CGI script that's evolving. Early on, it needed a single database access. Now it's doing two accesses (and it looks like I'll be adding more.) Rather than making a connection for each access, the script needs to connect once. To do this, I want to combine the CGI monad with the Reader monad. This version compiles cleanly: > module AppMonad (App (..), runApp) > where > > import Control.Exception (bracket) > import Control.Monad.Reader > import Network.CGI.Monad > import Network.CGI.Protocol > import System.IO (stdin, stdout) > import Database.HSQL.PostgreSQL > > newtype App a = App (ReaderT Connection (CGIT IO) a) > deriving (Monad, MonadIO, MonadReader Connection) > > runApp :: App CGIResult -> IO () > runApp (App a) = > bracket (connect "host" "dbname" "user" "password") > disconnect > (\c -> do { env <- getCGIVars > ; hRunCGI env stdin stdout (runCGIT (runReaderT a c)) > ; return () } ) Unfortunately, when another module tries to actually use the monad, I get warnings about "No instance for (MonadCGI App)". I tried making an instance: > instance MonadCGI App where > cgiAddHeader = ? > cgiGet = ? But I don't know how to define these functions. I tried various 'lift'ing combinations, but couldn't come up with a solution that would compile. I'm also disappointed that I had to break apart 'runCGI' (by cut-and-pasting its source) because I couldn't make it believe my monad looked enough like MonadCGI. My previous experiment with monad transformers was successful. It didn't use CGIT, however, so the 'run*' functions were simpler. Does anyone have an example of using CGIT (I didn't find any from Google)? Shouldn't I be able to use 'runCGI' with my monad? CGIT users shouldn't be required to re-implement 'runCGI", right? Any help or ideas is appreciated! -- Rich JID: rich(a)neswold.homeunix.net AIM: rnezzy

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Re: [Haskell-cafe] Re: Graph reduction [Was: Where is StackOverflow on the Wiki?]
by Andrew Coppin 24 Aug '07

24 Aug '07

Shiqi Cao wrote: > Check out this > > http://www.cas.mcmaster.ca/~kahl/HOPS/ > <http://www.cas.mcmaster.ca/%7Ekahl/HOPS/> > Heh. I was thinking about trying to build something *exactly like* this... OOC, how the heck did they make it work through a "document interface"? Surely that's impossible?

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IO inside CGI
by Adrian Neumann 24 Aug '07

24 Aug '07

-----BEGIN PGP SIGNED MESSAGE----- Hash: RIPEMD160 I'm toying around with web programming in Haskell. I'm trying to write a script which GETs an id and returns a couple of random numbers. Something like this: cgiMain :: CGI CGIResult cgiMain = do inp <- getInput "id" let gen = parse inp output $ take 10 (randoms gen) parse :: Maybe String-> StdGen parse (Just x) = read x parse Nothing = undefined Now I'd like to get a new StdGen, in case no id was supplied to the script. cgiMain :: CGI CGIResult cgiMain = do inp <- getInput "id" gen <- parse inp output $ take 10 (randoms gen) parse :: Maybe String-> IO StdGen parse (Just x) = return $ read x parse Nothing = getStdGen Obviously this doesn't work because I'm trying to do IO inside CGI (right?). Is there some incantation I can perform to make this possible? Like gen <- arcaneMagic parse inp As you probably have noticed I don't know very much about monads, all I did until now was reading or writing some files. Thanks in advance -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.7 (MingW32) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org iD8DBQFGzudU11V8mqIQMRsRA33RAJ9buZDHgz/eXi8Jw9OBwbTErDccRgCfbGrr 1WXiGHmxlTBe01E409yJyv8= =XSDj -----END PGP SIGNATURE-----

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2D game graphics library for Haskell?
by Ben Lippmeier 24 Aug '07

24 Aug '07

Hi Peter, The OpenGL/GLUT bindings support all the things you would want, but it's a bit too much pain for first year students. For the last couple of years at the ANU (Australian National University, Canberra) we've been using a front end library that I wrote which is similar to SOE/HGL but with support for images, animation, alpha blending and some other things. I think the real trick is hiding enough of the OpenGL/GLUT internals to make it suitable for first year students, while at the same time exposing enough functionality so they don't feel constrained by what they can do with the library. Usually we think of the project we want the students to do, then supply most of the infrastructure via the library - leaving the students to fill in the 'fun' stuff. There is the added benefit that the OpenGL/GLUT bindings (and hence our library also) compiles out of the box on both Linux and Windows. We use linux machines at uni for the student labs, but students have been able to take their code home and get it running on their home Windows PC's without much difficulty. You can get our library (with examples) from my homepage at http://cs.anu.edu.au/people/Ben.Lippmeier/ I've also got a simple asteroids game which I wrote with an extended version of it. There is a playable version, but unfortunately it's on my home machine that is now packed in storage while I'm at MSR, Cambridge. I'm getting back to Canberra late October so if you're still interested I can dig it out and send you a copy then. Cheers, Ben. From: haskell-cafe-bounces(a)haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of peterv Sent: 24 August 2007 11:32 To: haskell-cafe(a)haskell.org Subject: [Haskell-cafe] 2D game graphics library for Haskell? I'm currently playing around with SOE to make some simple interactive math exercises for students. This worked fine, although I could have done this much faster using C# (which I know very well), but since I'm addicted to Haskell now, I used the latter language ;) Furthermore, I hope that one day, I will know enough Haskell to learn it to the students, because I feel that functional programming should not be given in the last bachelor or master years, since most software engineering students then know OO programming extremely well and have a horrible time with FP (I currently did not meet anyone in my sector of game development that liked FP, and many of those people had a masters degree and some were PhDs) Anyway, SOE is great for learning Haskell, but it lacks a couple of fundamental functions to make it really attractive, like: - Support for images - Support for rendering to an "offscreen graphics surface" and reading the pixels from that surface (for pixel-wise collision detection) - Support for detecting non-ASCII key presses (cursor keys, etc) - Support for joysticks Concurrent Clean seems to have a nice 2D game library and PLT/DrScheme also has nice support for basic 2D graphics, but somehow I feel Haskell is more mature and more elegant. So before digging into "advanced" APIs (like GTK itself, which I know nothing about, I'm a Win32 GDI/XNA/WPF expert), I should ask the question if something similar exists? It has to be as simple as SOE. Would it be possible to extend the GTK SOE with support for the features mentioned above? Is this insanely difficult for someone like me who knows a lot about Win32 but little Haskell? Thanks, Peter Verswyvelen No virus found in this outgoing message. Checked by AVG Free Edition. Version: 7.5.484 / Virus Database: 269.12.4/969 - Release Date: 23/08/2007 16:04

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newbie : multi-parameter type classes
by Thomas Girod 24 Aug '07

24 Aug '07

Hi there. I'm trying to define a generic graph type here and don't understand on one error I get. Here I come. module Graph where class (Eq n, Eq e) => Topo a n e where empty :: a nodes :: a -> [n] edges :: a -> [e] data Node n = Node n deriving (Eq,Show) data Edge n e = Edge e (Node n) (Node n) deriving (Show) instance (Eq e) => Eq (Edge n e) where (Edge e1 _ _) == (Edge e2 _ _) = e1 == e2 data Graph n e = Graph [Node n] [Edge n e] deriving (Eq,Show) instance (Eq n, Eq e) => Topo (Graph n e) (Node n) (Edge n e) where empty = Graph [] [] nodes (Graph ns _) = ns edges (Graph _ es) = es My class Topo (for topography) is supposed to give the basic interface any graph should have. My instance is Topo (Graph n e) (Node n) (Edge n e), so the infered types of the functions should be : empty :: Graph n e nodes :: Graph n e -> [Nodes n] edges :: Graph n e -> [Edge n e] right ? When I load the code in GHCi, no errors. But then : *Graph> let g = Graph [Node 0, Node 1] [] *Graph> nodes g <interactive>:1:0: No instance for (Topo (Graph Integer e1) n e) arising from use of `nodes' at <interactive>:1:0-6 Possible fix: add an instance declaration for (Topo (Graph Integer e1) n e) In the expression: nodes g In the definition of `it': it = nodes g *Graph> And I don't understand this. How can he search for an instance of Topo (Graph integer e1) n e) ? regards, Thomas

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Re: [Haskell] Issue about use of WinHugs
by Neil Mitchell 24 Aug '07

24 Aug '07

Hi > I'm new to WinHugs, what's wrong with isUpper of my WinHugs? Nothing. The book/tutorial you are going from is out of date. Before using the isUpper/isLower functions you first have to type ":load Char": Hugs> :load Char Hugs> filter isUpper "ABCDEfgh" "ABCDE" The ":load Char" loads the Char module into scope, which provides the isUpper function. The haskell@ mailing list is mainly for announcements, so I've sent this email to haskell-cafe@ - which is the right place for questions like this. Thanks Neil

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latest cabal conflict?
by Luc TAESCH 24 Aug '07

24 Aug '07

when trying to build the latest cabal from darcs, I got luc@haserv:~/src/cabinstall/cabal$ runghc Setup.lhs configure Distribution/Simple.hs:110:7: Could not find module `System.FilePath': it was found in multiple packages: filepath-1.0 FilePath-0.11 luc@haserv:~/src/cabinstall/cabal$ sound like a conflict , i still have ghc 6.6, not 6.6.1. what should I do ? hide FilePath-0.11 in Distribution/Simple.hs imports ? ( and for all othe files? any way to just have FilePath out of the way via a Cabal statement? thks

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Haddock: documenting parameters of functional arguments
by Henning Thielemann 24 Aug '07

24 Aug '07

I like to write documentation comments like fix :: ( a {- ^ local argument -} -> a {- ^ local output -} ) -> a {- ^ global output -} but Haddock doesn't allow it. Or is there a trick to get it work?

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