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Re: [Haskell-cafe] Re: Graph reduction [Was: Where is StackOverflow on the Wiki?]
by Andrew Coppin 25 Aug '07

25 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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Newbie question: Where is StackOverflow on the Wiki?
by Peter Verswyvelen 24 Aug '07

24 Aug '07

When reading an article about tail recursion (http://themechanicalbride.blogspot.com/2007/04/haskell-for-c-3-programmers. html) I came across the follow statements: "If you can write a non-recursive function that uses the colon syntax it is probably better than a tail recursive one that doesn't. This is because Haskell's lazy evaluation enabled you to use the non-tail recursive version on an infinite stream without getting a stack overflow. " and ""Unfortunately", laziness "gets in the way". While transforming non-tail-recursive code to a tail-recursive form is important and useful for functional programming in general, dealing with laziness requires a little more care, and often "non-tail-recursive" versions are preferrable. flatten is an example of this, the first version is better in many ways. While I don't believe it happens in this case, oftentimes naively writing code "tail-recursively" in Haskell will actually -make- it overflow the stack. Another (actual) benefit of the first version of flatten is that it will work on infinite lists. http://www.haskell.org/hawiki/StackOverflow gives a simple example and some explanation." Unfortunately I can't find the StackOverflow page anymore. Now if I understand this correctly, this just means that when writing something like: foo n = if n<0 then [] else n : foo (n-1) bar n = aux 0 [] where aux i xs = if i>n then xs else aux (i+1) (i:xs) that foo is more efficient than bar because lazy evaluation of foo just puts the delayed computation in the "cdr" of the list, while lazy evaluation of bar has to keep track of all aux calls (the "closures") which gives much more overhead, maybe even stack overflow? Something like that? Thanks, Peter

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Re: [Haskell-cafe] Re: Newbie question: Where is StackOverflow on the Wiki?
by Peter Verswyvelen 24 Aug '07

24 Aug '07

I think DrScheme does that for Scheme. So a must have for Haskell! Maybe could be added to the Helium project? >----- Oorspronkelijk bericht ----- >Van: apfelmus [mailto:apfelmus@quantentunnel.de] >Verzonden: donderdag, augustus 23, 2007 08:00 PM >Aan: haskell-cafe(a)haskell.org >Onderwerp: [Haskell-cafe] Re: Newbie question: Where is StackOverflow on the Wiki? > >Neil Mitchell wrote: >>>> sum (enum 1 10) => >>>> sum' 0 (enum 1 10) => >>>> ... >>>> >>>> sum' 36 (9 : enum (9+1) 10) => >>>> (sum' $! (36+9)) (enum (9+1) 10) => >>>> sum' 45 (enum (9+1) 10) => >>>> sum' 45 [] => >>>> 45 >>>> >>>> (I need to find some way to automate making these trails :) ) >>> Yes! We'd need such an automatic tool for the wikibook, too. >> >> The problem is that Haskell is ridiculously complex, and the "small >> step" interpretation is much harder than you'd think. For example, sum >> may well be defined as foldl' (+) 0, which is a CAF, so gets reduced >> once. The 0 won't actually be a 0, but will be fromInteger 0, which >> will correspond to looking up an item in the dictionary and applying >> it. Dictionaries especially make the "simple" interpretation >> completely wrong. >> >> It's easy to do informally, but once you start being more precise, its >> very complex. > >Yeah, the precise details may vary, even :) But for teaching, an >automatic tool that does graph reduction would be great. I don't mind if >it's sloppy (directly apply definitions & pattern matching VS everything >is a lambda abstraction) and only does simply typed lambda calculus (no >type applications, no type classes). > >Regards, >apfelmus > >_______________________________________________ >Haskell-Cafe mailing list >Haskell-Cafe(a)haskell.org >http://www.haskell.org/mailman/listinfo/haskell-cafe > >

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help understanding lazy evaluation
by Xavier Noria 24 Aug '07

24 Aug '07

I am learning Haskell with "Programming in Haskell" (an excellent book BTW). I have background in several languages but none of them has lazy evaluation. By now I am getting along with the intuitive idea that things are not evaluated until needed, but there's an example I don't understand (which means the intuitive idea needs some revision :-). We have factors(), defined on page 39 like this[*]: factors :: Int -> [Int] factors n = [x | x <- [1..n], n `mod` x == 0] and we base prime() on it this way: prime :: Int -> Bool prime n = factors n == [1, n] Now, the books says prime does not necessarily compute all of the factors of n because of lazy evaluation. Meaning that if n is composite as soon as some non-trivial divisor appears we halt computation and return False. My vague intuition said "we either need factors or we don't, we do because we need to perform the test, so we compute it". That's wrong, so a posteriori the explanation must be something like this: 1. someone knows we want factor() to perform an equality test 2. someone knows an equality test between lists is False as soon as we have a mismatch, left to right 3. thus, instead of evaluating factors completely we are going to build sublists of the result and perform the tests on those ones against [1, n]. That's a lot of *context* about that particular evaluation of factors, in particular step puzzles me. Can anyone explain how lazy evaluation fits there? I suspect the key is the implementation of == together with the fact that list comprehensions are lazy themselves, is that right? -- fxn [*] Which notation do you use for functions in text? is f() ok?

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