June 2007 - Haskell-Cafe

Re: I just don't get it (data structures and OO)
by Stefan O'Rear 03 Jun '07

03 Jun '07

(Sorry to break the thread, but mutt somehow managed to eat the message I'm replying to...) Arie Peterson: > You could also use 'compositional functional references'. These are > introduced in the paper "A Functional Programming Technique for Forms in > Graphical User Interfaces" by Sander Evers, Peter Achten and Jan Kuper. > > === Introduction === > > There are two things one typically wants to do when working with a > substructure of some larger data structure: (1) extract the substructure; > and (2) change the larger structure by acting on the substructure. A 'Ref > cx t' encodes both of these functions (for a substructure of type 't' and > larger structure (context) of type 'cx'). > > > data Ref cx t > > = Ref > > { > > select :: cx -> t > > , update :: (t -> t) -> cx -> cx > > } ... > I've written a template haskell function to derive Refs from a data > structure definition (with record syntax): given I've implemented this in Derive[1] in 12 minutes, counting the time required to re-familiarize with the code. The patch is at [2] and has also been darcs sent. [1] http://www-users.cs.york.ac.uk/~ndm/derive [2] http://members.cox.net/stefanor/derive-Ref-patch Stefan

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happs tutorial
by Anatoly Yakovenko 03 Jun '07

03 Jun '07

can someone help me get the first part of the tutorial working with happs 0.8.8 http://happs.org/HAppS/doc/tutorial.html this part at least : module Main where import HAppS import Control.Monad.State data MyState = MyState String deriving (Read, Show) instance StartState MyState where startStateM = return $ MyState "Hello World!" instance Serialize MyState where typeString _ = "MyState" encodeStringM = defaultEncodeStringM decodeStringM = defaultDecodeStringM app :: Method -> Host -> [String] -> Ev MyState Request Result app _ _ _ = do MyState message <- get sresult 200 message main = stdMain $ simpleHTTP "" [] app :*: End I am having a hard time getting everything to compile. Also, why is > :t HAppS.simpleHTTP HAppS.simpleHTTP :: forall st. [ServerPart (Ev st Request) Request IO Result] -> Conf -> Handler st but the type in this http://happs.org/auto/apidoc/HAppS-Protocols-SimpleHTTP.html#v%3AsimpleFile… docs is simpleHTTP :: XSLPath -> [(String, String)] -> (Method -> Host -> [String] -> Ev st Request Result) -> Conf -> Handler st

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Re: [Haskell-cafe] Implementing Mathematica
by Chaddaï Fouché 03 Jun '07

03 Jun '07

---------- Forwarded message ---------- From: Chaddaï Fouché <chaddai.fouche(a)gmail.com> Date: 2 juin 2007 00:00 Subject: Re: [Haskell-cafe] Implementing Mathematica To: Andrew Coppin <andrewcoppin(a)btinternet.com> 2007/6/1, Andrew Coppin <andrewcoppin(a)btinternet.com>: > I looked, I didn't find anything interesting. Well maybe you should look one more time with your brain on... Your claim that : > Last I heard, Maple is simply another fast number-chrunking engine. Show that you never even bothered to read the feature list of the software ! http://www.maplesoft.com/products/Maple11/academic/index.aspx?D=3040 I tried both Mathematica and Maple and while I'll be the first to admit the Mathematica interface was more attractive, it's the only obvious point I concede to it (and my version of Maple was quite old so this might not even be true anymore). As others who knows the subject have said, Mathematica and Maple are quite close and outperforms each other regularly in their fields of excellence. -- Jedaï

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New (?) Partial Monad
by Christopher Lane Hinson 02 Jun '07

02 Jun '07

Some time ago I noticed that a function I was writing had the exact type as liftM2, and I so I ended up generalizing a monad. I thought I should document it really well, and the result is this: http://www.downstairspeople.org/darcs/partial/Partial.pdf $ darcs get http://www.downstairspeople.org/darcs/partial I would appreciate any feedback. Is this even original? --Lane

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OpenGL
by Jon Harrop 02 Jun '07

02 Jun '07

I've found HOpenGL and the Debian package libghc6-opengl-dev. The former seems to be very out of date (last release 2003) but I can't find any demos for the latter. Where should I go to get started with OpenGL and Haskell? -- Dr Jon D Harrop, Flying Frog Consultancy Ltd. OCaml for Scientists http://www.ffconsultancy.com/products/ocaml_for_scientists/?e

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Resolved: [Haskell-cafe] ffi linking problem
by jeff p 02 Jun '07

02 Jun '07

Hello, In case anyone else finds this useful... My linking problem was finally resolved by using the -fvia-C flag when compiling with ghc. Thanks to Stefan O'Rear who pointed out the possibility and wrote: > > Does using -fvia-C help at all? The C compiler understands header > files and is sometimes better equipped to resolve things. > regards, Jeff

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how do I pass customization items in syb code?
by Alex Jacobson 01 Jun '07

01 Jun '07

I'm looking at the XML SYB example http://www.cs.vu.nl/boilerplate/testsuite/xmlish/Main.hs I'd like to find a way to pass other type customizations as arguments to data2content and content2data. I modified data2Content as follows: data2content f = element `ext1Q` list `extQ` string `extQ` f But I can't figure out how to give f a type such that (data2Content $ myFooElem `extQ` myBarElem) operates on both Foo and Bar elements. Right now I am giving f the type f::(a->[Content]) which compiles but causes the wrong behavior. Any recommendations on how to fix this? -Alex-

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Haskell Quiry
by ashutosh dimri 01 Jun '07

01 Jun '07

I want to read some data from a file "Signature.ken" , for that I have written a program but its showing some errors (the file "signature.ken" contains data of the type = (122,152) ) main = do j <- readFile "sig1.key" let (x,y) = (read j) putStrLn ("value=" ++ (show (x,y))) ERROR file:.\-as.hs:5 - Unresolved top-level overloading *** Binding : main *** Outstanding context : (Read c, Read b, Show c, Show b)

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Has anyone looked into adding subtyping to Haskell?
by Al Falloon 01 Jun '07

01 Jun '07

OCaml has been getting a lot of mileage from its polymorphic variants (which allow structural subtyping on sum types) especially on problems relating to AST transformations and the infamous "expression problem". Has there been any work on extending Haskell's type system with structural subtyping? What is the canonical solution to the expression problem in Haskell? What techniques do Haskellers use to simulate subtyping where it is appropriate? I bring this up because I have been working on a Scheme compiler in Haskell for fun, and something like polymorphic variants would be quite convinent to allow you to specify versions of the AST (input ast, after closure conversion, after CPS transform, etc.), but allow you to write functions that work generically over all the ASTs (getting the free vars, pretty printing, etc.). -- Alan Falloon

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