September 2008 - Haskell-Cafe

Hyena Status? Re: [Haskell-cafe] Starting Haskell with a web application
by Donnie Jones 17 Sep '08

17 Sep '08

Hello Johan Tibell, Hyena looks very interesting. From the github tracking, you've been working... Maybe a release soon? Also, I saw your slides from the 'Left-fold enumerators' presentation at Galois. Maybe include the slides in the docs/ for a release? Thank you. __ Donnie On Thu, Mar 6, 2008 at 7:40 AM, Johan Tibell <johan.tibell(a)gmail.com> wrote: > > Do you (both) have repos that I could download from? I quite interested > > in both projects, esp. the WSGI clone. > > Yes and no. The code [1] is in my darcs repository but is in an > unusable state until I've fixed my incremental parser (in > Hyena/Parser.hs) which I plan to do next week. I would like the first > release to be nice and polished so I'm trying to not release anything > prematurely. > > 1. http://darcs.johantibell.com/hyena/ > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

2 2

Problem with hscurses
by david48 17 Sep '08

17 Sep '08

the getCh funtion is supposed to return an interpreted Key with values like KeyChar c, KeyReturn, KeyBackspace, etc. but in fact, it only ever returns KeyChar c values ! am I doing anything wrong ? Here's an example program : module Main where import UI.HSCurses.Curses import Text.Printf import System.IO main = do hSetBuffering stdout NoBuffering withCurses test1 withCurses f = do initScr echo False cBreak True res <- f endWin return res test1 = do c <- getCh printf "Touche : %s\r\n" (show c) test1

1 1

hircules resurrected (partially ;-)
by wman 17 Sep '08

17 Sep '08

I stumbled upon hircules-0.3 IRC client and decided to take it for a test drive. I managed to get it to compile under ghc-6.8.3/win32, converted it to use Text.Codec.Iconv (the old version of included Iconv.hs should also work), but it seems it doesn't work. It only gives an empty form and according to wireshark doesn't even attempt a connection. If somebody knew hircules in it's days of glory and/or wants to go bug-hunting, i'll gladly provide the modified version, as I probably won't spend more time on it in near future (at least until i get to know gtk2hs better).

1 0

Comparing GADTs for Eq and Ord
by Tom Hawkins 17 Sep '08

17 Sep '08

Hi, How do I compare a GADT type if a particular constructor returns a concrete type parameter? For example: data Expr :: * -> * where Const :: Expr a Equal :: Expr a -> Expr a -> Expr Bool -- If this read Expr a, the compiler has no problem. instance Eq (Expr a) where Const == Const = True (Equal a b) (Equal x y) = a == x && b == y _ == _ = False GHC throws: Couldn't match expected type `a1' against inferred type `a2' `a1' is a rigid type variable bound by the constructor `Equal' at Test.hs:9:3 `a2' is a rigid type variable bound by the constructor `Equal' at Test.hs:9:18 Expected type: Expr a1 Inferred type: Expr a2 In the second argument of `(==)', namely `x' In the first argument of `(&&)', namely `a == x' I believe I understand the reason for the problem; even though Equal has a type Expr Bool, two Equal expressions could have parameters of different types. But I'm not sure how to get around the problem. Any suggestions? Thanks, Tom

7 12

haskell blas bindings: does iomatrix gemv transposing of matrix a?
by Anatoly Yakovenko 17 Sep '08

17 Sep '08

Hey Patric, Thanks for your great work on the blas bidnings. I have a question on gemv. I thought its possible for blas to transpose the input matrix before doing the multiplication. Is it possible to do that with the haskell bindings? Or am I mistaken in how gemv is used Thanks, Anatoly

1 0

Semantic Domain, Function, and denotational model.....
by Daryoush Mehrtash 16 Sep '08

16 Sep '08

I have been told that for a Haskell/Functional programmer the process of design starts with defining Semantic Domain, Function, and denotational model of the problem. I have done some googling on the topic but haven't found a good reference on it. I would appreciate any good references on the topic. thanks, daryoush ps. I have found referneces like http://en.wikibooks.org/wiki/Haskell/Denotational_semantics which talks about semantic domain for "the Haskell programs 10, 9+1, 2*5" which doesn't do any good for me. I need something with a more real examples.

4 10

SYB with class: Bug in Derive.hs module
by Alexey Rodriguez Yakushev 16 Sep '08

16 Sep '08

Hi people (and Ralf and Alex), I found a bug in the SYB with class library when trying to implement generic equality. I am hoping that someone in the Cafe (maybe Ralf) can confirm it is a bug, or maybe show me that I am doing something wrong. I am using the "Scrap your boilerplate with class" library (http:// homepages.cwi.nl/~ralf/syb3/). More precisely, I am using the library distributed by the HAppS project, because it works with GHC 6.8 . You can get the repository as follows: darcs get http://happs.org/HAppS/syb-with-class However, the offending module (Derive.hs) produces broken instances in both distributions. The bug: ---------- Generic equality needs type safe casting when implemented in SYB3, I have tried both the gzipwith variant and using Pack datatypes (geq*.hs in the first distribution). However, both functions loop when applied to a tree value. This loop occurs when the functions try to cast one of the arguments. I have managed to reduce the error to a smaller source file that I send attached. It does the following: > main = print typeReps > > tree = (Bin (Leaf 1) (Leaf 2))::BinTree Int > > data Pack = forall x. Typeable x => Pack x > > packedChildren = gmapQ geqCtx Pack tree > > typeOfPack (Pack x) = typeOf x > > typeReps = map typeOfPack packedChildren Basically the tree is transformed into a list of Pack-ed values and then to a list of type representations. This program loops at "typeOf" when you call "main". The Data instance that Derive generates is as follows: > instance (Data ctx a, > Data ctx (BinTree a), > Sat (ctx (BinTree a))) => > Data ctx (BinTree a) where Note the recursive |Data ctx (BinTree a)| in the context. If I get rid of it (a correct manual instance is also included in the attachment) the example works. I thought of removing this from the context in the Derive source. But maybe I might break some other use cases. So I am asking for help! Should Derive be fixed? How? Cheers, Alexey

3 2

Hackage checks
by Andrea Rossato 16 Sep '08

16 Sep '08

Hi, the Hackage upload script performs some checks on the packages being uploaded. In my case, a library, I need to use the cabal generated Paths_package file to access some locale data stored in a data file directory. This requires to include in "hs-source-dirs" "dist/build/autogen", which doesn't exist in the source tree, since it's generated by cabal in the build process (the inclusion is made necessary by a bug in Cabal-1.2 which doesn't automatically search dist/build/autogen, see [1]). And so Hackage complains: I needed to manually include such an empty directory in the source code archive. Any way out? TIA Andrea [1] http://www.haskell.org/pipermail/libraries/2008-July/010078.html

3 3

Import qualified, inverse of hiding
by Mauricio 16 Sep '08

16 Sep '08

Hi, 'import' allows one to say 'hiding' to a list of names. Is it possible to do the opposite, i.e., list the names I want to import? Something like "import Module showing x"? Thanks, Maurício

4 7

Should `(flip (^^))` work?
by Mauricio 16 Sep '08

16 Sep '08

Hi, I tested the expression below and it doesn't work. Is there some way to achieve that (i.e., turning an expression inside parenthesis into an operator)? 2 `(flip (^^))` (3%4) Thanks, Maurício

5 5