May 2008 - Haskell-Cafe

type-level integers using type families
by Peter Gavin 03 Jun '08

03 Jun '08

Has anyone else tried implementing type-level integers using type families? I tried using a couple of other type level arithmetic libraries (including type-level on Hackage) and they felt a bit clumsy to use. I started looking at type families and realized I could pretty much build an entire Scheme-like language based on them. In short, I've got addition, subtraction, & multiplication working after just a days worth of hacking. I'm going to post the darcs archive sometime, sooner if anyone's interested. I really like the type-families based approach to this, it's a lot easier to understand, and you can think about things functionally instead of relationally. (Switching back and forth between Prolog-ish thinking and Haskell gets old quick.) Plus you can do type arithmetic directly in place, instead of using type classes everywhere. One thing that I'd like to be able to do is lazy unification on type instances, so that things like data True data False type family Cond x y z type instance Cond True y z = y type instance Cond False y z = z will work if the non-taken branch can't be unified with anything. Is this planned? Is it even feasible? I'm pretty sure it would be possible to implement a Lambda like this, but I'm not seeing it yet. Any ideas? Pete

7 9

Rotating backdrop (aka learning Haskell)
by Yann Golanski 03 Jun '08

03 Jun '08

To help me learn Haskell, I decided on a simple (AH!) problem: given a list of images, display a random one as my desktop backdrop. After some time, change the image. Simple? What I actually want to do is a little more specific: Read a list of images (one per line) from a file. Given that a working day is about 8 hours, I want to see all the images during the day. So, the time between changes should be (nbr_of_images) / (8 * 60 * 60) seconds. Of course, if the file changes (I add or remove any number of images) this need to change and be recalculated. Clearly, I want some interaction with a pseudo-random number generator. Because this is a learning exercise, I want to have a pretty GUI for this. Three buttons: Exit (which quits the application), Reset (re-reads the file whether it changed or not) and Next (display the next image). Then I want a counter and a progress bar telling me when the next change will occur. 1- Get a list out of a file: I managed to do that using the following: parseImageFile :: FilePath -> IO [String] parseImageFile file = do inpStr <- readFile file return $ filter (/="") (breaks (=='\n') inpStr) Nice, simple and I understand what it is doing. 2- Get a random element from a list and remove it: Okay, this I understand less well. I looked at the solutions of problems 23 and 20 in http://www.haskell.org/haskellwiki/99_questions so there is a skeleton there. However, my list is IO [String].... Hum, monads. Any pointers as to how to do that? 3- Wait and do something later.... How, I have no idea how to do that! Help? 4- I guess that progress bars and updating text will be somewhere in the GUI (I chose wxHaskell)... Again, no idea where. 5- How do you call an external program in Haskell? Either xv or Esetroot will do the job of displaying the image. Is there a Haskell native way to do that? Once this is done and I have commented to the code, I will be happy to put it onto the wiki as a teaching aid. Thanks. -- yann(a)kierun.org -= H+ =- www.kierun.org PGP: 009D 7287 C4A7 FD4F 1680 06E4 F751 7006 9DE2 6318

7 8

[Newbie question] -- Looping stdin until condition is met
by Martin Blais 02 Jun '08

02 Jun '08

Allright, this is a definitely a newbie question. I'm learning Haskell and going through the exercises in the beautiful Hutton book, and one of them requires for me to write a loop that queries a line from the user (stdin), looping until the user enters a valid integer (at least that's how I want to implement the interface to the exercise). I have tried tons of variants and modifications of code, and I can't find the way to implement this. Here is what my emacs buffer is at now:: import Text.Read import System.IO import qualified Control.Exception as C getNum :: IO Int getNum = do line <- (hGetLine stdin) x <- (C.catch (do return (read line :: Int)) (\e -> getNum)) return x main = do x <- getNum putStr ((show x) ++ "\n") Now, I've tried the Prelude's catch, the Control.Exception catch, I've tried moving it at the top of getnum, I've tried without catch, I've tried a ton of other shtuff, so much that I'm starting to think that Emacs is going to run out of electrons soon. I asked some half-newbie friends who are insanely enthousiastic about Haskell and they can't do it either (I'm starting to think that those enthousiastic friends are dating a beautiful girl with 3 PhDs, but she has a 2-inch thick green and gooey wart smack on her nose and they're so blindly in love that they can't admit that she does). I've asked some university profs and they sidetrack my question by saying I shouldn't do I/O so early. Can anyone here restore my faith in the Haskell section of humanity? 1. How do I catch the exception that is raised from "read"? 2. Where do I find the appropriate information I need in order to fix this? I'm probably just not searching in the right place. (Yes, I've seen the GHC docs, and it doesn't help, maybe I'm missing some background info.) 3. Please do not tell me I should solve the problem differently. Here is the problem I'm trying to solve, and nothing else: "Write a program that reads a line from the user, looping the query until the line contains a valid integer." It shouldn't be too hard i think. The best answer would be a two-liner code example that'll make me feel even more stupid than I already do. Thanks in advance.

5 8

modelling C in Haskell ..
by Galchin, Vasili 02 Jun '08

02 Jun '08

Hello, I don't want to write kludgy Haskell code! typedef struct blah { int val1; union { int val2; struct { int val3; int val4; } } } C_type; question: in Haskell, can I embed definition of the "union" inside of the C typedef, i.e. recursion definition? Or must I have a separate definition for the "union" which I "instantiate" inside the Haskell "typedef", i.e. Haskell "data"? Kind regards, Vasili

4 7

Damnit, we need a CPAN.
by Achim Schneider 02 Jun '08

02 Jun '08

No, wait, there's cabal-install. And it doesn't even depend on 20+ packages I'd have to install manually. My day, for now, is saved, at least if portage installs it without hesitation. Ah, there it is: >>> Compiling source >>> in /var/tmp/portage/dev-haskell/cabal-darcs-0/work/cabal-darcs-0 ... * Bootstrapping Cabal... <no location info>: can't find file: Setup.lhs * * ERROR: dev-haskell/cabal-darcs-0 failed. Ok, let's try it the other way 'round...: % wget \ http://hackage.haskell.org/packages/archive/cabal-install/0.4.0/cabal-insta… % tar xvzf cabal-install-0.4.0.tar.gz % cd cabal-install-0.4.0 % runhaskell Setup.lhs configure --prefix=/home/ksf/cabal-inst --user Configuring cabal-install-0.4.0... % runhaskell Setup.lhs install Installing: /home/ksf/cabal-inst/bin Setup.lhs: dist/build/cabal/cabal: copyFile: does not exist (No such file or directory) % runhaskell Setup.lhs build Preprocessing executables for cabal-install-0.4.0... Building cabal-install-0.4.0... [...] % runhaskell Setup.lhs install Installing: /home/ksf/cabal-inst/bin % cabal update % cabal install phooey [...] type 'make' to build wxhaskell. type 'make install' to install wxhaskell. type 'make help' to receive help on all other make targets Configure Succeeded. Indeed. It succeeded. Now what? Building and installing by hand, it seems. Building works, installing too, but registering fails: the Makefile does not seem to like --user: % runhaskell Setup.lhs register --user -=-= Cabal executing: make register=-=- cat config/wxcore.pkg | sed -e "s|\${wxhlibdir}|/home/ksf/cabal-inst/lib|" | ghc-pkg update - Reading package info from stdin ... done. WARNING: unversioned dependencies are deprecated, and will NOT be accepted by GHC 6.10: base haskell98 Unable to rename "/usr/lib64/ghc-6.8.2/package.conf" to "/usr/lib64/ghc-6.8.2/package.conf.old" Saving old package config file... ghc-pkg.bin: /usr/lib64/ghc-6.8.2/package.conf: renameFile: permission denied (Permission denied) Just a second... # emerge wxhaskell -pv These are the packages that would be merged, in order: Calculating dependencies... done! [ebuild UD] dev-lang/ghc-6.4.2 [6.8.2] USE="X%* doc opengl%* -binary -ghcbootstrap -openal% -test%(-bash-completion%)" 0 kB [0=>1] [ebuild N ] dev-haskell/wxhaskell-0.9.4 USE="doc" 0 kB In other words: No way. The only program that touches anything in /usr/ is portage. Well, at least ghc-pkg update works on a mangled wxcore.pkg. Rationale: We need a CPAN, a cabal that is smart enough to know what to to, even if building depends on make, grapefruit authors that commit to hackage, a cabal-inst that can install from darcs (or at least from a local directory), or maybe just a make-replacement for haskell, like java has with ant (which is really cool if you successfully do not notice the xml-syntax). I would already shut up if there were an ebuild generator, but then I'm not that egoistic ;) Anyway, three hours after wanting to try it out, grapefruit is up and running, and I did not even have to install its dependencies by hand. It seems like grapefruit could benefit from nestable cabal packages, though: It has one big script that installs many packages. You could also call them dependencies. I'm going to shut up now. -- (c) this sig last receiving data processing entity. Inspect headers for past copyright information. All rights reserved. Unauthorised copying, hiring, renting, public performance and/or broadcasting of this signature prohibited.

13 19

Default definitions for associated type synonyms
by Dreixel 02 Jun '08

02 Jun '08

Hello, Does anyone know if it is possible to specify a default definition for an associated type synonym? When I tried: class A a where > type B a = a > GHC (version 6.9.20080309) told me: "Type declaration in a class must be a kind signature or synonym default". However, when I look at the parser I see no way to specify such synonym default. Is this possible? Thanks, Zé Pedro

3 2

GHCi panic
by Andrew Coppin 02 Jun '08

02 Jun '08

I don't suppose this will surprise anybody greatly, but... Apparently if you write a Haskell module that is 400 KB in size and defines a single CAF consisting of a 45,000-element [String], GHCi panics when attempting to load it interpretted, and hits a stack overflow attempting to load it compiled. GHC also takes forever to compile it in the first place, and eventually spits out a 5 MB interface file later followed by a 16 MB object file. And attempting to compile a trivial module against it again causes a stack overflow. Presumably the designers of GHC just didn't expect anybody to try to do anything this weird? ;-) I was hoping that doing things this way round would be *more efficient*. But this is apparently not the case at all, so I'll just go back to reading the file at runtime instead... [Presumably if I was desparate I could convert the data into some kind of preinitialised C structure and manually link it in - if I was that determined.]

3 2

Images and GUIs in Haskell
by Ronald Guida 02 Jun '08

02 Jun '08

Two questions: 1. In a Haskell program, if all I want to do is output an image, like a graph or chart, what is the simplest library to use? N.B. Simpler := easier to get minimal functionality. I really don't want to wade through a bunch of boilerplate or climb a steep learning curve just to be able to plot a few lines or circles. 2. Suppose I want interactivity. For example, I want to plot a line graph, and then let the user click and drag the data points. From what I understand about GUIs, I would need to track mouse buttons (up and down), mouse movements, and possibly keystrokes. (I know this is the complete opposite extreme from my first question) In this case, what would be the best (not necessarily simplest) library to use? What would you recommend?

3 4

haskell compiler on NetBSD amd64
by Donn Cave 02 Jun '08

02 Jun '08

So here I am with 64 bit Athlon hardware, running amd64 NetBSD (a.k.a. x86_64), reasonably motivated to compile Haskell. From what I make of the porting instructions, my only hope is to install FreeBSD-amd64 on another partition and download the 6.6.1 ghc for that platform, use it to generate .hc files to build 6.6.1 on NetBSD, and then use 6.6.1 to build 6.8 or whatever the current version may be by the time I get there? -- Donn Cave <donn(a)avvanta.com>

4 13

Re: [Haskell-cafe] hmatrix
by Thomas Hartman 01 Jun '08

01 Jun '08

what package do you install/import to get at Vector? 2008/5/31 Thomas Hartman <tphyahoo(a)gmail.com>: > what package do you install/import to get at Vector? > > 2008/5/31 Anatoly Yakovenko <aeyakovenko(a)gmail.com>: >> What is the most efficient way to update a position in a matrix or a >> vector? I came up with this: >> >> updateVector :: Vector Double -> Int -> Double -> Vector Double >> updateVector vec pos val = vec `add` v2 >> where >> v2 = fromList $ (replicate (pos) 0.0) ++ ((val - (vec @> >> pos)):(replicate ((dim vec)- pos - 1) 0.0)) >> >> but this seems pretty inefficient to me. >> >> thanks, >> Anatoly >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> >

2 1