October 2009 - Haskell-Cafe

Re: Finally tagless and ghc-extensions
by oleg＠okmij.org 30 Oct '09

30 Oct '09

> In both, the JFP & APLAS paper, the authors note that no fancy type > extension are needed for this approach. But in almost every (Haskell) > example that is a bit more involved the authors do make use of > functional dependencies and / or type families. GADTs are also being > used, although *not* in the syntax definition... Just to dispel the doubts, here is the Haskell98 version of the previously posted code CB.hs (tagless final interpreter for call-by-name, call-by-value and call-by-need lambda-calculus with integers and constants) http://okmij.org/ftp/Haskell/CB98.hs It may appear somewhat less convenient. How less convenient -- well, you decide. The code can be easily re-written in OCaml, which has no typeclasses, type families and other bleeding edge features. Talking about it here is probably not appropriate. (I wonder if someone has ever cross-posted to the Haskell and Caml lists). P.S. > I'm still digesting the "Finally Tagless" approach from Oleg, Jacques > and Chen. You probably mean Chung-chieh Shan, whose last name is Shan and the first name is Chung-chieh. http://en.wikipedia.org/wiki/Chinese_name

1 0

Singapore Functional Programmer Group First Meetup
by kenny lu 30 Oct '09

30 Oct '09

Sorry for the late notice. We are organizing an informal meeting for the Functional Programmer Group in Singapore on 2 Nov 2009. Since this is the first meeting, the theme will be mainly 'meet and greet' and discuss our interests in functional programming languages. We have participants coming from both the industry and the academia. If you happen to be in Singapore and interested in sharing your experience and interests, please contact Kenny at luzhuomi_AT_gmail_DOT_com. Best Regards, Kenny Zhuo Ming Lu

1 0

wxMac/wxHaskell focus problem, a quick solution
by Iain Barnett 30 Oct '09

30 Oct '09

I was trying to go through some of the wxHaskell examples, and they wouldn't work on my (Tiger) mac. Same thing happened with a few other wx apps I downloaded from Hackage. A bit of a rummage on the internet turned up an issue with wxMac[1]. You need to put the haskell executable in an OSX Application Bundle or you can't get focus on the window. Since I don't know how to use XCode (and quickly got bored of trying to find what I needed to do with it for my needs) I setup an applescript that builds a bare bones app bundle, with a basic plist file too. This is probably something others have gone though and, just in case it might be some use to others, I stuck it here (might be a little bit of overkill using Sourceforge, but I wasn't go to pay for the download space on my blog considering I have zero cash right now). http://sourceforge.net/projects/makebundle/ The applescript has a perl script in it too, so you'll need perl on your machine for it to work. Everything you need to know is in the readme. Iain [1] http://wiki.wxwidgets.org/WxMac_Issues#My_app_can. 27t_be_brought_to_the_front.21

6 10

Marshalling arrays without flakiness?
by Ron de Bruijn 29 Oct '09

29 Oct '09

Hi, I would like to know whether there is a good way to marshal the following structure to C without using pointer arithmetic done by a programmer (as opposed to a tool). typedef struct{ double a[10]; double b[10]; double b[10]; } foo; I don't need this functionality, but it would make Haskell -> C interfaces less flaky. Basically, writing Haskell -> C interfaces seem to be a very unpopular thing to do (as opposed to C -> Haskell interfaces). For example, on Hackage (actually a checkout of a few months ago) there is only one package using pokeArray and it is not pretty; it uses pointer arithmetic and all other kinds of hard-coded addresses. If I change the order of the C fields it will break, if I change the length of the buffer they use, it breaks. Just look at it, and it will break ;) If you are a bit creative, it is possible to make it work already, by using the best features of both hsc2hs and c2hs, but it is hardly elegant. I suppose the real solution would be to extend c2hs with an offsetof function (for which patches already exist, btw), but it might be that I have missed some other solution. -- Best Regards, Ron de Bruijn, Developer, Gamr7

2 2

Command-line options idea
by Robin Green 29 Oct '09

29 Oct '09

This is not in any way specific to Haskell, but I know a number of packages on Hackage have executables with command-line options, so: The non-Haskell program get_iplayer has an excellent idea for command-line options parsing. Rather than having to remember two separate syntaxes, one for command line options and one for a preferences file, the user can simply type get_iplayer --prefs-add --OPTION[=VALUE] which will add the equivalent of --OPTION[=VALUE] to the get_iplayer preferences file (and do nothing else). So it's like a special "preferences file manipulation mode" of the program. There are also the following options, as described by --help: --prefs-clear Remove *ALL* saved user or preset options --prefs-del Remove specified saved user or preset options --prefs-show Show saved user or preset options (This also means that the user doesn't have to remember the filename of the preferences file.) I think this sort of thing would be a good idea for *any* program that has command-line options that some user might want to hold constant across several runs. -- Robin

2 1

frag game - compilation fixes
by Phil Jones 29 Oct '09

29 Oct '09

I've hacked through (senselessly) the various compilation errors (I think they were all related to GLfloat vs. Float, etc.) Frag now compiles and works, but I think I may have introduced some bugs (the weapon doesn't appear on the screen?) Unfortunately, I did the whole job on an unpacked cabal package from hackage, and also I've never used darcs. So here's the resulting package tree (excluding the big files). If anyone knows how to turn it into a darcs working copy and create a patch from it, please do!

3 4

ANNOUNCE: GeBoP 1.7
by Henk-Jan van Tuyl 29 Oct '09

29 Oct '09

L.S., I have updated and cabalized Daan Löffler's GeBoP and uploaded it to Hackage. GeBoP, the General Boardgames Player, offers a set of board games: Ataxx, Bamp, Halma, Hez, Kram, Nim, Reversi, TicTacToe, and Zenix. The application also features a brain viewer which enables you to explore the computer's internal game tree. The homepage is: http://www.haskell.org/haskellwiki/GeBoP Regards, Henk-Jan van Tuyl -- http://functor.bamikanarie.com http://Van.Tuyl.eu/ --

1 0

Finally tagless and ghc-extensions
by Günther Schmidt 28 Oct '09

28 Oct '09

Hi all, ok, I haven't let go on the EDSL subject just yet. I'm still digesting the "Finally Tagless" approach from Oleg, Jacques and Chen. In both, the JFP & APLAS paper, the authors note that no fancy type extension are needed for this approach. But in almost every (Haskell) example that is a bit more involved the authors do make use of functional dependencies and / or type families. GADTs are also being used, although *not* in the syntax definition but merely in the semantic instances, I'm cool with that :) . I haven't touched base yet with either functional dependencies nor type families as I understand the edges on neither are yet worked out, so I'd prefer being able to avoid either when using the tagless approach even in more involved settings. Is that possible? Günther

1 0

cabal probems with binary/containers packages
by brian 28 Oct '09

28 Oct '09

It all started with this: Loading package binary-0.5.0.1 ... can't load .so/.DLL for: HSbinary-0.5.0.1 (dlopen(libHSbinary-0.5.0.1.dylib, 9): image not found) so I tried > cabal upgrade binary Resolving dependencies... Configuring containers-0.2.0.1... Preprocessing library containers-0.2.0.1... Building containers-0.2.0.1... Data/IntMap.hs:182:7: Could not find module `Data.Data': it is a member of package base, which is hidden cabal: Error: some packages failed to install: binary-0.5.0.2 depends on containers-0.2.0.1 which failed to install. containers-0.2.0.1 failed during the building phase. The exception was: exit: ExitFailure 1 help.... Thanks, Brian

2 5

simple parsing
by satorisanitarium 28 Oct '09

28 Oct '09

I'm trying to parse a list of numbers plus four diferent signs (+-*/) in this way: Lets say the list is "32+5/46" result would be "2+5/4" I get: "2+5/4*** Exception: geneticSimple.hs:(55,0)-(65,35): Non-exhaustive patterns in function chromoResult If the list is "32+5**6" result would be "2+5*6" I get: "2+5/*** Exception: geneticSimple.hs:(55,0)-(65,35): Non-exhaustive patterns in function chromoResult If the list is "32+-72" resoult would be "2+7" I get: "2+*** Exception: geneticSimple.hs:(55,0)-(65,35): Non-exhaustive patterns in function chromoResult code: chromoResult [] = [] chromoResult (a:b:c:xs) | elem a "0123456789" && elem b "0123456789" && elem c "0123456789" = chromoResult (c:xs) | elem a "0123456789" && elem b "0123456789" && elem c "+-*/" = b:c: chromoResult xs | elem a "0123456789" && elem b "+-*/" && elem c "0123456789" = a:b:c : chromoResult xs | elem a "0123456789" && elem b "+-*/" && elem c "+-*/" = a:b : chromoResult (c:xs) | elem a "+-*/" && elem b "0123456789" && elem c "0123456789" = chromoResult (b:c:xs) | elem a "+-*/" && elem b "0123456789" && elem c "+-*/" = b:c : chromoResult xs | elem a "+-*/" && elem b "+-*/" && elem c "0123456789" = chromoResult (c:xs) | elem a "+-*/" && elem b "+-*/" && elem c "+-*/" = chromoResult xs | otherwise = chromoResult (b:c:xs) I suspect my approach is flawed but i have exausted my ideas. I need a fresh approach so if anybody would be kind enough and just give me a hint how to approach the problem. Thx in advance.

4 3