April 2007 - Haskell-Cafe

A wish for relaxed layout syntax
by Benjamin Franksen 21 May '07

21 May '07

Hi, I often run into the following issue: I want to write a list of lengthy items like this mylist = [ quite_lengthy_list_item_number_one, quite_lengthy_list_item_number_two, quite_lengthy_list_item_number_three ] With the current layout rules this is a parse error (at the closing bracket). Normally I avoid this by indenting everything one level more as in mylist = [ quite_lengthy_list_item_number_one, quite_lengthy_list_item_number_two, quite_lengthy_list_item_number_three ] but I think this is a little ugly. Same issue comes up with parenthesized do-blocks, I would like to write when (condition met) (do first thing second thing ) So my wish is for a revised layout rule that allows closing brackets (of all sorts: ']', ')', '}') to be on the same indent level as the start of the definition/expression that contains the corresponding opening bracket. Cheers Ben

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Compilling GHC on Vista
by Monique Monteiro 13 May '07

13 May '07

Hi all, I'm trying to compile GHC on Windows Vista, but I encountered the following error when running ./configure --host=i386-unknown-mingw32 --with-gcc=c:/MinGW/bin/gcc: configure: error: C compiler cannot create executables See `config.log' for more details. ./configure: line 3404: cannot create temp file for here document: No such file or directory ./configure: line 3441: cannot create temp file for here document: No such file or directory Has anyone any idea about what may be happening? Thanks in advance, Monique -- __________________________________________________________ Monique Monteiro, MSc MCP .NET Framework 2.0 / SCJP / IBM OOAD Project Manager Recife Microsoft Innovation Center +55 81 34198137 http://www.cin.ufpe.br/~mlbm http://thespoke.net/blogs/moniquelouise/default.aspx monique(a)qualiti.com.br MSN: monique_louise(a)msn.com

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Re: [Haskell-cafe] generate Haskell code from model
by Brian Smith 11 May '07

11 May '07

On 4/14/07, Steffen Mazanek <steffen.mazanek(a)googlemail.com> wrote: > > Brian, but don't you think that you have to write a lot > of boilerplate code in Haskell? I have never felt I was writing a lot of boilerplate. There are a lot of abstraction mechanisms in Haskell to avoid boilerplate. Second, if Haskell should be more successful in the > real world there has to be a way of demonstrating > basic ideas of a big program to "customers". How > would you do this? Everybody knows UML class > diagrams, for example. In contrast, nobody knows > about termgraphs or lambda *g*. I've never had to show a UML or ER diagram to any business people--usually they want a slideshow that is far simpler and a little prettier. The fact that "nobody knows about termgraphs or lambda" in your group means that you probably shouldn't be considering Haskell (for the same reason my bosses always asked me to document everything--"in case you get hit by a bus"). Thank you very much for contributing to the discussion. > Please assume, that you have to generate the code from > a model. Further assume, that you have no choice and > are not allowed to discuss the sense of this approach :-) > How should the code look like? I am not sure if you are trying to solve a real problem or not. If you are solving a real problem, where you already happen to have an EMF model which you are required to generate code from, then I recommend to just do everything in Java using the existing tools built for EMF. If you decide to still keep working in Haskell, and it works out well, please share your solution because I think many people here will be very interested. wxHaskell, OOHaskell, and O'Haskell are all starting points for this type of project. - Brian

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Grabbing list of functions from current module
by Joel Reymont 10 May '07

10 May '07

Folks, Does anyone have code that can grab a list of functions named with a certain prefix from the current (or given) module? I want to find functions named, say, ast_* and produce a list of tuples like ("input1", ast_input1). Thanks, Joel -- http://wagerlabs.com/

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Listing package dependencies from GHC
by Dougal Stanton 08 May '07

08 May '07

If I'm doing development between ghci and vim, all the different dependencies I need get linked in when required without me asking. Similarly if I call "ghc --make" from the command line. But I have to write them in manually to my *.cabal file otherwise the compilation process will fail. Until now I've just loaded the program in to ghci and noted down all the dependencies it links in. Am I missing a trick? There must be an easier way, especially for multiple source files. cheers, D.

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Is Excel a FP language?
by Tony Morris 06 May '07

06 May '07

In a debate I proposed "Excel is a functional language". It was refuted and I'd like to know what some of you clever Haskellers might think :) My opposition proposed (after some "weeding out") that there is a distinction between Excel, the application, the GUI and Excel, the language (which we eventually agreed (I think) manifested itself as a .xls file). Similarly, VB is both a language and a development environment and referring to VB is a potential ambiguity. I disagree with this analogy on the grounds that the very definition of Excel (proposed by Microsoft) makes no distinction. Further, it is impossible to draw a boundary around one and not the other. I also pointed to the paper by Simon Peyton-Jones titled, "Improving the world's most popular functional language: user-defined functions in Excel", which quite clearly refers to Excel as a [popular] functional language. The debate started when I referred to the fact that financial institutions change their functional language from Excel to something like OCaml or Haskell. Of course, there is no doubting that these companies can replace their entire use of Excel with a functional language, which I think is almost enough to fully support my position (emphasis on "almost"). -- Tony Morris http://tmorris.net/

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Displaying infered type signature of 'offside' functions
by Georg Sauthoff 05 May '07

05 May '07

Hi, I like the strong static type system of Haskell for various reasons. One reason is, that it makes easier to understand new code. I.e. when I read code I type ':t foo' in ghci/hugs from time to time, to check my own idea of the type signature, if it is not included in the source code. Well, ':t ' doesn't work withwith non-top-level/local functions (or does it?). Thus I searched for a way to get this information for functions, which are defined offside (of the main indentation level). So, what tools do you use to get the inferred type signature of local functions? I first looked at the GHC-API, but I found this comment in http://darcs.haskell.org/ghc/compiler/main/GHC.hs : - NOTE: -- - things that aren't in the output of the typechecker right now: [..] -- - type signatures Hm, is this comment in sync with the current GHC source? Then I looked at the ghc options. '-ddump-types' only dumps the signatures of global functions ... But '-ddump-tc' includes type signatures of offside defined functions. I.e. something like ghc -ddump-tc Foo.lhs 2>&1 \ | awk '/^[A-Z].+lhs/, ! // { print $0; }' \ | awk '/ :: / , /\[\]/ { if (!($0 ~ /\[\]/)) print $0; }' \ | less does the trick. But as the haskell/ghc wiki mentions, the -ddump-* output is optimized for human reading and not for some kind of 'automatic use'. For automatic use one should look at the GHC-API ... Well, I mention this, because I would like to integrate some lookup feature (for type signatures) into vim (if it doesn't exist yet). Best regards Georg Sauthoff

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Poor first impression
by Fernando Cassia 04 May '07

04 May '07

I admit in shame never having heard about Haskell before. I know about PHP, Python, IBM' s REXX, TCL, TCL/TK, perl... but Haskell, never. So, here's how I landed in Haskell-land: I was looking for a simple ncurses-based text mode mp3 player with some sort of basic GUI and found "HMP3" written in, you guessed it, Haskell. So I follow the directions and download the huge 30MB+ ghc-6.6.1-i386-unknown-linux.tar.bz2. Bunzip2 it. tar xvf it. ./configure and "make install". So far, so good. and I get the following message, supposedly telling me that the haskell compiler was installed OK... ======================================================================= Installation of ghc-6.6.1 was successful. To use, add /usr/local/bin to your PATH. For documentation, see /usr/local/share/ghc-6.6.1/html/index.html ======================================================================= (/usr/local/bin is already in my path) So I decide to call the ghc compiler with no arguments to see if it was indeed installed, and I get this: [root@localhost ghc-6.6.1]# ghc /usr/local/lib/ghc-6.6.1/ghc-6.6.1: error while loading shared libraries: libreadline.so.4: cannot open shared object file: No such file or directory # So, I conclude that Haskell is not ready for prime time, if it cannot install itself correclty including shared libs in a standard Fedora Core 6 system. Goodbye Haskell, I just wanted to compile a MP3 player, and perhaps if the compiler installed OK with no issues, I'd have taken a look at the language. But as of right now, I don't have time to waste with broken compiler installers. Byebye FC

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'Proper' use of the State monad
by Denis Volk 03 May '07

03 May '07

Hello all, I am trying to make a (turn-based) game in Haskell and need to pass around quite a bit of information, so using the State monad seems most appropriate. My question is, which is a better idea: 1) Using State GameState r and then call execState for each game event (i.e. user input) so I can do IO 2) Using StateT GameState IO () and have the entire game live in one big execStateT call. (I note XMonad does something similar.) There are difficulties with the first option, including keeping even more state about what we're doing (for instance, are we in a menu?), and adding stuff later would possibly require substantial rewrites. Other than the fact that I would have IO in places where it perhaps shouldn't be, the second option seems better, but I am new to all this and it may be that I'm missing something.

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Bloom Filter
by tom 02 May '07

02 May '07

Hi all, I'm pretty new to Haskell, I've been working on a Bloom filter[1] implementation as a learning exercise. I'd really appreciate it if someone more experienced would comment on the code. I'm sure there's plenty of places where I'm doing things in silly or overly complex ways. I've packaged it all with Cabal because I wanted to see how that all worked, I don't think it's ready for release yet! I would like to at some point get it polished up and release it if anyone thinks it might be useful. You can download it at: http://www.almostobsolete.net/bloom-0.0.tar.gz I've also put the Haddock docs for it online at: http://www.almostobsolete.net/doc/html/Data-yBloom.html All comments will be very much appreciated :p Thanks Tom [1] There's a nice description of what a Bloom filter is here: http://www.cs.wisc.edu/~cao/papers/summary-cache/node8.html

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