June 2007 - Haskell-Cafe

Name Decoration and Undefined References
by SevenThunders 01 Jul '07

01 Jul '07

I have a very complex project that has to play nice with a lot of C code written by other people. My own code is half Haskell and half C. My build process is rather complex since I generate about 5 different libraries, some of them export Haskell routines. A supreme effort was made to try to make Haskell somewhat invisible, although ultimately it requires a lot of gcc command line work with a bunch of undefines and a links to Haskell static libraries. In the end I'm not sure if it's such a good idea and probably would be better off completing the final link stage just using ghc. In the process of doing all this I attempted to link one of my libraries to some test code written in Haskell and I proceeded to get a few unresolved links whilst running ghc. What happened is that I have a Haskell binary, lets say called foo.o generated from foo.hs that appears in two libraries say called libmyh.a and libmyc.a and that contains a Haskell function called bar. Library libmyh.a ultimately gets built using cabal and libmyc.a gets built using gcc directly. The binary is compiled twice (for complicated reasons) prior to linking to each library. However in the library libmyh.a, the bar symbol is prepended with an ascii string that contains the library name (say libmyh). However the other library shows that the symbol does not have this name decoration since it was linked directly with gcc. It appears however when trying to link against libmyc.a that ghc expects to see the original name decoration for bar. I presume this is due to some side information, perhaps what's contained in foo.hi that informs ghc what names to actually link to. I now ask the question, how can I control this process? Build tools such as CMake and Cabal seem to have their own ideas about what directories to place binaries. It seems like I need to somehow make sure that all files related to the compilation process of a given library are isolated in the same directory or something. Thus it's not enough to specify binaries on the command line of ghc, it's got to have access to some other files; I presume the .hi files. This makes things a bit confusing I'm afraid. -- View this message in context: http://www.nabble.com/Name-Decoration-and-Undefined-References-tf4003458.ht… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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"Read-only" functions in State Monad
by Ken Takusagawa 01 Jul '07

01 Jul '07

I'd like to have a state monad with the feature that I can somehow annotate using the type system that some functions are only going to read the state and not modify it. Such read-only functions are only permitted to call other read-only functions, whereas state-modifying functions can call both read-only and other state-modifying functions. How can I do this? It does not seem to be straightforwardly doable with Control.Monad.State. --ken

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Fwd: Re: [Haskell-cafe] avoiding command window with wxHaskell on Windows?
by Dean Herington 30 Jun '07

30 Jun '07

>Date: Mon, 25 Jun 2007 20:19:50 -0400 >To: Jens Fisseler <Jens.Fisseler(a)FernUni-Hagen.de> >From: Dean Herington <heringtonlacey(a)mindspring.com> >Subject: Re: [Haskell-cafe] avoiding command window with wxHaskell on Windows? >Cc: haskell-cafe(a)haskell.org, wxhaskell-users(a)lists.sourceforge.net > >At 10:10 PM +0200 6/23/07, Jens Fisseler wrote: >>On Sat, 23 Jun 2007, Dean Herington wrote: >> >>> But if I double-click the .exe file, I get a command window that >>>hangs around >>> (but doesn't appear to do anything, fortunately) until the >>>program terminates. >>> How can I avoid this command window? >> >>With gtk2hs, using "-optl-mwindows" as a command line option for GHC lets >>me get rid of this window. Perhaps it will do the same for wxHaskell? > >Yes, that did the trick! Thanks a lot! But now I've discovered that using "-optl-mwindows" creates a program that doesn't work when invoked from a command line. Is there any way to create a program that can work when invoked either from a command line or through double-clicking? Dean

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advice: instantiating/duplicating modules
by Nicolas Frisby 30 Jun '07

30 Jun '07

I wrote a combination reader/writer monad (a la the RWS monad in the mtl) and I find myself wanting to use multiple instances of it in the same stack of transformers. The functional dependencies prevent this from working out. The class is called MonadRW and the transformer is called RWT. I find myself wishing I could import the same module twice, but instead of having two names for the same entity, I want the definitions proper of the module to be duplicated: I want two separate MonadRW classes and two separate RWT transformers. Since the module integrates the MonadRW and RWT transformer with the mtl, I would then only need to lift the instances of the instantiated MonadRW classes through the other RWTs. I'm rather unfamiliar with Template Haskell, but it sounds like it might fit the bill. Although, if I recall correctly, instances and type declarations splicing are yet to be implemented in GHC? Does anyone have any advice how to do this? I'm envisioning some preprocessing. I'd like to know if someone has developed a tool for this already or if this path has been attempted. Thanks for your time, Nick PS - If this doesn't work out, I'm aware that I could use type-indexed products and coproducts to pull it off, but I think that would drastically reduce the number of people who could understand/maintain the code without having to learn such cool stuff.

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ANNOUNCE: hiccup, a toy tcl impersonator in haskell
by Kyle Consalus 30 Jun '07

30 Jun '07

A while back I saw a toy tcl interpreter in 550 lines of C called 'picol'. I was looking for a simple language to implement in haskell, so I made my own toy tcl interpreter. It was surprisingly easy to make, thanks to the magic of Haskell and Bytestrings. :) It handles a few things incorrectly, and it is not feature complete, but I thought maybe somebody else might find it interesting. For the record (though it means nothing), my interpreter is about half the size of picol and runs about 30% faster in the few tests I've run, despite having a few more features. (it was certainly less than half the size of picol, but I keep tacking on functions when I get bored). Anyway, you can check it out at: http://code.google.com/p/hiccup/ I plan on making it more complete and faithful to tcl, but I'm branching into a different project for this one. However, any suggestions to make hiccup more efficient, elegant, or correct are certainly appreciated. Cheers, Kyle

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Organising teams for ICFP 2007
by dons＠cse.unsw.edu.au 30 Jun '07

30 Jun '07

Interested in competing in the ICFP 2007 programming contest -- the hackers contest of choice! http://www.icfpcontest.org There's 2000 people on this mailing list, and 350 people in #haskell, we must be able to put together a few decent teams out of that talent pool.... To help people get organised, I've created a page on the haskell.org wiki to help Haskell (and FP) people find teams: http://haskell.org/haskellwiki/ICFP_Programming_Contest/Teams_2007 If you've got a team, but need more people, or need a team, add your details, and hopefully people will be able to find you. The more teams the better. Also, drop by the #haskell-icfp07 IRC channel to talk to people and organise. -- Don

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let vs do?
by Greg Meredith 30 Jun '07

30 Jun '07

Haskellians, Once you have a polymorphic let, why do you need 'let' in the base language, at all? Is it possible to formulate Haskell entirely with do-notation where there is a standard monad for let environments? Probably this was all discussed before in the design deliberations for the language standard. Pointers would be very much appreciated. Best wishes, --greg -- L.G. Meredith Managing Partner Biosimilarity LLC 505 N 72nd St Seattle, WA 98103 +1 206.650.3740 http://biosimilarity.blogspot.com

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Updating haskell.org robots.txt
by Justin Bailey 29 Jun '07

29 Jun '07

All, Referencing this thread ( http://www.haskell.org/pipermail/haskell-cafe/2007-May/025769.html) I have prepared a small update to robots.txt. It doesn't do much, but should exclude all dynamic pages from being spidered. Please vet it and let me know of any concerns. Note I'm not really interested in reconfiguring the wiki to have nicer URLs - what is there is what I'm working with right now. Here's the updated robots.txt (availabe as http://www.haskell.org/robots2.txt right now): User-agent: * Disallow: /haskellwiki/Special: Disallow: /haskellwiki/Special%3A Disallow: /haskellwiki/? Disallow: /haskellwiki/%3f Any feedback, let me know. Thanks! Justin

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HGL on Windows
by peterv 29 Jun '07

29 Jun '07

I vaguely remember to have read that HGL is (currently) not supported on Windows, but I can't find this information any more. Is this correct? It seems not to be included in GHC 6.6

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Best idiom for avoiding Defaulting warnings with ghc -Wall -Werror ??
by Dave Bayer 29 Jun '07

29 Jun '07

Hi all, I've been going over my code trying to get it all to compile with "ghc -Wall -Werror", without introducing constructs that would make my code the laughing stock of the dynamic typing community. They already think we're nuts; my daydreams are of a more computer literate society where Jessie Helms stands up in the U.S. Senate to read aloud my type declarations to the derisive laughter of the Ruby and Lisp parties. There's a fine line between my opinion as to how GHC should issue warnings, and a legitimate bug report. I've already submitted a bug report for the need to declare the type of the wildcard pattern, because I believe that the case is clear. Here, I'm seeking guidance. Perhaps I just don't know the most elegant construct to use? My sample code is this: > {-# OPTIONS_GHC -Wall -Werror #-} > > module Main where > > import Prelude hiding ((^)) > import qualified Prelude ((^)) > > default (Int) > > infixr 8 ^ > (^) :: Num a => a -> Int -> a > x ^ n = x Prelude.^ n > > main :: IO () > main = > let r = pi :: Double > x = r ^ (3 :: Int) > y = r ^ 3 > z = r Prelude.^ 3 > in putStrLn $ show (x,y,z) > GHC issues a "Warning: Defaulting the following constraint(s) to type `Int'" for the definition of z. The definition of y glides through, so a qualified import and redefinition of each ambiguous operator does provide a work-around, but the code is lame. (I could always encapsulate it in a module Qualude.) If I import a module that I don't use, then "ghc -Wall -Werror" rightly complains. By analogy, if I use "default (Int)" to ask GHC to default to Int but the situation never arises, then GHC should rightly complain. Instead, if I use "default (Int)", GHC complains about defaulting anyways. In my opinion, this is a bug, but I'd like guidance before reporting it. Is there a more elegant way to handle the numeric type classes with "ghc -Wall -Werror" ? No one is forced to use "ghc -Wall -Werror", but it should be a practical choice. I've enjoyed the recent typing discussions here. On one hand, there's little difference between using dynamic typing, and writing incomplete patterns in a strongly typed language. On the other hand, how is an incomplete pattern any different from code that potentially divides by zero? One quickly gets into decidability issues, dependent types, Turing-complete type systems. My personal compromise is to use "ghc -Wall -Werror", live with the consequences, and get back to work. Perhaps I'll get over it, but that's a slippery slope back to Lisp.

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