December 2008 - Haskell-Cafe

Data.ByteString vs Data.ByteString.Lazy vs Data.ByteString.Char8
by Galchin, Vasili 03 Dec '08

03 Dec '08

Hello, Some mention is made in corresponding web pages about implementation difference of these three different DataString impl. Any advice? Regards, Vasili

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Re: What causes <<loop>>?
by Martin Hofmann 03 Dec '08

03 Dec '08

I was not sure about it, so I just speculated. Anyway, thanks a lot. martin

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What causes <<loop>>?
by Martin Hofmann 03 Dec '08

03 Dec '08

I've already posted this mail on haskell-cafe, but apparently the subject suggested a too simple question, so I try it here again. I am picking up a discussion with the same topic from haskell-users on 8th November. Thunks with reference on themselves was mentioned as main reason for <<loop>>. > A safe recursive definition would be > let x = Foo (x+1) > However, if you leave out the constructor, > let x = x + 1 > you get a <<loop>> (or a deadlock). > Are there any other reasons? I am trying to debug monadic code which stores state information in a record maintaining several Data.Maps, but in vain so far. A state is modified/changed in several steps by a compound function i.e. changeA $ changeB $ changeC state Could this also lead to a deadlock? If so, can I prevent this using CPS? If the state transformation is the main purpose/computation of the monad at all, is it better to use CPS? Is code using mtl, records or Data.Map more prone to <<loop>> or does this not matter at all. How can I identify self-referencing thunks more easily? Is there any rule of thumb to prevent <<loop>>? Are there any less obvious newbie mistakes causing <<loop> then the one above? Sorry, a lot of questions at once, but I am kind of helpless because I can't find any reason of my <<loop>>, so any comments, experience, and tips are highly appreciated. Thanks, Martin

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Install Yi editor with GHC 6.10.1
by Andy Stewart 03 Dec '08

03 Dec '08

Hi all, I have install GHC 6.10.1 and Gtk2hs (darcs version) in Debian. So i want to ask, have anyone install Yi (darcs version) with GHC 6.10.1 successfully? And the best way to install darcs version Yi? Thank you very much! -- Andy

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Deriving
by Daryoush Mehrtash 03 Dec '08

03 Dec '08

What happens when a type adds driving such as: newtype SupplyT s m a = SupplyT (StateT [s] m a) deriving (Functor <http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#t:Funct…>, Monad <http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#t:Monad>, MonadTrans, MonadIO) Two questions: How does the deriving implement the instance? Is there a way for me to add my own classes in the deriving? for example newtype ..... deriving( xyz) Thanks

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Compatible problem with GHC 6.10.1
by Andy Stewart 02 Dec '08

02 Dec '08

Hi all, I use Debian and GHC 6.8.2-7 Have install cabal, xmonad, gtk2hs with GHC 6.8.2-7 But when upgrade to GHC 6.10.1, i failed to install XMonad 0.8 and gtk2hs 0.9.13 I heard someone install success with develop version. So anyone success install XMonad and gtk2hs with GHC 6.10.1? BTW, i want to know the details compatible problem between 6.8 with 6.10? Is most libraries have compatible with GHC 6.10.1? Any suggestions? Thanks. -- Andy

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The Knight's Tour: solutions please
by Don Stewart 02 Dec '08

02 Dec '08

Lee Pike forwarded the following: "Solving the Knight's Tour Puzzle In 60 Lines of Python" http://developers.slashdot.org/article.pl?sid=08/11/30/1722203 Seems that perhaps (someone expert in) Haskell could do even better? Maybe even parallelize the problem? :) So, team, anyone want to implement a Knight's Tour solver in a list monad/list comprehension one liner? These little puzzles are made for fast languages with backtracking monads.... Post solutions to the wiki.. -- Don

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Moving the messages for compiler to another file?
by nml 02 Dec '08

02 Dec '08

How about moving the messages for compiler to an additional file? My motivation is that we often face a trade-off between aesthetical(elegant code) and practical(efficient code). Like pragmas and strictness annotations, I often feel they make the source code ugly. They don't affect the semantics of our programs, do they? Some people would say this beautifying should be accomplished by an editor, like hiding/showing option for those information. But such separation has additional benefits. For instance, making the source code more compiler-independent. (yeah, this is not the case with language-extensions) And we avoid dispersing those information among our lines. (or even files) In some cases, it would be convenient. I'm not sure if this idea is reasonable, reachable or just naive. Suggestions? Best regards -nml

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GHC 6.10 and OpenGL
by Paul L 02 Dec '08

02 Dec '08

Hi everyone, It's sad to see the OpenGL binding being dropped from GHC binary installers starting from 6.10. Though this issue has been brought up and discussed before, I'm sure a lot of people who based their work on OpenGL would share the same sympathy. I'm not here to argue whether this decision by GHC dev team is right or wrong, but what's really causing the pain is that the OpenGL binding doesn't have its own binary installer for Windows, and compilation from source on this platform is non-trivial. I wouldn't recommend doing it for ordinary users. So my question is, are we going to see a binary installer for OpenGL binding provided separately from either HOpenGL site, or Hackage? Or even the GHC installation page? -- Regards, Paul Liu Yale Haskell Group http://www.haskell.org/yale

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global variables for foreign C functions
by Andrea Rossato 02 Dec '08

02 Dec '08

Hello, I'm writing the bindings to a C library which uses, in some functions, global variables. To make it clearer, those functions need a global variable to be defined. A C program using my_function, one of the library functions, would look like: char progname[] = "a_program_name"; int main( int argc, char *argv[] ) { param p; my_function ( &p ) etc. I've been searching the ML, the wiki, the net, etc. without finding some examples on how such things are dealt with in Haskell - is it possible, BTW? If I import those functions without defining the global variable I get a linker error: /usr/lib/mylib.a(cfile.o): In function `my_function': cfile.c:(.text+0x510): undefined reference to `progname' I hope the issue is clear. Any help would be greatly appreciated. Andrea

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