March 2007 - Haskell-Cafe

Program Command Line Arguments
by unknown＠example.com 10 Mar '07

10 Mar '07

My apologies if this is a question with a trivial answer. Command line args in C are accessed via argc and argv[] defined as arguments to main();. How are command line arguments to a ghc-compiled program accessed in Haskell? Or is that even possible? Thanks.

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literate Haskell newbie question
by Immanuel Normann 10 Mar '07

10 Mar '07

I am a newbie to literate Haskell and these are my two simple questions: How do I compile a literate haskell file foo.lhs (using ghc-6.6)? Is there a tool that translates foo.lhs to foo.hs? Surprisingly I don't find the answer in http://haskell.org/haskellwiki/Literate_programming whereas a lot about translation into tex-files can be found. Thanks, Immanuel

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Implementation of "Dynamic" datatype
by Isaac Dupree 09 Mar '07

09 Mar '07

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Currently we have data Dynamic = Dynamic TypeRep Obj where Obj is {-in GHC up to 6.6-} type Obj = forall a . a {-in GHC 6.7.something-} type Obj = Any -- Use GHC's primitive 'Any' type to hold the dynamically typed value. {-in some other compilers (like Hugs)-} data Obj = Obj ( According to this comment, it is not data Obj = Obj in GHC now because: -- In GHC's new eval/apply execution model this type must not -- look like a data type. If it did, GHC would use the -- constructor convention when evaluating it, and this will go -- wrong if the object is really a function. Using Any [or, -- earlier comments said, a polymorphic type] forces GHC to use -- a fallback convention for evaluating it that works for all -- types. ) So, my question is: why doesn't it just use an existential? : data Dynamic = forall a. Dynamic TypeRep a According to http://hackage.haskell.org/trac/haskell-prime/wiki/HaskellExtensions , all important implementations support existentials (and it will probably go into Haskell'). It seems semantically the most accurate, doesn't require an unsafeCoerce when _constructing_ a Dynamic, and doesn't require unsafeCoercing between unequal types (which had caused problems in GHC as noted above), so it seems like it could be a portable implementation. A quick copy of Dynamic.hs with these modifications seemed to work fine in ghc6.4.2, ghc6.6, and hugs20050308 (which is all haskell implementations that are working for me at the moment (nhc98 is broken for me) and that have Data.Typeable (not yhc currently, it seems)). ( Also I wonder about the performance implications of changing it to data Dynamic = forall a. Typeable a => Dynamic a or making the TypeRep be a strict field (which shouldn't have any semantic impact since typeOf always succeeds(assuming no-one defines a horrible instance of Typeable) and typeOf's result is always what is put into that field. (Separately, should seq'ing a Dynamic seq the encapsulated object, or should Dynamic act as a box as it does now?)). ) Anyway, how would I go about seriously testing this in ghc, hugs... to see if it breaks anything (and also see how it affects Dynamic's performance)? (Since I am interested in the refactoring of the base/core libraries, a little something like this seems like a good place to start learning how to test what I have.) Isaac - - Was this a good mailing list to choose for this question? (And was it a good question? what about all the little question-ideas branching off of it? I can never be sure...) -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.3 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org iD8DBQFF6LVSHgcxvIWYTTURAoeTAKCP8XIn4pVXnJsimEX8aAgIcXLb1ACbBOw2 elKn59TdPKiAJq8VA4MOd0o= =NF4R -----END PGP SIGNATURE-----

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Google summer of code
by Malcolm Wallace 09 Mar '07

09 Mar '07

Google Summer of Code As many of you will already know, Google is running its "Summer of Code" project again this year, and haskell.org is once again going to apply to be a mentoring organisation. Are you a student who would like to earn money for hacking in Haskell? Or are you a non-student who has a cool idea for a coding project but no time to do it yourself? Well, our wiki to gather ideas is now up-and-running again: http://hackage.haskell.org/trac/summer-of-code Add yourself to the list of interested people! There are some ideas still there from last year. Add new ones! Google will start accepting student applications in just over a week's time, but now is the time to start using our own wiki to match up interesting ideas with interested people. The official timeline is as follows: March 12: Mentoring organization application deadline March 14: List of accepted mentoring organizations published March 14: Student application period opens March 24: Student application deadline Interim Period: we review and rank student proposals April 9: List of accepted student applications published Interim Period: Students learn more about their project communities May 28: Students begin coding; Google begins issuing initial payments July 9: Students upload code to code.google.com/hosting; July 16: Google begins issuing mid-term payments August 20: Students upload code to code.google.com/hosting; August 31: Final evaluation deadline; Google begins issuing final payments Regards, Malcolm

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auto-completion for Vim 7
by Stephan Walter 09 Mar '07

09 Mar '07

Hi, I started to write a Vim script for the new omni-completion feature of Vim 7. If you use Vim you might want to check it out: * download http://stephan.walter.name/files/haskellcomplete.vim and put it in ~/.vim/plugin * :set omnifunc=haskellcomplete#CompleteHaskell * press <c-x><c-o> to complete (The advantage of omni-completion over normal completion using <c-n> and <c-p> is that you can write you own function which does some smart guessing to offer good completions) So far the functionality is *very* limited. It will autocomplete a few functions from the prelude as well as functions in the current file that have a type declaration. If you look through the source code of the script, you'll see that I have included some function names and types from the standard libraries. Before I continue this (rather tedious) work, I wanted to know what the cafe patrons think about it. If someone knows of a simple way to generate a function/type list of the standard modules, I'd love to hear about it. Generating such a list on the fly each time is probably not an option, as it would be too slow. -Stephan

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library for generating HTML and LaTeX
by Henning Thielemann 09 Mar '07

09 Mar '07

We have Text.Html in the standard libraries for generating HTML code. Is there also a library for generating LaTeX code? Or even better: Is there a library which can emit the same document with markup in HTML, LaTeX or a different markup language? Frequently I like to present results of some computations and then I start hacking output for LaTeX and I wonder if there is a simpler solution.

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Fwd: [Haskell-cafe] Re: Haskell spacing problem.
by Mark Goldman 09 Mar '07

09 Mar '07

Oops, think I didn't send this to cafe, forgive any duplicates please. -mdg ---------- Forwarded message ---------- From: Mark Goldman <bitshifter(a)gmail.com> Date: Mar 9, 2007 8:37 AM Subject: Re: [Haskell-cafe] Re: Haskell spacing problem. To: Frozz <dj_frozz(a)hotmail.com> >From a different angle, are you looking at the text file with a proportional font instead of a fixed width font? With a proportional font each letter takes up a different amount of horizontal space which mucks up most ASCII based formating. -mdg On 3/9/07, Frozz <dj_frozz(a)hotmail.com> wrote: > prettyPrint seems complicated.. > > Dan Mead <d.w.mead <at> gmail.com> writes: > > > > > > > that seems like the "wrong" waycheck out prettyprint in the standard library > > > > On 3/9/07, Frozz < > > dj_frozz <at> hotmail.com> wrote:Hi Haskell ppl,I'm trying to solve a > problem that had been bothering me for a long time. I'm > > trying to create index and display the index in Hugs as well as an output > textfile.Below is my code. In Hugs, the index managed to get displayed well > with nicealignments. However, for the text file, the alignment is off. The > bracket with > > the numbers are messy. I really don't know why this would happen.Desired > output:Anna [1]Ben[2]Shane[3]dispTable :: [([Int], Word)] -> > StringdispTable = unlines . map (\ (num,word) -> word ++ > > take (25 - (length word)) (repeat ' ')++ show num)Did i do sth > wrong? Thanks in > advance._______________________________________________Haskell-Cafe mailing > list > > Haskell-Cafe <at> haskell.orghttp://www.haskell.org/mailman/listinfo/haskell- > cafe > > > > > > > > > > > > _______________________________________________ > > Haskell-Cafe mailing list > > Haskell-Cafe <at> haskell.org > > http://www.haskell.org/mailman/listinfo/haskell-cafe > > > > > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > -- Our problems are mostly behind us, now all we have to do is fight the solutions. -- Our problems are mostly behind us, now all we have to do is fight the solutions.

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Haskell spacing problem.
by Frozz 09 Mar '07

09 Mar '07

Hi Haskell ppl, I'm trying to solve a problem that had been bothering me for a long time. I'm trying to create index and display the index in Hugs as well as an output text file. Below is my code. In Hugs, the index managed to get displayed well with nice alignments. However, for the text file, the alignment is off. The bracket with the numbers are messy. I really don't know why this would happen. Desired output: Anna [1] Ben [2] Shane [3] dispTable :: [([Int], Word)] -> String dispTable = unlines . map (\ (num,word) -> word ++ take (25 - (length word)) (repeat ' ') ++ show num) Did i do sth wrong? Thanks in advance.

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Distributing a GHC-compiled binary for Macs (x86)
by Arjun Guha 09 Mar '07

09 Mar '07

Hi, I compiled a binary using GHC 6.6 on my Mac (specifically, using ghc --make). This binary seems to depend on the GNU MP framework--I imagine GHC uses it to implement its numeric tower. However, Macs that don't have GHC installed don't seem to have GMP, so I'll guess that it was installed along with the GHC binary. Is there any way to get around this GMP dependency? I expect the binary to be used on Macs that don't have GHC installed. It would be acceptable if I instructed Mac-users to download a binary installer for GMP, but I haven't been able to find one. Any hints or suggestions would be appreciated. Thanks. Arjun

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A different Maybe maybe
by Joachim Breitner 09 Mar '07

09 Mar '07

[Also on http://www.joachim-breitner.de/blog/archives/229-A-different-Maybe-maybe.ht…] Hi, For a while I have been thinking: Isn’t there a way to get rid of the intermediate Maybe construct in a common expression like “fromMaybe default . lookup”. It seems that a way to do that would be to pass more information to the Maybe-generating function: What to do with a Just-Value, and what to return in case of Nothing. This leads to a new definion of the Maybe data type as a function. Later I discovered that this seems to work for any algebraic data type. This is probably nothing new, but was offline at the time of writing, so I didn’t check. This means that this might also be total rubbish. Enjoy. * “Algebraic Data Type Done Differently” (PDF-File) at http://www.joachim-breitner.de/various/FunctionalMaybe.pdf * “Algebraic Data Type Done Differently” (Literate Haskell Source) at http://www.joachim-breitner.de/various/FunctionalMaybe.lhs Comments are appreciated. Greetings from my holidays, Joachim -- Joachim "nomeata" Breitner mail: mail(a)joachim-breitner.de | ICQ# 74513189 | GPG-Key: 4743206C JID: joachimbreitner(a)amessage.de | http://www.joachim-breitner.de/ Debian Developer: nomeata(a)debian.org

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