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question on types
by Daryoush Mehrtash 18 Feb '09

18 Feb '09

Is there a way to define a type with qualification on top of existing type (e.g. prime numbers)? Say for example I want to define a computation that takes a prime number and generates a string. Is there any way I can do that in Haskell? thanks, Daryoush

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How to create an online poll
by Bulat Ziganshin 18 Feb '09

18 Feb '09

Hello haskell-cafe, http://zohopolls.com/ -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

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ANN: The Typeclassopedia, and request for feedback
by Brent Yorgey 18 Feb '09

18 Feb '09

Hi all, If you've noticed the lack of a HWN this week, that's because I've been doggedly finishing my article entitled 'The Typeclassopedia', which I have just submitted for publication in the Monad.Reader. Here's the abstract: The standard Haskell libraries feature a number of type classes with algebraic or categorical underpinnings. Becoming a fluent Haskell hacker requires intimate familiarity with them all, yet acquiring this familiarity often involves combing through a mountain of tutorials, blog posts, mailing list archives, and IRC logs. The goal of this article is to serve as a starting point for the student of Haskell wishing to gain a firm grasp of its standard type classes. The essentials of each type class are introduced, with examples, commentary, and extensive references for further reading. My hope is that this will be a valuable resource to the Haskell community, especially those who are learning. Any feedback would be greatly appreciated, especially if it helps improve the article before publication. A draft can be found here: http://www.cis.upenn.edu/~byorgey/papers/typeclassopedia-draft-090216.pdf Also see my blog post for a bit more info: http://byorgey.wordpress.com/2009/02/16/the-typeclassopedia-request-for-fee… happy haskelling! -Brent

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what is [::]
by Daniel van den Eijkel 18 Feb '09

18 Feb '09

Dear Haskellers, please can anybody tell me what [::] means or where to read about it? A few days ago I saw this for the first time in my life, at the list of instances of the Functor class, and I don't know where to look for information. The search engines I tried didn't produce results for [::]. Thanks, Daniel

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estimating the speed of operation
by Belka 18 Feb '09

18 Feb '09

Hello, communion people! I have a little problem and ask for an advice. I'm trying to estimate the performance speed of one pure function, but get some strange results. ------------------------------------- import qualified module Data.ByteString.Lazy.Char8 as L8 import module Data.Digest.Pure.MD5 import module System.IO.Unsafe import module System.Time import module System.Random import module Data.Char clockTime2Tuple (TOD sec pic) = (sec, pic) getClockTimeTuple = getClockTime >>= (\ x -> return $ clockTime2Tuple x) subTimeTuples (s2,ps2) (s1,ps1) = let dps = ps2-ps1 in let (s,ps) = (s2-s1-(if dps<0 then 1 else 0), if dps<0 then dps+(10^12) else dps) in (s, (div) ps (10^6)) test_md5 test_input n = flip mapM test_input ( \ input_row -> do t1 <- getClockTimeTuple sequence_ (replicate n (return $ md5 input_row)) t2 <- getClockTimeTuple dt <- subTimeTuples t2 t1 return (dt) ) test_list = [ pack "Hello world!"] ++ [L8.replicate 1000000 ':'] ------------------------------------- The results are (iterations_count, microseconds): (1000, [105,105]) (+/- 10) (10000, [1000,950]) (+/- 50) (100000, [9050,9000]) (+/- 50) (1000000, [89200,89150]) (+/- 100) ------------------------------------- I suspect following problems, which make the results non-objective: 1. I cant get out of laziness 2. I don't turn off GHC internal optimizers So, perhaps, GHC evaluates the MD5 once, but cycles on something else?.. For now I can only guess. Could anybody, please clarify and maybe suggest configuration, which would allow objective speed estimation? Thanks in advance, Best regards, Belka -- View this message in context: http://www.nabble.com/estimating-the-speed-of-operation-tp22075843p22075843… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Bug in Cabal?
by Martin Huschenbett 18 Feb '09

18 Feb '09

Hello haskell-cafe, trying to install ghci-haskeline I got the following error message: $ cabal install ghci-haskeline Resolving dependencies... cabal.exe: dependencies conflict: ghc-6.10.1 requires process ==1.0.1.1 however process-1.0.1.1 was excluded because ghc-6.10.1 requires process ==1.0.1.0 Looks like this is a bug in Cabal or cabal-install? Regards, Martin.

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concurrent haskell debugging
by Jianzhou Zhao 18 Feb '09

18 Feb '09

Hi Folks, If I am using Control.Concurrent package, does GHC have any static or runtime tools or debuggers to detect problems, such as deadlock, data races? Cheers Jianzhou

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Fwd: [Haskell-cafe] question on types
by Alberto G. Corona 18 Feb '09

18 Feb '09

As allways, I forgot to forward the message to the list, but the Conor response makes it irrelevant: ---------- Forwarded message ---------- From: Alberto G. Corona <agocorona(a)gmail.com> Date: 2009/2/18 Subject: Re: [Haskell-cafe] question on types To: Ryan Ingram <ryani.spam(a)gmail.com> > Seriously, I love undecidable instances, but there's gotta be a way to > make type-level programming less painful. GHC team: please give us > type-level integers that don't suck! If I ever have to see S (S (S (S > Z))) again it's too soon. I´m starting to read about dependent types and it´s very promising. Maybe my suggestion makes sense: is to "compress" recursive types such are S(S(S(S Z))) into internal compiler representations S <n> Z. This would type match faster and will generate much more clear and compact compilation errors.

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Gtk2HS 0.10.0 Released
by Peter Gavin 18 Feb '09

18 Feb '09

Hi everyone, Oh, dear... it seems I've forgotten how to spell "cafe", and sent this message to haskell-caft(a)haskell.org the first time around. I resent it to all the lists again (just to make sure everyone interested receives it), so I apologize for any duplicated messages you might have received. In any case... I'd like to release the announcement of Gtk2HS 0.10.0. A lot of new stuff has gone into this release, including: - Support for GHC 6.10 - Bindings to GIO and GtkSourceView-2.0 - Full switch to the new model-view implementation using a Haskell model - Support for many more model-based widgets such as IconView and an updated binding for ComboBox - Full Drag-and-Drop support - Better support for Attributes in Pango - Replaced Event for EventM monad, thereby improving efficiency and convenience - Functions for interaction between Cairo and Pixbuf drawing - Lots of bug fixes, code cleanups, and portability improvements With this release, the bindings to GnomeVFS and GtkSourceView-1.0 have been deprecated. The TreeList modules have been deprecated from the Gtk+ bindings. Source and Win32 binaries are available at: https://sourceforge.net/project/showfiles.php?group_id=49207package_id=4244… Thanks to everyone who submitted bug fixes and features this time around! Thanks, Peter Gavin Gtk2HS Release Manager

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"first class" tuples?
by Peter Verswyvelen 18 Feb '09

18 Feb '09

Tuples in Haskell always have annoyed me a bit since each tuple of different dimension is hardcoded (I guess compilers enforce a maximum dimension on tuples?) Since a tuple represents a fixed size data structure with different types at each coordinate, it feels as it should be possible to have a couple of type and data constructors to build a tuple, and to use recursion at the type level to have functions operate on tuples of any dimension. E.g. one could then have a tmap function that takes a tuple of functions and a tuple of values and applies the function at coordinate N to the value at coordinate N. Is something like this possible today in Haskell, e.g. using new features like type families, GADTs, template haskell, etc? Or do we need dependent types for it? In C++x0 I believe it is now possible to do so, since they even allow a variable number of template arguments, using recursion at compile time (see e.g. http://publib.boulder.ibm.com/infocenter/comphelp/v9v111/index.jsp?topic=/c… ) Grapefruit has something like first class records, so I guess it should be possible (and simpler) to do this for tuples?

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